Relative and ‘absolute’ dating of land surfaces

Relative and ‘absolute’ dating of land surfaces

A.L. Watchman a,1, C.R. Twidale b,*

aSchool of Anthropology, Archaeology and Sociology, James Cook University, Townsville, Queensland 4811, AustraliabDepartment of Geology and Geophysics, University of Adelaide, Adelaide, South Australia 5005, Australia

Received 25 April 2000; accepted 9 July 2001

Abstract

The dating of land surfaces has long posed problems for geomorphologists. Relative methods (stratigraphic, geomorphic,

topographic) are sound and convincing. Exhumed forms may complicate identification and relationships, for both epigene and

etch forms have been buried, and exhumed, but in tectonically undisturbed areas, the higher surfaces are older than those

preserved at lower levels. Also, surfaces have an age range. The relationship of surfaces with volcanic deposits, old shorelines,

and genetically related sedimentary sequences provides sound ages, and correlation with dated duricrusts and faults is also

useful. There are no temporal limits to relative dating, for the methods are equally applicable to the dating, say, of Proterozoic

surfaces as of those of Pleistocene age. The disadvantage of such methods is that the necessary evidence is frequently either not

preserved or not exposed.

The so-called ‘absolute’ (physical, numerical) methods, and especially those based on exposure age dating with in situ

cosmogenic radionuclides, are appealing because they produce direct numerical ages, and appear to be widely applicable, but

there are severe temporal limitations, and sampling problems complicate, and may invalidate, interpretation. Absolute age

determinations must be consistent with the stratigraphic and geomorphologic settings. The best results are obtained when

physicists and earth scientists pool their knowledge and experience. A background in local and regional geology is especially

important. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: definition of age; types of surface; relative dating; absolute dating; value of dating

1. Introduction

Land surfaces can be regarded as including vari-

ous landform assemblages or suites. They can be

classified genetically as erosional or depositional, and

the former as epigene, etch or exhumed (Twidale,

1985).

Epigene surfaces are formed by various agencies

operating at the land surface. Some workers refer to

them as ‘‘subaerial’’. Most extensive epigene plains

have been shaped by rivers, for both marine and

aeolian planation, though effective, are inherently

limited in both their distribution and the areal extent

of their end-products. Thus, though various workers

have advocated extensive marine planation (Ramsay,

1846; Cushing, 1913; Barrell, 1920; Bascom, 1921;

Olmsted and Little, 1946), negative feedback im-

poses limits on the efficacy of wave action, an

important component of the marine planation equa-

tion. Shore platforms more than a few hundred (ca.

0012-8252/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0012 -8252 (01 )00080 -0

* Corresponding author. Tel.: +61-8-8303-5392 (Direct), +61-

8-8303-5376 (Department); fax: +61-8-8303-4347 (Department).

E-mail addresses: [email protected]

(A.L. Watchman), [email protected] (C.R. Twidale).1 Fax: + 61-7-4781-4045.

www.elsevier.com/locate/earscirev

Earth-Science Reviews 58 (2002) 1–49

1200) metres width are unlikely save in the optimal

circumstance of a slowly rising relative sea level

(King, 1963). Similarly, though wind action is argu-

ably responsible for yardangs and some major defla-

tion hollows (e.g., Bobek, 1969; Breed et al., 1989),

it is limited to extremely arid areas and outcrops of

weak sediments.

Many depositional plains are also riverine and,

whether due to lateral corrasion or to vertical accre-

tion, are distinguished from their erosional counter-

parts by their extreme flatness (Fig. 1a and b).

Though all till plains must be, to some extent,

fluvioglacial, they are extensive but characteristically

hummocky and irregular, and fluvioglacial outwash

plains are of limited extent and distribution. Dune-

fields occupy huge areas in the midlatitude deserts as

well as being developed in narrow zones in many

coastal settings. Such dunefields are topographically

differentiated, and though some sand plains display

little relief, even there broad swells and swales are

discernible.

Whatever their origin, surfaces exposed to the

elements are weathered, and weathering produces an

altered mantle or regolith. The lower limit of weath-

ering is the weathering front (Mabbutt, 1961a) and as

most regoliths are less resistant to erosion than cohe-

sive bedrock, the front is in many places exposed as

an etch surface (Fig. 1c). Even where weathering has

produced a duricrust, stream incision leads to the

preferential erosion of the underlying weathered

zones, in turn resulting in the undermining of the

capping, and backwearing of the slopes. Baselevel

permitting, such scarp retreat has in many areas

effectively caused stripping of the regolith down to

the level of the weathering front, which is thereby

exposed. Epigene and etch surfaces have been buried

by sediments or by volcanic rocks but later re-exposed

as exhumed forms (Fig. 1d).

These various types of surface may all occur in

close proximity and require close examination before

they can be identified and differentiated. For exam-

ple, in the vicinity of Scrubby Outstation some 35

km north of Nonning Homestead, in the central

Gawler Ranges, South Australia (see Fig. 2 for loca-

tion of this and other places mentioned in the text),

the plains are partly erosional, in the form of mantled

pediments cut in the Gawler Range Volcanics, a

siliceous effusive but welded volcanic rock of Meso-

proterozoic (ca. 1592 Ma) age; and partly depositio-

nal, with narrow fill terraces along valley floors, and

quite extensive areas of sand plain and degraded relic

dunes of probable Late Pleistocene age. Standing up

to 10 m higher than the plains are mesas capped by

silcrete overlying weathered rock. They are relics of

epigene plains rising from bedrock platforms or low

rises (Fig. 3).

Two types of bedrock plain can be distinguished.

In some mesas, the weathered material is altered

volcanic material (dacite, dacitic rhyolite) and as the

base of the regolith is coincident with the adjacent

bedrock surface, the latter is of etch type and is in one

sense of the same age as the regolith, but in another

post-dates it: it has two ages of origin. Others, how-

ever are underlain by up to 2 m of quartzite and

cobblestones of the Pandurra Formation, a fluviatile

sequence deposited about 1424 Ma. In places the

strata are weathered but elsewhere fresh. In either

instance, however, the adjacent platforms cut in vol-

canic rock are the surfaces upon which the Pandurra

deposits were laid down during the Mesoproterozoic,

but which have since been re-exposed or exhumed as a

result of the partial stripping of the cover in post

silcrete times.

Silcrete has not yet been dated directly (though U-

series or cosmogenic nuclide procedures—see below,

Section 5—surely offer possibilities?) but its topo-

graphic position is such that it postdates both the

volcanics and the Pandurra strata, for it is developed

on both, but it predates the incision of the surface on

which it is formed and is also older than the sand and

the pedimented plains. Regional correlations suggest a

Tertiary age—Eocene, or about 60 Ma old, if related

to the silcrete preserved to the west or, and more

likely, Miocene (ca. 20–24 Ma old) if developed

during the phase of silicification which affected sur-

faces in the Lake Eyre basin, Flinders Ranges and

adjacent areas at that time.

Thus, surfaces can be dated using relative (geo-

logic, stratigraphic, geomorphic, topographic) and

absolute (physical, numerical) methods, each having

its advantages and drawbacks. In the Scrubby Out-

station landscape, the numerical ages given for the

Gawler Range Volcanics, the Pandurra Formation,

the putative Late Pleistocene age of the sand plains

and dunes, have been derived from radioactive iso-

tope ratios and provide limiting ages for events and

A.L. Watchman, C.R. Twidale / Earth-Science Reviews 58 (2002) 1–492

Fig. 1. (a) Featureless alluvial plain of the Georgina River near Bedourie, southwest Queensland. (b) Broadly rolling plain cut in schist, with quartz-buttressed inselbergs, which are

part of The Pinnacles, just west of Broken Hill, western New South Wales. (c) Dissected etch surface in schist exposed from beneath a massive calcrete capping, Kuiseb Valley, central

Namibia. (d) The subCretaceous unconformity exposed in a shore platform at Bullocky Point, Fannie Bay, Darwin, Northern Territory. Precambrian strata are exposed in the platform,

Cretaceous in the backing cliff.

A.L.Watch

man,C.R.Twidale

/Earth

-Scien

ceReview

s58(2002)1–49

3

forms, but the topographic positions of the volcanics,

sediments and regoliths provide evidence for the

relative ages, and also determine the type of surface.

Relative methods applied to epigene and etch

surfaces include the use of correlative deposits, rela-

tionships between land surface and dated deposits,

date of fault dislocation, the relationship of surfaces to

dated volcanic materials, and the dating and correla-

tion of duricrusts formed on surfaces. Several of these

methods are commonly applicable in the same region,

thereby providing useful cross checks.

The relative or absolute ages of strata or intrusive

bodies transected by epigene or etch surfaces does

limiting ages, and stratigraphic bracketing provides

the same also for exhumed surfaces. The principles

underlying these methods are sound, and largely

involve common sense. In theory, surfaces of any

age can be dated, but the necessary evidence is

frequently either not preserved or not exposed.

Absolute dating methods (Table 1) applied to

surfaces or forms appear to offer precision and seem-

ing certainty but are bedevilled by theoretical and

technical problems: hence the revisions of chronolog-

ical data that have occurred over the past several

decades and the adjustments of individual ages and

sequences of dates that still take place. Nevertheless,

absolute age determinations have provided closely

constrained orders of magnitude to the geological

time scale and continue to offer close approximations

of ages for events and surfaces that would otherwise

be subject to speculation and obfuscation.

2. What is the age of a landform or surface?

Like strata, most landforms and the surfaces they

comprise are developed not instantly but over time,

and so have an age range (Twidale, 1956; King, 1962,

pp. 332–334). This is most obviously true of a plain

resulting from long distance scarp-retreat (Fisher,

1866; King, 1942, 1957) for the resultant surface is

manifestly diachronic, the scarp-foot plain being

younger than that formed near the valley axis. But

even plains produced by the lowering of slopes—the

peneplains of Davis (1899, 1909, pp. 249–278)—are

also developed over a period rather than an instant of

time, for the grading of the surface commences at the

coast and adjacent to the major river channels, and

Fig. 2. Location maps: (a) World, (b) Australia.


extends inland and headwards through time. Land-

scapes in a steady state (Hack, 1960), though morpho-

logically unchanging during any major phase of equi-

librium, are nevertheless constantly weathered and

eroded, with the waves of fluvial regradation extend-

ing from valley axes upslope.

Fig. 2 (continued ).

A.L. Watchman, C.R. Twidale / Earth-Science Reviews 58 (2002) 1–49 5

Also, as with strata, the age of a surface refers to the

time of its origin. The age of a Neocomian bed

originating in the Early Cretaceous, about 130–140

Ma, is not measured but deduced from its position vis-

a-vis adjacent strata, its fossil content, age of datable

detrital minerals and its relationship to dated igneous

bodies. Since deposition, it has not remained in its

pristine state, but has, for instance, suffered diagenesis.

Yet, it persists and is thus also a contemporary feature.

Similarly, all landforms are components of the

contemporary landscape, but the ‘age’ of a surface or

form conventionally refers to its date of origin. Just as

a wine is appropriately dated according to its vintage,

with all that that implies for the climatic conditions that

obtained in that year, so a surface is referred to in terms

of its period of initiation. (A similar convention has

been suggested for pedogenic accumulations such as

calcrete: Netterberg, 1969). In this way its origin may

either become more comprehensible in terms of, or it

may clarify, the environment in which it evolved.

Thus, Ayers Rock (Uluru) is a component of the

present landscape of central Australia, but its origin

can be traced back some 70 Ma to the Maastrichtian

when what is now the summit bevel was the crest of a

hill, then probably with a regolithic cover, bounded by

quite gentle slopes. This bevel can be dated by

projecting it a few kilometres into unconformity

beneath a broad shallow valley floored by paludal

deposits of Maastrichtian age (Twidale, 1978; Harris

and Twidale, 1991). At that time the climate was warm

Fig. 3. Diagrammatic section to show the different types of surface developed and preserved in a small area (ca. 5-km diameter) north of

Nonning Homestead, in the central Gawler Ranges of South Australia. V—Gawler Range Volcanics, S—silcrete, 1a— weathered volcanics,

1b—weathered Pandurra Formation, and X—approximate limit of Pandurra deposition, D—Late Pleistocene dune sand and local alluvium,

L—lacustrine and alluvial beds.

Table 1

Summary of the application ranges, materials and minerals used and the time ranges for absolute dating methods

Dating method Application range Materials/minerals Time span

Radiocarbon Sediments, lacustrine carbonates, alluvial fans,

glacial deposits, fault movement rates,

archaeology

Charcoal, cellulose, soil

organic carbon, plant fibres,

wood, shell, bone

< 40 ka

Cosmogenic isotopes Sedimentation, volcanic events, meteorite

impacts, rapid erosion, landslides

Quartz, rocks, olivine,

orthopyroxene

0.01–20 Ma

Luminescence Sediments (colluvial, alluvial and aeolian) Quartz, feldspar < 120 ka

(possibly

400 ka)

Uranium series Travertines, speleothems, corals Uranium-bearing minerals

deposited from water

< 500 ka

Argon Formation of igneous and metamorphic rocks,

landscape elevation and exhumation,

tephra deposits, start of weathering

Feldspar, other rock-forming

minerals

> 100 ka

Electron spin resonance Sedimentation, fault activity, loess, sands Foraminifera, flint, speleothems,

travertine, shell, coral,

quartz-rich volcanics

< 100 Ma

Palaeomagnetism Volcanic rocks, laterites Iron oxides < 70 Ma


and humid. The Cainozoic has seen successive stages

of scarp-foot weathering and erosion resulting in the

steepening of the marginal slopes to produce the bluffs

which delimit the residual, and in detail the gaping-

mouth caves and the flared slopes preserved on and at

the base of the flanks, especially the southern scarp.

In similar fashion, extant rivers are referred to ac-

cording to the age of demonstrable initiation. The River

Torrens, which runs to the sea through Adelaide, in

South Australia, and several creeks or streams in the

southern Flinders Ranges (Mt. Arden, Kanyaka, Wir-

reanda) were in existence in the Eocene. Though still

active, and apparently continuously active (though only

in geological terms, for all the Flinders Ranges rivers

are episodic) since initiation, they are of at least Eocene

age and are described in those terms (Twidale, 1997).

Once formed, a surface or landform does not

necessarily remain unchanged and unchanging, but

may be weathered and eroded or covered by detritus

as a result of exposure to the elements. Some forms

and surfaces have, however, manifestly survived long

aeons of exposure (e.g., Hossfeld, 1926; Craft, 1932;

Hills, 1934; Dixey, 1938; King, 1942; Twidale, 1976,

1994; Young, 1983; Partridge and Maud, 1987). Per-

sistence of a landform or surface as an element of the

modern landscape reflects the incapacity of the agents

of weathering and of erosion, operative at the partic-

ular site or in the particular area at the relevant time, to

eliminate or substantially change the original. Water is

the key (e.g., Twidale and Bourne, 2000a,b). For

instance, the flanks of Ayers Rock have become

steeper as a result of alternations of water-related

scarp-foot weathering and erosion through the Tertiary,

although the summit bevel which has been exposed

and which has shed water has endured, with the result

that the residual has stood in relief for some 70 Ma.

The relationship of a form with respect to its

surroundings may also change in time. A hill may be

reduced, but relief amplitude may also increase

through time because of unequal activity resulting in

the preferential lowering of valleys and plains (Crick-

may, 1932, 1976; Twidale, 1991a; Campbell and

Twidale, 1991). Thus, on northern Eyre Peninsula,

South Australia, reinforcement or positive feedback

effects (Twidale et al., 1974) have caused the plains to

be lowered more rapidly than the hills, causing an

increase in relief amplitude or elevation differential

(Twidale and Bourne, 1975; Twidale, 1982a). The

crestal bevels of some high inselbergs, such as Mt.

Wudinna, and Carappee and Ucontitchie hills, all on

northern Eyre Peninsula, can be correlated with very

old surfaces (Cretaceous or even older), which are

preserved in the general region (e.g., Campbell and

Twidale, 1991).

The present Mt. Wudinna, for instance, may be

construed as simulating the form of the original, but at

a lower level. Alternatively, and more probably, its

crestal areas, displaying the dimpled and grooved

morphology typical of granitic weathering fronts,

may be interpreted as the little-changed etch equiv-

alent of the original (see also Hills, 1975, p. 300). Just

as a fossil is not in its original pristine condition, but

may be replaced by minerals or sediment, or have lost

its detailed morphology, so a landform or land surface

is not unchanged; but both fossil and surface are

recognisable for what they originally were. On the

other hand, even slight surface changes may be

critical in absolute dating considerations.

3. Why date land surfaces?

All of the landforms which constitute the Earth’s

continental surfaces developed at some time in the

past, and all are still evolving, albeit at very different

rates. Some forms originated within living memory

and can be dated with precision. For example, the

Lochiel Landslip (170 km north of Adelaide) origi-

nated during the night of 9–10 August 1974 when a

large mass of quartzite slipped downslope along bed-

ding planes (Twidale, 1986a). Knowing when the

feature originated directed attention to earthquake

and rainfall records. Though no major seismicity was

recorded in the region the possibility of a minor tremor

cannot be ruled out. On the other hand, the slippage

followed tree clearing more than a century earlier to

permit or enhance agricultural and pastoral activities,

ingress of water during a particularly wet winter, and

formation of a crack across the slope at the site of the

eventual tension scar a few months earlier. Thus, being

able to date the landform permitted a closer, though not

quite conclusive, analysis of its origin.

Similarly but even more precisely, a series of minor

tectonic features (fault scarps, A-tents, displaced

slabs) was formed on Minnipa Hill, a low granite

dome located on northwestern Eyre Peninsula, South


Australia, at 1137 CSST on 19 January 1999. The

event was witnessed at the site and the formation of

the neotectonic suite coincided with a small earth

tremor epicentred some 80 km to the east (Twidale

and Bourne, 2000a).

Knowing the time of formation of a landform can

also focus attention on the nuances of its origin. For

example, accelerated soil erosion manifested in gullies

is a common result of vegetation clearance preparatory

to farming, but accelerated erosion does not immedi-

ately follow. Clearing woodland deprives the soil not

only of protection through the umbrella effect (rain-

drop impact) and root binding, but also greatly reduces

the supply of humus formed by the decay of leaf litter.

Soil humus decays at various rates according to envi-

ronment (Waksman, 1938), but after a few decades the

soils of cleared areas would have lost the organic com-

ponent vital to soil structure and cohesion, and would

be vulnerable to the next big rain, seepage and runoff.

Knowing the age of a form also permits estimates

to be made of the rate at which various processes

operate. Thus, various shore platforms, many of them

in a Late Pleistocene dune calcarenite, have formed

along the coasts of southern and southeastern Aus-

tralia in the last 6000–7000 years in relation to the

present stand of the sea. Knowing their width (com-

monly 200 m but as wide as 300 m), the annual

average rate of erosion involved can be calculated. Of

course it is an average and in all likelihood the erosion

is due mainly to a few major storms, but an order of

magnitude of change becomes feasible.

In terms of a broader time scale, dated landform and

sedimentary assemblages with a known climatic con-

text, add greatly to palaeogeographic interpretation

and to the derivation of a chronology of climatic

change. Features associated with climatic extremes

are especially useful in this regard. Thus, glaciated

pavements, like those reported from various Precam-

brian and Palaeozoic periods, and from many parts of

Gondwana, the Triassic barchans preserved beneath

basalt in the Parana Basin of South America (Almeida,

1953), and the fields of transverse dunes buried by

Early Cretaceous (133 Ma) lavas in Namibia (Jerram et

al., 2000), each constitutes proof of the environment at

the time and place in which the assemblage developed.

Various other useful deductions derive from the

age of surfaces and related events being known, and

they are noted in appropriate places in the text, but the

most important single aspect of having dated land-

forms and surfaces is that they are an integral part of

the geological record of a region: the erosional history

of a landscape complements the depositional chronol-

ogy. Like rock sequences the erosional landscape

reflects the chronology of events—so long as its

messages can be read and translated. Erosional surfa-

ces also mirror major climatic and sea level changes,

as do unconformities within sedimentary sequences

(e.g., Vail et al., 1977, 1984; Vail, 1984; see also van

Andel, 1994, pp. 186 et seq.). The age, and hence the

dating, of surfaces is, or ought to be, an integral aspect

of any regional geological study.

Here relative dating methods are first reviewed.

This is followed by a consideration of absolute dating

principles and techniques, and a discussion of the

strengths and weaknesses of the various procedures.

4. Relative dating methods

The various stratigraphic, geomorphic and topo-

graphic criteria that can be used to establish relative

and geological ages of land surfaces can economically

be reviewed in terms of the various types of surface

previously identified.

4.1. Depositional surfaces

Depositional or constructional surfaces are compa-

ratively easy to date in a general way, for deposits

frequently contain fossils of known stratigraphic age.

For example, the Miocene age of mesas standing a

few tens of metres higher than the Todd River plains

(Fig. 4) and in the piedmont of the Ooraminna

Ranges, southeast of Alice Springs, in central Aus-

tralia, is provided by the fossiliferous siliceous lacus-

trine beds which cap the residuals (Wells, 1969;

Stewart et al., 1978). The amount of post-Miocene

lowering of the surrounding plain can also be calcu-

lated, for the siliceous strata accumulated in shallow

lake basins within the then plains.

4.2. Epigene surfaces

An epigene or ‘subaerial’ surface is one shaped by

processes and agencies active at and near the land

surface. In general terms the topographic principle


concerning relative ages of surfaces is the converse of

the Law of Superposition, for unless there has been

regional deformation, the higher a surface stands, the

older it is (Rutimeyer, 1769; Baulig, 1952; Twidale,

1956). Thus, in the absence of evidence to the con-

trary, on stepped inselbergs (Fig. 5) the higher step

Fig. 4. Mesas capped by Miocene lacustrine siliceous rocks overlying folded Late Palaeozoic beds stand prominently above the Todd River

plains southeast of Alice Springs, Northern Territory; one example is seen here beyond the Palaeozoic outcrop and in the middle distance.

The Macdonnell Ranges can be seen in the distance.

Fig. 5. Stepped northwestern face of Yarwondutta Rock, near Minnipa, northwestern Eyre Peninsula, South Australia. Note the concave, or

flared, steps.


can be taken as predating the lower, for the host mass

has been exposed in phases and the steps mark pauses

in the exposure and apparent growth of the residual

(Twidale and Bourne, 1975; Twidale, 1982a). The

interpretation holds whether the unequal incision is

due to tectonic uplift, sea level or other (regional)

baselevel lowering, or simply to the weathering of the

plains around the residual. Folding and exhumation

(q.v.) introduce complications, just as tectonically

disturbed sequences introduce departures from the

Law of Superposition, but by and large the elevational

principle is useful at a variety of scales.

‘Useful’, but not foolproof, and in particular, topo-

graphic levels per se (as opposed to the relative

position of a surface or form in a landscape) offer

no basis for correlation. Even a gentle slope implies

considerable elevational change (a 1B slope implies a

vertical difference of some 17.5 m/km), and strati-

graphic evidence must constantly be kept in mind.

Thus, in the Flinders Ranges the northernmost sector

of the widespread summit high plain is of exhumed

type and preCretaceous, whereas what is physically a

contiguous surface is probably of epigene type and

Cretaceous in age (q.v.). Also, and in detail, it is

difficult to correlate zones of flared slopes developed

on various inselbergs on northwestern Eyre Peninsula

because of local variations in exposure related to

drainage developments that are obscured and compli-

cated by dune and calcrete formation in the recent

past. On the other hand, the relative ages of treads (or

platforms) preserved on a given inselberg are, in the

absence of evidence of fault displacement, indicated

by their topographic position, and as suggested else-

where (q.v.; Twidale and Bourne, 1998a,b), and used

with care, duricrusts provide good local and regional

morphostratigraphic markers.

In many instances, the regional setting of a partic-

ular hill or massif provides a relative or absolute age

for the form. Thus, the antiquity of Enchanted Rock, a

well-known granitic bornhardt located in the Llano of

central Texas, USA, is demonstrated by the basal

Cambrian strata lapping against its present base. The

palaeotopography at that time was of the same order

of today’s (Barnes, 1952; Klier, 1988; Petersen,

1989). The residual survived the Cretaceous marine

transgression and remains a prominent local land-

mark. Early Cretaceous shorelines located marginal

to the Arnhem Land (Kakadu) massif (Needham,

1982; Frakes and Bolton, 1984) show that the sand-

stone upland, with its prominent summit surface(s),

was already in existence when the marine transgres-

sion occurred.

The Arcoona Plateau, in the arid interior of South

Australia, is underlain by a gently northward-dipping

sequence of Neoproterozoic strata, which include

several thin but resistant quartzites. The plateau is

dissected and is also truncated by faulting on its

eastern side. The sedimentary sequence deposited on

the downthrown eastern block includes basal Eocene

strata, suggesting a minimum Eocene age for the

Plateau. But this is 40–60 Ma too young, for the

distribution of Early Cretaceous marine sediments in

the valley floors demonstrates that the Arcoona Pla-

teau was already in existence and dissected at that

time (Johns, 1968; Twidale, 1994).

The ridge and valley topography of the central and

southern Flinders Ranges can be dated by reference to

the northern Willochra Basin, an intermontane basin

which occupies a breached and downfaulted regional

anticlinorium in the southern Flinders Ranges. The

topographic basin associated with the structure was in

existence by the (?)Middle Eocene, when a lake was

impounded in the northern half of the depression

(Harris, 1970). Lacustrine beds, which are superfi-

cially silicified, tongue up the valleys of such rivers as

the Mt. Arden, Kanyaka, Wirreanda and Boolcunda

creeks (Twidale, 1966, 1991b; Twidale and Bourne,

1996), indicating that the framework of the present

landform assemblage must have been in existence

when the lake was impounded, and that it is therefore

at least of Middle Eocene age. The crestal bevels that

are widely developed in the upland, most evidently in

the Mt. Remarkable syncline, in Wilpena Pound

adjacent to Edeowie Gorge, and in the Willow Springs

area, stand well above the level of the system of valley

floors affected by the lacustrine invasion, and must be

older. It has been suggested that they are of Early

Cretaceous age, having been eroded by rivers graded

to the Early Cretaceous seas that extended over the

northernmost part of the Ranges, around Mt. Babbage,

and which invaded the valleys of the Arcoona Plateau

(q.v.).

In many parts of the Eastern Uplands of Australia,

the high plain or plains predate the lava flows, many

of them Eocene, which ran down already incised

valleys (e.g., Baragwanath, 1925; Hills, 1934; Young


and McDougall, 1985). They show that the valleys

they occupy predate the volcanic extrusions putatively

dated as early Tertiary. The high plains into which the

valleys are incised must be still older. Both Craft

(1932, 1933) and Hills (1934) deduced that the upland

surfaces are probably Cretaceous in age, and this has

been sustained by the later radiometric dating (Eocene)

of many of the volcanics.

In some areas long-distance correlation of surface

and associated basin sediments has been used. For

instance, Jones (1931) suggested that the prominent

summit surface preserved in the Welsh massif (see

also Brown, 1950, 1957) is of Triassic age. Certainly,

the massif was a land mass throughout that period and

was a source area for the strata accumulated in

adjacent basins (Audley-Charles, 1970). Erosion is a

corollary of clastic deposition so that Triassic plana-

tion surfaces must have developed there and remnants

may persist: it is a matter of whether they have

survived and whether they can be dated. Again,

periods of planation in the Appalachians can be

identified from sedimentological evidence preserved

in marine sections drilled beneath the adjacent Atlan-

tic shelf (Poag and Sevon, 1989), and they suggest

periods of planation in the source area and possible

ages for the well known, but temporally enigmatic,

surfaces of the area (see e.g., Davis, 1889; Johnson,

1931; Thornbury, 1965). The Roraima Plateau of

northern Brazil and Venezuela has been dated as

Cretaceous by correlation with a subEocene uncon-

formity exposed in a distant coastal section (Briceno

and Schubert, 1990). Such long-distance correlations

are useful, though closer source-basin linkages are

more convincing.

King (1962) correlated marine Miocene deposits

exposed on the KwaZulu/Natal coast with his African

Surface which in these terms was early-mid Tertiary in

age (but see also Frankel, 1960). On that basis, he

suggested that the much higher Drakensberg surface,

which cuts across Jurassic basalts, was a later Meso-

zoic (Gondwanan) feature. The great antiquity of this

high plain has been denied (Partridge and Maud,

1987), but on balance the evidence and argument

favour King’s interpretation (Twidale, 1990; Birken-

hauer, 1991). One of the problems raised by Partridge

and Maud concerns the kimberlite pipes. Most are of

Cretaceous age with maxima in early, mid and late,

though some may date from the mid Tertiary (see

Moore, 1999; A. Moore, personal communication,

April 2001). They are intrusive into the Stormberg

Basalts. The latter are mainly Jurassic (ca. 180 Ma—

Marsh et al., 1997) though the volcanics of parts of

Namibia are Cretaceous in age (Erlank et al., 1984);

and very minor extrusive contemporary activity has

been recorded (Maud et al., 1998). Such pipes have a

well known morphology, with a bulge located a few

hundred metres beneath the surface (Hawthorne,

1975), and it was pointed out that on the high

Drakensberg the bulge was intersected by the land

surface indicating about 300 m of erosion. But this

does not negate King’s argument for the pipe could

have intruded a weathered land surface and been

eroded as the regolith was stripped to produce the

present high etch surface.

Attempts have been made to date surfaces at

various scales from high plains to terrace sequences

by estimating the baselevels to which they were

related. The various planation surface remnants of

north Wales, for example, were separated and treated

in this manner (for review, see Beckinsale and Chor-

ley, 1991, p. 331 et seq.). A relative dating is indicated

by, and presumably baselevels, once established,

could be related to, known levels of the sea (as

indicated in the precursors of the Vail or Exxon sea

level change curve: see e.g., Haq et al., 1987, also van

Andel, 1994). Also, many river terraces were dated by

projection to an assumed sea level.

The method is inherently unsound. No part of the

Earth’s surface is tectonically stable and even slight

warping introduces marked elevational changes. Also

planation surfaces shaped by weathering and river

erosion are not level but sloping; even fluvial deposi-

tional plains display a gradient. Also, unpaired terra-

ces are not related to a stable baselevel; the geometry

of the lower sectors of a thalweg reconstructed by

projection from terrace remnants is uncertain as to

shape and length, for the level of the sea and hence the

position of the coast was not known so that the

termination of the projected profile was indeterminate.

A prime example of a model being so widely and

uncritically accepted, that data was accommodated to

it, is that concerning Late Cainozoic stands of the

sea and due to Deperet (1918). The so-called Med-

iterranean scheme of sea level change was always

flawed, if only because the eponymous type area

was, and was known to be, tectonically active (e.g.,


Castany and Ottmann, 1957; see also Ward, 1965;

Twidale et al., 1967). Long distance projections to

known genetically related deposits are reasonable

though questionable, but blind extension to unknown

termini are fraught with problems and form no fit

basis for correlation.

4.3. Etch surfaces

Bedrock is attacked or ‘etched’ by shallow ground-

waters charged with chemicals and biota, and for this

reason the resultant surfaces are known as etch surfa-

ces. They are also known as two-stage forms because

they have developed as a result first, of subsurface

weathering and the formation of a regolith and a

weathering front, and second, of the stripping of the

regolith and the exposure of the front as an etch

surface. Such etch surfaces have two ages, one indicat-

ing the period of subsurface weathering and shaping of

the bedrock at the weathering front, the other the

period of stripping of the regolith and exposure of

the bedrock forms (see e.g., Twidale, 1990, 1994).

Thus, the summit high plain of the Gawler Ranges

(Fig. 6), in the interior of South Australia, is an etch

plain. The massif is developed on the Gawler Range

Volcanics, a complex of dacitic and rhyolitic ignim-

brites extruded in Mesoproterozoic times just under

1600 Ma. The prominent summit surface can be

closely dated using correlative deposits preserved in

the adjacent Eromanga Basin (Wopfner, 1969; Camp-

bell and Twidale, 1991). Following upfaulting on its

southern margin and tilting of the upland down to the

north or northeast, boulders and cobbles of the dis-

tinctive volcanic rocks exposed in the Ranges were

transported by rejuvenated rivers and deposited in the

Mt. Anna Sandstone which is intercalated with various

strata of the Cadna-owie Formation, of Neocomian–

Aptian age (Wopfner, 1969, pp. 152–156). This

relationship dates the stripping of the earlier devel-

oped regolith and the resultant etch surface, which is

now widely exposed in the upland. The weathering of

the regolith, which has been stripped to expose the

bedrock high plain, must have taken place prior to the

Early Cretaceous, but later than the Permian when the

region was overridden by ice sheets. That the massif

has stood with little change since the Early Cretaceous

uplift and tilting is demonstrated first by the extent of

the summit surface, second by the preservation both at

Fig. 6. Summit bevel cutting across ignimbritic dacite and dacitic rhyolite in the southern Gawler Ranges, South Australia (E.M. Campbell).


the margins of the upland and in valley floors within

the Ranges of remnants of silcrete of putative Eocene

age (Firman, 1983); and third by the paucity of Cre-

taceous and Cainozoic sediments in adjacent basins.

The principle that erosion in one area results in

deposition in a genetically linked area also has been

used to date surfaces by means of riverine sediments.

In the Hamersley Range of northwestern Western

Australia the dating of the summit high plain eroded

in mostly flat-lying Proterozoic Banded Iron Forma-

tion strata is direct and precise. The ferruginous

regolith developed on the plain was stripped and

reprecipitated in the valleys draining the area. The

ironstone so formed—the Robe River Pisolite-trapped

riverine silts containing plant fossils of Eocene age,

which is also the age of the stripped or etch high plain.

Since deposition, the unprotected former valley-side

walls have been eroded, leaving the valley floors

inverted in the local relief. They now take the form

of elongate sinuous mesas standing 40–60 m above

the present plains and valleys (Fig. 7); this is the

amount of lowering achieved by exogenic agencies in

the last 60 Ma. The planation and weathering of the

land surface must have occurred in pre-Eocene, i.e.

Cretaceous, or earlier, times (Twidale et al., 1985).

This is consistent with regional stratigraphy (e.g.,

Hocking et al., 1987).

Plains dominate the topography of several of the

continents, and some, including several of the most

extraordinarily flat, are of etch type. The Nullarbor

Plain for example is notorious for its lack of relief.

The crystalline Miocene Nullarbor Limestone under-

lies it and this, in turn, is underlain by the Eocene

Wilson Bluff Limestone, which is a calcarenite. Early

workers (e.g., Tate, 1879) considered the Plain to be a

single bedding plane, an uplifted seafloor untouched

by weathering and erosion by virtue of its being

underlain by limestone and the present arid climate.

Nevertheless, it was soon recognised that more humid

conditions prevailed in the earlier Pleistocene and that

there has been erosion, for several tens of metres of

stratigraphic section are missing at the southern edge

of the Plain (Jennings, 1963; Lowry, 1970). Despite

its name, there are scattered trees over some areas of

the southern Nullarbor Plain, and there are also

patches of a thin soil cover. But the soil is thin and

Fig. 7. Part of the southern flank of the Hamersley Range, south of Mt. Ward. The sinuous mesas are capped by ferruginous pisolite and

have been stripped from the high plain of the upland.


discontinuous, suggesting that some has been evac-

uated. Such calcareous dust may have contributed to

the Pleistocene calcrete found extensively developed

to the east, in the Lake Eyre basin for instance. The

former presence of a soil cover could account for the

extreme flatness of the Nullarbor surface, for it would

retain moisture and rapidly smooth off any irregu-

larities in the limestone with which it came into

contact. Thus, it is suggested that the Nullarbor Plain

is of etch origin and Pleistocene age. Partly because of

present aridity, and partly because of the pervious

character of the country rock, this etch surface is essen-

tially stable.

The Bushmanland surface of northern Namaqua-

land (Western Cape Province) and central Namibia is

also flat and virtually featureless (Fig. 8a). Again, a

few inselbergs and duricrusted mesas stand above the

general level of a plain eroded in granite, gneiss,

schist and sandstone. Near Platbakkies, in Namaqua-

land, a silcrete-capped mesa stands above the plain

(Fig. 8b). In plan the mesa is elongate and sinuous: it

is an old valley floor preserved by silcrete. The base

of the underlying kaolinised zone is coincident with

the level of the plain, in which fresh granite and

granite gneiss are exposed. The age of the duricrust

is the key to dating. According to Partridge and Maud

(1987) the silcrete is part of King’s (1950, 1962)

African cycle (also capped, e.g., in KwaZulu/Natal,

by laterite) and of early-mid Tertiary age. Thus, the

bedrock plain produced by the erosion of the regolith

is a later Cainozoic feature.

Etch surfaces are more common than was realised

half a century ago (see e.g., Thornbury, 1954, p. 193).

Such is the efficiency of subsurface moisture attack,

acting either on susceptible bedrock or over immense

periods of time, that some etch surfaces are almost as

flat and featureless as depositional plains. In the

present context, however, it is noted that they have

two ages, one referring to the period of subsurface

initiation, the other to the phase of erosion or strip-

ping when the weathering front is exposed as a land-

form.

4.4. Duricrusts

Duricrusts are widely developed and preserved in

various parts of the world. They differ in composition

(Fig. 9) but most such duricrusted surfaces are, strictly

speaking, etch surfaces, for commonly, though by no

means everywhere, the friable and easily eroded sandy

or silty A-horizon has been evacuated to expose the

indurated horizon.

Calcrete (Fig. 9a) is well known in many semiarid

parts of the world (Africa, Australia, India) and also as

caliche in the United States (e.g., Netterberg, 1969,

1971; Milnes and Hutton, 1983). Travertine is calcrete

deposited not in a soil but precipitated from spring or

stream waters. It is also preserved in the stratigraphic

record where it is referred to as cornstone. Gypcrete

(Fig. 9b) occupies large areas of northern Iraq and the

southwestern Lake Eyre basin in central Australia

(e.g., Wopfner and Twidale, 1967; Butler and Church-

ward, 1983; Tucker, 1987). Both of these encrusta-

tions are essentially later Quaternary in age and both

are still forming. Ferricrete, taken to be a surficial

ferruginous accumulation, is also a contemporary as

well as a relic development. Ferricretes dated strati-

graphically as of Pliocene age have been recorded

from Yorke Peninsula, South Australia (Horwitz and

Daily, 1958), and as noted previously, in the Hamers-

ley Range, but they are also found on very young

colluvial surfaces below iron-rich cappings, as for

instance in the ‘canga’ deposits, also of the Hamersley

Range (e.g., MacLeod, 1966); (see also Fig. 9c).

Silcrete (Fig. 9c) is extensively preserved in central

Australia (see map in Twidale, 1983) and widely in

southern Africa. The well-known sarsen stones of

southern England and northern France and surface

orthoquartzites (Frankel and Kent, 1937) are similar.

Silcrete is recorded in the stratigraphic column (e.g.,

in the Jurassic, see Wopfner, 1978), but exposed

silcretes are of various Cainozoic ages (e.g., Wopfner

and Twidale, 1967; Firman, 1983; see also Langford-

Smith, 1978).

Laterite (Fig. 9d) is widely distributed in many

parts of Africa, every state of Australia, tropical and

subtropical South America and many parts of southern

and southeastern Asia; whence they were first

described. Minor occurrences have been noted from

the USA. (For an account of laterite distribution, see

e.g., Maignien, 1966.)

In addition to their possible environmental signifi-

cance, duricrusts have been used as morphostrati-

graphic markers (see Twidale and Bourne, 1998b).

Also, many duricrusts are relic and dissected, and

though the surficial carapace after which they are


Fig. 8. (a) Part of the Bushmanland Surface, here cutting across schists, in central Namibia. (b) Silcrete-capped mesa with kaolinitic alterites

exposed in the flank, near Platbakkies, Namaqualand, Western Cape Province. The base of weathering, the weathering front, is at the same level

as the adjacent plain eroded in intrinsically fresh gneiss (J.A. Van Zyl).


Fig. 9. (a) Calcrete-capped plateau, Lake Eyre basin, northern South Australia. (b) Gypcrete-capped western scarp of the Lake Eyre salina. (c) Silcrete-capped mesa at The

Breakaway, near Coober Pedy, northern South Australia. Note the iron-encrusted slope segments below the caprock: though predominantly siliceous, the latter contains enough

mobile iron oxides (see, e.g., Thiry and Milnes, 1991; also chemical analyses published in various chapters in Langford-Smith, 1978) to indurate gully floors, which were later

left in relief by the dissection of the non-protected slope sectors—an example of gully gravure (see Bryan, 1940; Twidale and Campbell, 1986). (d) Dissected laterite, capping

mesas eroded in Cretaceous strata, Kynuna Plateau, northwest Queensland (CSIRO).

A.L.Watch

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/Earth

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ceReview

s58(2002)1–49

16

named is resistant to weathering and erosion, so that

dissected duricrusts, no matter of what composition,

give rise to plateau landscapes (Fig. 9).

Not only do the various duricrusts vary in age, but

representatives of the same type also differ in texture

and mineralogy. For example, some silcrete occurs in

essentially massive, if usually elongate sheets, but a

chemically contrasted type forms skins on blocks and

boulders (Hutton et al., 1972). In the Lake Eyre basin

of central Australia the widespread mid Tertiary

silcrete has a crystalline matrix, whereas that of the

lower and younger (Pleistocene) silcrete capping

plains to the southwest of the Lake is amorphous

(Wopfner and Twidale, 1967). Again, McFarlane

(1971) noted that in Uganda the so-called high level

laterite of the Buganda Surface in fact occurs at two

levels. A vermiform ferruginous zone, regarded as

mature, is preserved on higher remnants and formed

in association with a stable water table. It has been

truncated and reworked to form continuous sheets of

(immature) packed pisoliths at a lower level in asso-

ciation with a gradually lowering water table. In this

instance the two levels of laterite are attributed to

secular water-table lowering and hence, whatever the

reason for water-table lowering, are of different ages.

Similar sequences of higher vermiform and lower

spaced and packed pisolitic laterites occur also lower

in the landscape.

Nevertheless, local correlation between surfaces in

similar topographic settings but carrying dissimilar,

though probably related encrustations (e.g., silcrete

identified by porphyroclastic shards, and orthoquart-

zite) seems reasonable (e.g., Alley, 1973; see also

Langford-Smith and Dury, 1965); as it does also

where topographic relationships between different

types of accumulation are consistent. For example,

ferruginous and siliceous duricrusts which stand in

similar topographic settings in the Macdonnell Ranges

of central Australia have been interpreted as being of

similar age (e.g., Mabbutt, 1966; Woodburne, 1967).

In southern Africa, and as mentioned, the African

Surface in places carries a capping which is ferrugi-

nous in the east (KwaZulu/Natal) but siliceous in the

west (Namaqualand, Western Cape).

Many of the silcretes previously thought to be

remnants of dissected sheets are narrow and sinuous

in plan. They contain rounded exotic cobbles (mainly

quartz) and many form shallow basins in cross-sec-

tion. They are, almost certainly, river valley deposits

that have been left high and dry by the preferential

weathering and erosion of the adjacent unprotected

valley-side slopes; an example of relief inversion (see

also Miller, 1937; for an account of inversion involv-

ing travertine, q.v.). In many instances, the clays

which frequently underlie the resistant capping are

weak. The crusts are readily undermined and the

whole profile has been widely stripped, leading to

the formation of extensive etch plains in previously

duricrusted landscapes.

The Yilgarn Craton of the southwest of Western

Australia is an example of a duricrusted landscape

dated using relative methods. The Craton comprises

granites, gneisses and ‘greenstones’ (basic volcanics)

of Archaean age. It has long been recognised (Jutson,

1914; see also Mabbutt, 1961b; Finkl and Church-

ward, 1973; Mulcahy, 1973) that the landscape devel-

oped on it is dominated by two planation surfaces, the

Old Plateau which in most places is underlain by a

laterite, and the New Plateau (really a high plain)

resulting from the dissection and stripping of the

duricrust formed beneath the Old. Clearly the Old

reflects its name and is older than the New for it

stands higher in an undisturbed erosional landscape.

The weathered mantle of the Old predates Eocene

palaeochannel deposits (e.g., Van De Graaff et al.,

1977; Commander, 1989; Clarke, 1994), but the sur-

face is diachronic for whereas the sediments deposited

in the main channels are of Eocene age they are

younger (Miocene and Pliocene) in the headward

reaches (e.g., Commander, 1989; Clarke, 1994;

Waterhouse et al., 1995; Salama, 1997). But the

Eocene strata are the important ones for they indicate

the minimum age of stream incision and also indicate

a minimum Cretaceous age for the Old Plateau and its

laterite (Twidale and Bourne, 1998a).

Laterite dominated the Old Plateau. In the south the

Darling Range carries a carapace of laterite and

bauxite, and dissected remnants of the ferruginised

surface extends as far as the southeastern margin of

the Yilgarn Craton. To the north, however, as for

example, north of Meekatharra in the Killara district,

the duricrust capping the Old Plateau remnants is

siliceous in places. According to Fairbridge and Finkl

(1978) the silcrete is of mid Tertiary (Oligocene–

Miocene) age, raising the possibility that the Old

Plateau, like the New, was a diachronic surface.


Having a relatively determined age for the lateri-

tised surface allows dating of associated forms. Insel-

bergs such as King Rocks, The Humps (Fig. 10),

Pingaring Rock and the upper sectors of Boyagin

Rock, and several other residuals, stand higher than

the adjacent plains capped by primary laterite and are

therefore of at least the same age, i.e. Cretaceous; but

the stepped morphology of The Humps, for example,

suggests either repeated piedmont weathering and

erosion, or phases of exposure, implying an even

older developmental chronology. Residuals such as

Hyden Rock, on the other hand, standing lower than

the laterite surface, are younger than the Old Plateau.

The combination of topographic and stratigraphic

data shows that rivers had incised deep valleys in the

Old Plateau by the Eocene and that headward exten-

sion of the regraded channels has been slow and of the

same order (ca. 100 km in 60 Ma) as noted in the

hinterland of the southeast coast of New South Wales

(Taylor et al., 1985).

Perhaps the most controversial duricrust dating is

that involving the laterite preserved on the plateau

which occupies much of Kangaroo Island. The laterite

is developed and preserved on strata of Proterozoic,

Cambrian and Permian ages and therefore postdates

the Permian. In the vicinity of Kingscote the kaoli-

nised mottled and pallid C-horizons are overlain by a

basalt, the Wisanger Basalt, which, to the west of

Penneshaw, rests on the ferruginous B-horizon of the

duricrust (Tilley, 1921; Daily et al., 1974; Daily et al.,

1979). The laterite clearly is older than the basalt.

When first mapped (Sprigg et al., 1954) it was thought

to be linked with the volcanic activity of the South

East district of South Australia and of western Victoria

(e.g., Hills, 1940; Sprigg, 1952) and thus to be of

Pleistocene and Holocene age. In those terms the

laterite was considered to be Pliocene which compared

well with the date derived for the laterites of northwest

Queensland (Whitehouse, 1940; Northcote, 1946).

This scheme was upset when the Wisanger Basalt

was dated radiometrically as of Middle Jurassic age

(with absolute ages about 176 Ma: Wellman, 1971;

McDougall and Wellman, 1976). As the stratigraphic

relations established by Sprigg et al. (1954) remain

valid, the laterite must predate the Middle Jurassic and

be younger than the Permian. If laterite is a warm

humid development, as it mostly is (but see Taylor et

al., 1992; Young et al., 1994), then the best strati-

graphic niche for the laterite of Kangaroo Island is the

Triassic (Daily et al., 1974). Though incompatible

Fig. 10. The Humps is a complex granite–gneiss bornhardt on which several steps or platforms backed by flared slopes are preserved. All

are older than the laterite capping the plain (and exposed in the shallow borrow pit in the foreground) on which the inselberg stands.


with palaeomagnetic data (Schmidt et al., 1976), such

a date is consistent with the topographic, tectonic and

stratigraphic relations of the laterite of the southern

Mt. Lofty Ranges and the Lincoln Upland, on south-

ern Eyre Peninsula, both also in the Gulfs region

(Miles, 1952; Daily et al., 1974) and where the laterite

also occurs on plateaus and high plains. Oxygen

isotope dating of the kaolinite suggests the weathering

is of Mesozoic age (Bird and Chivas, 1988, 1993).

Like the faults delimiting the Arcoona Plateau on

its eastern margin, the age of the faulting to which the

Mt. Lofty horst block is due, and which caused the

lateritised surface to be uplifted, provide only a

minimum age for the dislocated surface. The Middle

Eocene and later marine embayments marginal to the

Mt. Lofty Ranges suggest a minimum Early Eocene

age for the summit surface of the upland, whereas the

relative evidence cited here suggests a much greater

age (Miles, 1952; Campana, 1958; Glaessner and

Wade, 1958).

In passing, it may be noted that though at odds

with other evidence and argument concerning the area

just discussed, palaeomagnetic dating has proved

useful elsewhere, e.g., in India (Schmidt et al.,

1983; Widdowson, 1997), and also that absolute

dating of weathering profiles, including laterites,

using K–Ar and 40Ar/39Ar is possible (e.g., Vascon-

celos, 1999). Such weathering chronologies provide

rates of erosion and age estimates which, in northwest

Queensland, for example, are comparable to those

derived from relative dating (stratigraphy). They are,

however, incompatible with apatite fission-track ther-

mochronology (AFTT) chronologies for the same

region. Similar discrepancies have been noted in

southeastern Australia (Bishop and Goldrick, 1998).

Thus, several criteria and concepts can be used,

singly or in combination, to date land surfaces in

relative terms. Correlative deposits, dated shorelines,

faulting, dated regoliths, the link between deposition

and erosion, have all been used but other specific lines

of evidence, such as the morphology of kimberlite

pipes, can also enter into consideration.

4.5. Exhumed surfaces and forms

An unconformity denotes an hiatus, or time gap, in

the geological record. Exhumed land surfaces or

forms are unconformities that have been buried by

sediments or volcanic rocks and then re-exposed. The

most common examples are associated with periods of

global marine transgressions, such as the Cambrian,

Cretaceous and Miocene. Exhumed surfaces have

been reported from all over the world. They derive

from every geological period from the Late Archaean

to the Late Pleistocene, with forms and surfaces dating

from the sub-Cambrian and the sub-Cretaceous espe-

cially well represented.

Exhumed forms are useful in several respects. That

bornhardts developed in preCainozoic times (indeed

back into the Archaean) shows that, however they

form, the mechanisms have been operative and effec-

tive throughout geological time. In some areas, such

as the Isa Highlands of northwest Queensland, the

behaviour or disposition of exhumed surfaces demon-

strates the nature of tectonism. In that particular

instance upwarping and erosion of the exhumed sur-

face, leaving scattered remnants of the cover sedi-

ments, occurs high in the relief in the central part of

the upland but declines in elevation on both sides (Fig.

11a–d). In the east, the uplands disappear beneath the

Mesozoic formations of the Carpentaria Basin and re-

emerge at the base of the Einasleigh Uplands and rise

to form summit high plains in the region between the

Basin and the Pacific coast. In broader view they

confirm Hills’ (1961, p. 88) (see also Twidale and

Campbell, 1992) contention that some tectonic blocks

have been uplifted repeatedly, whereas intervening

basins have suffered recurrent subsidence.

Surfaces are preserved by burial in basins. On

uplifted blocks, water is shed, the sites are compara-

tively dry, and surfaces tend to survive. Uplift com-

monly induces a renewal of erosion, but this is one

way in which tectonism is conservative.

Exhumed forms and associated surfaces also pose

problems. Many unconformities, like that exposed in

the Grand Canyon in the southwestern USA (see

Powell, 1875; Sharp, 1940), and hence exhumed

equivalents, are remarkably flat, even in detail. Is this

because the surface eventually buried was featureless

(possibly as a result of long-continued etching) or

because groundwaters concentrated at the unconform-

ity eliminated any irregularities? Does the ‘regolith’,

found at some contacts, predate burial or does it

reflect post burial weathering (see e.g., Van Hise,

1896; Weidman, 1903; Thwaites, 1931)? Where a

regolith is preserved does it reflect the character of


the burial agency: is preservation facilitated by basal-

tic flows, ash deposits, or by aeolian deposition,

whereas, in contrast, regoliths are largely eliminated

during glaciation or marine sedimentation? What is

the explanation of occasional local exceptions to these

general rules? The answers to these questions can

provide important information about the character of

ancient surfaces and about the nature of subsurface

weathering.

Exhumed forms and surfaces are readily dated by

stratigraphic bracketing: the surface is younger than

the youngest rocks it transects, but older than the basal

member of the cover sequence. At first sight, it might

be thought that such a procedure would leave too

Fig. 11. (a) Section across the Carpentaria Basin from the Isa Highlands in the west to the Einasleigh Uplands and the Pacific coast in the

east, and showing the sub-Cretaceous surface. (b) The sub-Cretaceous surface represented by bevelled quartzite ridges in the Isa Highlands

near Mt. Isa. (c) The unconformity between the lateritised Cretaceous (or possibly uppermost Jurassic) strata and Precambrian granite, and

standing high in the relief in the central southern Isa Highlands, and (d) similar Mesozoic strata over dipping phyllite low in the relief near

the eastern margin of the upland.


great an hiatus into which the period of landform

development might fit. The interval indicated may be

enormous, as for example on the west coast of Eyre

Peninsula where an hiatus of some 1500–1600 Ma

separated the Mesoproterozoic granite –gneiss –

amphibolite and arkose ‘basement’ dated by U/Pb

and K/Ar ratios, from the overlying Middle Pleisto-

cene calcarenite, dated by thermoluminescence (q.v.)

(Wilson, 1991). A ferruginised regolith is developed

on a surface eroded across the older rocks. It is widely

preserved in unconformity. Given the evidence of

survival of landscapes and forms for periods of

100–200 Ma (e.g., Twidale, 1976, 1978, 2000;

Campbell and Twidale, 1991; Twidale and Bourne,

1998a,b), exhumed surfaces may have originated long

before burial and the ferruginous duricrust may have

developed anytime in the hiatus represented by the

unconformity. On the other hand, it seems reasonable

to correlate this ferruginised surface with similar

ferricrete regoliths, preserved in unconformity and

of Pliocene age, on Yorke Peninsula. Ferricrete-cap-

ped remnants of that order of age are preserved

elsewhere on Eyre Peninsula (see e.g., Horwitz and

Daily, 1958; Twidale et al., 1976; Molina Ballesteros

et al., 1995). In these terms, the old land surface

immediately predates the calcarenite and is of later

Pliocene or early Pleistocene age.

Cowie (1961) and Ambrose (1964) have described

exhumed surfaces of various Precambrian ages from

Greenland and northern Canada. Sub-Cambrian sur-

faces are reported from Wisconsin (Van Hise, 1896;

Weidman, 1903; Thwaites, 1931), and in the St Franc�oisMountains of the Ozark region of Missouri, where

a rugged relief developed on Precambrian granite and

rhyolite is, in part at least, exposed from beneath the

Cambrian and other Early Palaeozoic strata that

underlie the Salem Plateau (Bretz, 1965). A sub-

Cambrian inselberg landscape of considerable extent

is preserved in south Namibia and Namaqualand, in

Western Cape Province (du Toit, 1937, p. 230), and

planate surfaces and bevels of similar ages are

reported from Sweden (Rudberg, 1970) and northern

Australia (Opik, 1961; Young, 1992). Much of the

topography developed on Palaeozoic strata on either

side of the Bristol Channel, in southwestern Britain,

and on Precambrian rocks in the English Midlands, is

exhumed from beneath a Triassic cover (Watts, 1903;

Strahan and Cantrill, 1904; Jones, 1931).

The various Cretaceous transgressions (Frakes,

1988) at one stage or another covered about 40% of

the present Australian continent, and similarly exten-

sive transgressions affected other parts of the world at

this time (e.g., Reyment, 1989). Related exhumed

surfaces include many minor occurrences (Fig. 1d)

but also an extensive, and as yet only generally

delimited, surface of extreme low relief at the south-

western margin of the Eromanga Basin (Jack, 1931;

Wopfner, 1964); inselberg landscapes around Kulgera,

south of Alice Springs, and in the northern Pilbara

region of Western Australia; and the gorge cut by the

Murchison River where it crosses the Victoria Plateau,

near Ajana, in Western Australia.

The Tindal Plain in the Northern Territory is eroded

in Cambrian limestone and has been re-exposed as the

result of the almost, but fortunately not quite, com-

plete stripping of the former early Cretaceous sand-

stone cover. The surface poses intriguing problems as

to how even minor solutional features survived marine

transgression (Twidale, 1984). There is a gap of some

400 Ma between the two stratigraphic units but the

plain was probably in existence in the latest Jurassic

or earliest Cretaceous, though it may have com-

menced development much earlier.

Where the Early Cretaceous (Neocomian) seas

lapped against what is now the northernmost Flinders

Ranges, the single sedimentary remnant associated

with the transgression and surviving in the upland,

that at Mt. Babbage (Fig. 12), not only demonstrates

exhumation of the surrounding high plains but also

provides a date for the exhumed surface (Woodard,

1955; Twidale and Bourne, 1996). Also, because of

the littoral character of the contained plant fossils, the

local limit of the transgression is evidenced in broad

terms, and this clarifies the nature of the related

planation surface preserved as crestal bevels in the

central and southern Flinders Ranges (Twidale, 1980;

Alley and Lemon, 1988).

The Cretaceous transgression also affected other

continents. Thus, the classical inselberg landscape

described by Bornhardt (1900) from what is now

Tanzania is an exhumed ‘‘preCretaceous’’ terrain (Wil-

lis, 1936, p. 113 et seq.). Willis pointed to the great

thickness of Cretaceous sandstones exposed in west-

facing escarpments and, citing also the work of Von

Staff and Brantford Muff (later Maufe), went on to

suggest that ‘‘there is reason to recognize the fact that


they formerly covered the area of the present plain’’

(Willis, 1936, p. 113). He speculated on the origin of

that plain and concluded that it is exhumed from

beneath a Late Jurassic–Early Cretaceous cover, but

also that it is of etch character. In Scania and adjacent

areas of southern Sweden, sub-Cretaceous surfaces are

well preserved, as is a ‘‘preCretaceous’’ gorge (Lid-

mar-Bergstrom, 1989; Lidmar-Bergstrom et al., 1997).

‘Exhumed’ glaciated pavements of ages ranging

from various Precambrian episodes to Late Palaeozoic

have been recorded from various parts of the world.

But are they exhumed? An exhumed form is one that

has been buried and re-exposed. A glaciated pavement

originates at the base of the glacier and may then be

covered by detritus from the melting ice, before it is

ever exposed. The erosion of the glacigene strata has

exposed the glaciated pavement or surface for the first

time. What then is the evidence for the exposure of the

pavements, their burial and subsequent re-exposure,

and hence the justification for the application of the

adjective ‘exhumed’?

4.6. Summary

Relative dating is secure and appropriate for dating

the surfaces and forms that occupy most of the

continental areas. Numerical approximations derive

from stratigraphy, the subdivisions of which have

been fixed in time by a combination of physical dating

of intrusive and extrusive masses and of included

durable minerals, plus the Law of Superposition as

expressed in stratigraphic bracketing. Approximations

are appropriate to the dating of most forms and

surfaces because they evolve over time and are best

allocated an age range rather than a specific moment

of time. One advantage of relative dating is that the

methods apply to all geological ages.

The value of such dating in reconstructing regional

denudation chronologies cannot be overstated. In

addition, however, dating of the incised surface allows

estimates to be made of rates of erosion both in

lowering by, and headward regression of, rivers. For

instance, just as rates of lowering through the Caino-

zoic can be estimated in the Hamersley Range, so,

given the age of the travertine capping, could the rate

of lowering in aridity be calculated for the Dahran

area of the eastern Arabian Peninsula where carbo-

nate-capped inverted river channels are preserved

(Miller, 1937).

Difficulties arise where appropriate evidence is

either not preserved, not exposed or not recognised,

and also where catastrophic forms such as fault scarps

Fig. 12. Mt. Babbage, a small remnant of silicified Lower Cretaceous marine strata preserved on the exposed unconformity cut in various

Precambrian rocks in the northernmost Flinders Ranges, South Australia.


and rock avalanches have not involved or produced

stratigraphically datable materials. Here recourse

increasingly is made to procedures arising from

advances in the physical sciences. They involve radio-

isotopes which allow direct dating of surfaces or

constituent materials.

5. Absolute methods

Established absolute dating methods such as ura-

nium–lead and potassium–argon have long been used

to give temporal reality to what was previously a

relative stratigraphic column. Such age approxima-

tions have been and remain fundamental for they

provide temporal datums that allow the age ranges

of land surfaces to be inferred.

The dating of gypsum deposits using strontium

isotopes is a case in point. The variation in sulphur

and strontium isotopic ratios (34S/32S and 87Sr/86Sr) in

seawater has formed the basis for dating some ancient

evaporite deposits (Claypool et al., 1984; Denison et

al., 1998). Application of this dating method requires

that the isotopic record reflects open marine condi-

tions and that this signal has remained unchanged

throughout later diagenetic alteration. The principle of

the method is based on work by Long et al. (1997),

who showed that the strontium content of clays in

evaporites forms a closed system, even during dia-

genesis, and this can be used to estimate the age of

sedimentation. This new approach to dating sulphate-

rich sediments replaces previous attempts that were

based on palaeontological records of the overlying

and underlying units, which bracket the stratigraphic

age of a sulphate deposit (Holser, 1992).

Rhenium–osmium dating of black shales provides

another example of innovation in the dating of sedi-

ments. When used as a chronometer (Ravizza and

Turekian, 1989), the pair of isotopes 187Re–187Os can

indicate closed-system behaviour for Re and Os and

provide isochron ages consistent with fossil evidence

(Cohen et al., 1999; Singh et al., 1999).

These procedures are largely geological dating

tools but are useful for estimating the age of a land-

form by providing a limiting (maximum) age for

features or products, such as solutional caves or

associated speleothems developed in the evaporitic

materials. Thus, gypsum caves and associated forms

cannot be older than the host sediments, and a

planation surface cannot be older than the youngest

material it transects.

Procedures that offer a more direct means of dating

forms and surfaces are considered in the following

sections (see also Table 1). Absolute methods for

dating surfaces depend on the ability to date minerals

or elements that (a) occur in bedrock or other materi-

als (regolith, alluvium) exposed at the surface, and (b)

are clearly genetically related to the surface. Many

land surfaces are not readily datable because the

processes responsible for them are complex and

additive, and take place slowly over time: they evolve

rather than forming in an instant. Moreover, short

duration but intensive storm and catastrophic events

are superimposed on long-term gradual processes, so

that rates of erosion vary in time, and what is measured

is a combination of a long-term average and a short-

term spike.

An eroding surface may lose or may have lost

material which could have been used to ascertain its

age. Dating methods are based on certain assump-

tions, use different isotopes, elements or radionuclides

and require specific sampling strategies, pretreatment

steps and analytical devices. Datable material col-

lected from a surface may not necessarily provide an

age of formation for that surface or produce a reliable

estimate of its rate of degradation because of contam-

ination or inheritance problems. Thus, dating involves

interpretation in light of other factors and information:

numbers derived from analysis do not stand un-

changed but if considered pertinent to the problem

are modified in terms of theoretical constants appro-

priate to the procedure.

There are also conceptual conflicts between geo-

morphologists who want to know the age of a surface

and the rate of a land surface process, and chronol-

ogists who collect and analyse samples associated

with a surface to determine the extent of decay or

accumulation of an isotope. This is especially so with

the relatively modern method of ‘exposure’ or cos-

mogenic nuclide dating techniques. In geomorphol-

ogy, the major problem is to equate a chronometric

estimate of age from a mineral or rock sample, with

the geological age of formation of the sampling lo-

cation.

While these differences may appear insignificant,

there are real issues to be considered in exposure


dating, questions that may be overlooked in the quest

of chronology. A chronometric age determination

provides an estimate of the apparent time since an

isotope started to accumulate (in situ stable cosmo-

genic isotope, e.g., 3He, 21Ne) or decay (in situ

radiogenic cosmogenic nuclide 10Be, 26Al, 36Cl or

radiocarbon 14C) within material found on or in a

surface. This single apparent age is compared to the

long-term average age derived from the interactions of

many geological and geomorphological processes

leading to formation of the surface. The issue relates

to different perceptions of the constancy of a geo-

morphological process, the stability of a surface, and

the time frame over which the chronometric measure-

ment applies, compared to the period of isotopic

accumulation or decay on that surface.

For example, in in-situ cosmogenic nuclide dating,

‘age’, ‘erosion rate’ and ‘surface’ carry distinct con-

notations which differ from the geological understand-

ing of these terms. The age of a geomorphological

surface is the length of time that has elapsed since its

initiation. Thus, extracting and dating minerals and

elements from rock, saprolite or soil on a surface may

provide an age estimate different to the one required

by the geomorphologist. Determining the length of

time that cosmogenic nuclides have been forming in a

rock or mineral on a surface may not indicate the age

of the surface or landform, but the time since exposure

to cosmic rays (or a minimum age if the sample were

saturated). For a geochronologist the ‘surface’ may be

a layer of rock several metres thick, but it is the age of

the outcrop or exposure that is of immediate concern

to the geomorphologist. Views about erosion rates are

also likely to differ because chronometric measure-

ments are impossible to obtain from material no

longer on a surface, unless from sedimentation rates

downslope from the catchment of which the surface

under analysis is part. Approximate rates of erosion

and ages of surfaces can be inferred, but the under-

lying assumptions used in measuring ages and rates

must be well described and their implications under-

stood.

5.1. Cation ratios

Dark iron–manganese rock coatings, called rock

varnishes, are developed on stable rock surfaces in

many different geomorphic and environmental set-

tings. The age of a surface with varnish-coated

cobbles or pavements can be established if the onset

of varnishing can be determined. The cation-ratio

method was developed and used for dating varnishes

consisting of fine laminations (Dorn, 1983), but it

has subsequently been shown to be unreliable.

Correlations between major elements and cations

used in ratio calculations frequently reflect geochem-

ical affinities that are unrelated to time (Reneau et

al., 1992), negating a major assumption of this

method. Geochemical heterogeneity in micro-sites

also creates large cation-ratio ranges (Watchman,

1992b) thereby minimising application to homoge-

neous rock varnishes (Harrington and Whitney,

1995). Moreover, this dating method cannot be

applied universally (Watchman, 1992a, 1999)

because of multiple varnish types (Dorn, 1994a),

contamination problems (Watchman, 1993), interfer-

ence from barium in energy dispersive analyses

(Bierman and Gillespie, 1991), and calibration prob-

lems where ratios must be measured on surfaces of

known ages (which is in itself a circular argument).

The Accelerator Mass Spectrometry (AMS) radio-

carbon method has replaced cation-ratio dating of

rock varnishes.

5.2. Radiocarbon dating

Radiocarbon or 14C is a cosmogenic isotope

which is continually being formed in the atmos-

phere by the interaction between neutrons, produced

by cosmic rays, and nitrogen (Libby et al., 1949).

Rapid combination of this radioactive isotope with

oxygen forms carbon dioxide, which is chemically

similar to carbon dioxide containing either of the

other two carbon isotopes (12C and 13C). Mixing of

the radiogenic carbon dioxide throughout the atmos-

phere and accumulation in the biosphere, under

conditions of constant production, leads to dynamic

equilibrium and therefore a uniform concentration

of 14C in the environment. Theoretically there is

a constant 14C activity in living organisms that

reflects the equilibrium concentration in the atmos-

phere. On the death of an organism, exchange of

carbon between the organism and the equilibrium14C concentration in the atmosphere ceases, and the

level of the radioactive isotope decreases exponen-

tially with time according to its half-life (t1/2 = 5730


years). Application of this technique is therefore

limited to ca. 40 ka and therefore to relatively recent

geological and geomorphological events and surfaces.

In principle, the time that has elapsed since death of

the organism can be determined by measuring the

equilibrium activity of 14C and the number of 14C

atoms remaining in the dead organism.

A range of suitable datable materials, for example

woody plants, grasses and seeds, peat, charcoal,

shell, bone, soft animal tissues, insects, atomic car-

bon, coral and microorganisms, is available for

dating. But complications arise from assumptions

concerning their contexts in the landscape and diffi-

culties in sampling, while chemical problems pose

challenging laboratory purification procedures. In ad-

dition there are the fundamental assumptions of the

method itself (variations in natural production rates

through time, source or reservoir effects, alteration

effects and contamination: see Bowman, 1990), and

these may radically affect the reliability of some age

determinations. Radiocarbon was at one time widely

used for dating calcrete, and through that human

culture stages (e.g., Netterberg, 1969), but the possi-

bilities of contamination from circulating groundwa-

ters (recent exchange with CO2 in the atmosphere,

and chemical and biotic fractionations) are too great

and the use of such materials for this procedure is not

recommended.

The 14C activity is measured either by using

conventional gas or liquid scintillation counters to

measure the decay of electrons (h particles: Polach,

1974) or by single atom counting using AMS (Lither-

land, 1980; Wolfli, 1984). Apart from the basic differ-

ences in the physics of these methods there is also a

mass limitation. Generally, more than 5 mg of carbo-

naceous material is needed for h-counting whereas

much smaller targets (exceeding 25 Ag graphite) are

necessary for AMS.

The advent of AMS radiocarbon measurements on

microgram quantities of graphite produced from small

samples of carbon-bearing substances has stimulated a

good deal of lively, even controversial, geomorpho-

logical and archaeological research. Examples of this

application are the dating of sediments (Fowler et al.,

1986), lacustrine carbonates (Benson, 1993), the for-

mation of alluvial fans (Gillespie et al., 1994; Reheis

et al., 1996), and determination of glacial chronology

(Dorn et al., 1990). Where possible, charcoal has been

used to establish the approximate age of recent

alluvial landforms, but separate carbon components

of soil organic matter can also be dated (O’Brien and

Stout, 1978; Huang et al., 1996; Wang et al., 1996).

Particles of plant fibres and cellulose materials, and

other organic compounds (microbiological remains,

pollen) contained in rock varnishes have been used to

date the segmentation of alluvial fans (Hooke and

Dorn, 1992), petroglyphs (Watchman, 1992a) and

rates of fault movement. Some exorbitant claims of

antiquity for petroglyphs (Whitley and Dorn, 1987;

Nobbs and Dorn, 1993) and rock surfaces (Dorn et al.,

1990; Dorn, 1994b) have been challenged recently

because of allegedly dubious sample integrity (Beck

et al., 1998).

A critical and insoluble shortcoming of the method

is its inherent incapacity to determine ages beyond the

detectable limit of 14C, restricting dating to surfaces

younger than about 40,000 years. Within this time

range, the major uncertainty of the AMS radiocarbon

method is determining whether the datable material is

contemporaneous with the landform or process under

investigation. Charcoal is inert and can be recycled

from one unconsolidated formation into a new de-

posit. Dating the carbon in charcoal should there-

fore not be regarded as reliable until evidence is found

to support its contemporaneous coexistence. Close

field observations and other analytical information

can be used to assess whether or not a radiocarbon

age relates to the date of deposition. Reliable dating

can be achieved where it is possible to extract carbon-

bearing compounds in stratigraphically conformable

layers, even in extremely thin rock surface deposits

(Watchman et al., 1993; Watchman and Campbell,

1996) or by showing that the individual components

are coeval.

5.3. Cosmogenic isotopes

The Earth is exposed to cosmic rays (mainly

protons and alpha particles) originating in the Sun

and also outside the Solar System. These rays induce

a variety of nuclear interactions in the atmosphere

including a cascade of energetic particles, a secon-

dary flux of neutrons and other interacting particles.

Atmospheric cosmogenic radionuclides 10Be, 14C

(described above), 26Al and 36Cl are produced in

air mostly by neutron-induced spallation reactions


with 14N, 16O and 40Ar (Lal and Peters, 1967; Reedy

et al., 1990; Shibata et al., 1993). While production

of 26Al in the atmosphere is weak because of the low

abundance of argon, 10Be and 36Cl are formed in

much larger and significant quantities, and unless

removed from rock samples will affect measurements

of their in situ counterparts. Cosmogenic beryllium

produced in the atmosphere has been used to estab-

lish a model for sedimentation rates along continental

margins and sediment recycling at subduction-vol-

canic arc zones by comparing isotopic concentrations

of pelagic and continental sediments (Lee et al.,

1993).

The secondary flux of particles with sufficient

energy, such as thermal and fast neutrons and muons,

can reach the Earth’s surface and generate long-lived

cosmogenic nuclides in rocks and minerals. These in

situ-produced stable nuclides (3He and 21Ne) and

radionuclides such as 10Be, (half life, t1/2 = 1.5 Ma),26Al (t1/2 = 0.7 Ma) and 36Cl (t1/2 = 0.3 Ma) are

generally produced within about a metre or two of

the surface. Spallation and muon interactions pro-

duce 3He, 21Ne (Marti and Craig, 1987), 10Be, 26Al

and 36Cl, and thermal neutron activation also leads to36Cl (Lal and Peters, 1967; Yokoyama et al., 1977).

At increasing depth within the Earth, spallogenic

production decreases while muon production of iso-

topes increases because longer distances or denser

rock materials are needed to attenuate these particles.

Therefore, in situ cosmogenic isotopes can form at

depths exceeding 3 m depending on the composition

of the rock. Such deep production of isotopes within

the regolith has implications for determining expo-

sure ages of land surfaces and in estimating the

effect of isotopic inheritance in transported boulders

and clasts.

Computation of production rates and measurement

of concentrations of in situ nuclides in the near-

surface zone allows a ‘surface exposure history’ to

be estimated (Lal, 1988). Stable cosmogenic isotopes

seem ideal for dating young volcanic events (Cerling,

1990; Kurz, 1986; Kurz et al., 1990), and in ideal

circumstances the half-lives of the radionuclides

allow for age estimates to be made for occurrences

within the last 2 or 8 Ma. The basis of this surface

exposure history model is that the material to be

sampled and analysed was once located beyond the

range of cosmic-ray bombardment and thus was

originally devoid of cosmogenic nuclides. For stable

nuclides, it is the eruption and cooling of a lava and

exposure to cosmic radiation of the olivine and

orthopyroxene crystals (plagioclase and quartz do

not retain helium: Cerling, 1990) that starts the

cosmogenic nuclide clock. For radionuclides, either

following an instantaneous catastrophic event, e.g.,

faulting, meteorite impact (Phillips et al., 1991),

volcanic eruption (Zreda et al., 1993), rapid erosion

(Zreda and Phillips, 1994; Zreda et al., 1994; Bier-

man and Steig, 1996) or deposition (Gosse et al.,

1995; Liu et al., 1996), the surface to be sampled is

instantly exposed to cosmic rays. As exposure con-

tinues, the concentration of in situ cosmogenic

nuclides increases. For the radionuclides, the increase

is initially linear but then the rate of accumulation

falls off slowly as radioactive decay begins to take

effect. After four or five half-lives, the radionuclide

system reaches secular equilibrium or saturation in

which production and decay of radionuclides are

balanced and the cosmogenic chronometer no longer

functions.

In the context of geomorphology, measuring the

rates of erosion or deposition and exposure to air are

difficult because of practical limitations introduced by

factors that affect production of in situ cosmogenic

nuclides: factors such as the shielding effects created

by snow, ice, weathered bedrock (regolith), moraines

and other transient sediments, the introduction of

materials that already contain nuclides from previous

exposures (inheritance) or by the removal of rocks

with accumulated nuclides. However, to simplify

calculations it is often assumed that (1) the geometry

of the surface has not changed, (2) the surface is

static, and (3) the sampled rock has not been previ-

ously exposed. These assumptions are incorporated

within the in situ exposure model (Nishiizumi et al.,

1991a). This model of exposure history is constrained

by two distinct boundaries—a minimum age under

zero erosion, and a maximum erosion rate under

exposure long enough for the nuclide concentration

to attain equilibrium.

Cosmogenic concentrations are highest with zero

erosion, and the model age is well defined, represent-

ing the exposure time of the material on the surface.

The in situ age relates to a modified version of the

original surface and not to the true original surface,

because the present surface is actually an old layer


that was present at shallow depth under the original

surface. The new surface replaced the former by

becoming exposed through wasting of the original

surface. Even an erosional ‘surface’ in dynamic equi-

librium is not the original surface, for it originated at

depth and has been progressively exposed as a result

of surface lowering.

In geomorphological nomenclature, and in the

time scale implied by the half lives of cosmogenic

nuclides, the term ‘surface’ is interpreted as a feature

that is actively evolving. Therefore, the in situ

exposure model age is actually the mean effective

irradiation time or the apparent age of the surface

(Lal, 1991). This is the time needed for erosion to

remove a thickness of rock equivalent to the mean

attenuation length of cosmic rays. This is defined as

the penetration depth where the cosmic ray flux is

reduced by f 50%, typically about 50 cm, but for

muons it could also be 3 m or more. The apparent

numerical age is a measure of the mean time required

for the contemporary surface to accumulate the meas-

ured equilibrium concentration of a cosmogenic iso-

tope. Thus, the cosmogenic isotopic age estimate is

not necessarily an indication of the geomorphological

age of the surface.

A ‘long-term average’ erosion rate of a surface can

be measured using the surface exposure chronology

model (Granger et al., 1996). If erosion is the dom-

inant mechanism by which radionuclides are removed

from a surface in dynamic equilibrium, then the

attenuation length of the cosmic rays describes the

average rate of surface lowering. Conversely, where

erosion is small, the half-life of the radionuclide and

the apparent age of the surface set the time scale,

which approaches the minimum exposure age.

Many studies have applied the in situ ‘dating’

methods in a variety of geomorphic settings (Cerling,

1990; Cerling and Craig, 1994; Nishiizumi et al.,

1991b; Liu et al., 1994a,b; Gosse et al., 1995). Most

of these are centred on the experimental determination

and calibration of production rates of in situ cosmo-

genic radionuclides (Nishiizumi et al., 1989; Stone et

al., 1994; Phillips et al., 1996a) and modelling their

production below the surface (Dep et al., 1994; Liu et

al., 1994b). Accurate determination of surface pro-

duction rates is of paramount importance, yet some

discrepancies in estimating production rates exist

(Gosse et al., 1996). Selection of cosmogenic nuclide

sampling locations and age calibration sites is critical,

with those being targeted that have evidence of

minimal erosion, well-determined exposure histories,

close geographical proximity and similar geomorpho-

logical contexts. Scaling factor uncertainties in nor-

malising production rates to sea level, latitude and

angle of surface can also cause additional deviations.

Finding multiple calibration sites and ensuring cross

calibration of production rates for different cosmo-

genic nuclides is the target of much current research

(Gosse et al., 1996).

A single time-independent production rate is not

viable and the uncertainty of the calculated rates for

each nuclide stems not only from the reliability in age

control, but from the fact that these production rates

have varied in the past. Variations in the Earth’s

geomagnetic field strength, dipole axis position and

solar activity directly affect the intensity of the

cosmic ray flux entering the atmosphere (Laj et al.,

1996). Of these, the long-term variability in geo-

magnetic field strength is the most relevant (Masarik

and Reedy, 1995; Robinson et al., 1995). Production

rates have varied through time especially during

periods of weak magnetic strength when higher

fluxes of lower energy particles have enhanced in

situ cosmogenic isotope production. The selection of

a well characterised geological formation with known

exposure age for calibrating in situ production rates

will give rates that suffice only for other study sites

with similar geomagnetic latitude and similar age

(Clapp and Bierman, 1995; Clark et al., 1995). Thus,

the use of in situ cosmogenic nuclides for dating old

landforms requires production rates calculated from

calibrations based on coeval known-age formations

(Reedy et al., 1994). The importance of this complex

problem is indicated by the attention now devoted to

it (Bierman and Clapp, 1996; Gosse et al., 1996;

Phillips et al., 1996b).

Initially, single nuclides were used to estimate the

age of surfaces, but Lal (1991), Nishiizumi et al.

(1991a,b), and Gillespie and Bierman (1995) pro-

posed that two in situ cosmogenic radionuclides

(10Be and 26Al) be used. This approach has the

advantage of constraining age determinations and

limiting age ranges for surfaces, landforms and pro-

cesses (Brown et al., 1991; Nishiizumi et al.,

1993a,b; Albrecht et al., 1993; Bierman and Turner,

1995; Clark et al., 1995; Stone et al., 1998). The ratio


26Al/10Be when plotted against either 10Be or 26Al

concentration indicates anomalous samples that

diverge from a history of steady state erosion (the

so-called ’banana’ plots). The build up of cosmogenic10Be and 3He in beach terraces is another example

which illustrates the potential for combining stable

and radionuclides to constrain age estimates (Trull et

al., 1995). Combinations of in situ cosmogenic,

luminescence and radiocarbon dating techniques are

also in vogue (Markewich et al., 1998), and in future

many more of these dating combinations should

provide better resolution and accuracy in geomorpho-

logical studies.

5.4. Luminescence methods

The thermoluminescence method was first used

for dating pottery (Aitken, 1974) and then applied to

the age-determination of sediments (Wintle and Hunt-

ley, 1980; Aitken, 1985). The principle of the method

is that crystalline materials such as rocks and sedi-

ments, contain small amounts of radioactive elements

(U, Th and 40K) and these decay by emitting alpha,

beta and gamma radiation. These energetic rays dis-

place electrons from the crystalline structures of

minerals into crystal lattice imperfections (light

traps). More and more electrons are displaced into

these traps with time. Quartz and feldspar crystals are

used for luminescence dating. They are separated

from the other minerals and analysed either in ali-

quots or as single grains. Calcite presents problems

because of its complex luminescence (Liritzis et al.,

1996). The amount of luminescent energy released

when the trapped electrons escape to lower energy

levels by heating the grains to 500 BC provides a

measure of the time over which the electron energy

has accumulated (thermoluminescence: LT). The

background radiation level in the material needs to

be measured and assumed to have been constant

throughout the period involved. Additionally, there

is the fundamental assumption that the crystals were

either exposed to sunlight or heated to reset the

electrons to their lower energy levels prior to the

period of radiation.

The method of stimulating the trapped electrons

was subsequently modified and the heat source

replaced by light in optically stimulated luminescence

(OSL; Aitken, 1994). Either green light for quartz

grains (Huntley et al., 1985) or infrared light for

feldspars (Mathur et al., 1986) can be used to stim-

ulate light sensitive traps. For quartz and feldspars,

the age range in natural sediments has been measured

back to approximately 120 ka. This has been ex-

tended to 400 ka based on micro-inclusions in quartz

(Huntley et al., 1993) and controversially to 800 ka

(Berger et al., 1992; Wintle et al., 1993). Techniques

of analysis have also changed from measuring the

light emitted from multiple aliquots of grains to single

aliquots and recently to single grains (Roberts et al.,

1998).

Problems of partial bleaching of the electron traps,

in situ disintegration of minerals bearing electron traps

that are saturated with a prevailing geological signal,

anomalous fading and the unknown variability in

radiation dose through time complicate the measure-

ment of age. Colluvial, alluvial and aeolian deposits

have been dated using these luminescent methods

(Clarke et al., 1996; Porat et al., 1996; Rendell and

Sheffer, 1996). The method offers potential for deter-

mining rates of accumulation of sediments, but ero-

sion rates can only be inferred from deposits adjacent

to the eroding terrains and they indicate averages for

the whole erosional catchment, whereas in reality

erosion may well be limited to narrow linear zones

in and near channels, as for instance in many gully

developments (Fig. 13).

5.5. Uranium series

This dating method, well described by Faure

(1986) and Neymark et al. (2000), is based on the

chemistry of water-soluble uranium isotopes and their

insoluble daughter-decay products. The isotopes 238U

and 235U are soluble in water and can be remobilised

and re-deposited in sediment, such as travertine,

speleothems or tufa, or coralline limestone. Once

deposited these radioactive isotopes decay to form230Th and 231Pa and the dating clock begins. The

method assumes that relatively insoluble thorium was

not deposited with uranium and that no leaching of

isotopes subsequently occurred. The amount of tho-

rium gradually increases with time as more uranium

decays. Application of the method has been demon-

strated for uranium-bearing materials less than about

350 ka (Burnett and Veeh, 1992), and combinations

with 14C (Lin et al., 1998) and thermoluminescence


Fig. 13. (a) A gully cut in Late Pleistocene fanglomerate at Sellicks Hill, about 40 km south of Adelaide, South Australia: an example of

intense but localised erosion during the last 150 years, and (b) narrow gorge eroded in Late Tertiary basalts (note the structural benches

marking flow junctions) north of Hughenden, north Queensland (CSIRO). Clearly, any sediment trap data concerning such catchments

would provide misleading data concerning the character of the erosion to which the deposits are related.


(Nanson et al., 1991) methods show strong agreement

in recent deposits.

The dating method is therefore applicable in

situations where components are accreting. The value

of uranium-series dating is that it can be applied in

both closed and open systems, provided sufficient

samples are collected and cross-checks are made

between different isotopes. The concordance of appa-

rent ages determined by 206Pb/238U and 207Pb/235U is

an important check on the closed system nature of

the materials analysed because it provides evidence

of gains or losses of uranium and the daughter

nuclides. In a closed system of radionuclides the

accumulation of daughters with respect to uranium

provides the chronometer; 226Ra/238U, 230Th/234U

and 231Pa/235U. For an open system, plots of either230Th/234U (Ivanovich et al., 1992, p. 91) or230Th/238U (Simpson and Grun, 1998, p. 1019), and231Pa/235U will reveal the extent of discordance. A

series of samples from the same deposit must be used

to resolve the uranium mobility question (and see

later Section 6.1). Rates of deposition can be com-

puted from a series of measurements from a strati-

graphic sequence, but land surface ages and erosion

rates cannot be accurately determined unless partic-

ularly associated with build-up of such deposits,

which is rare or unlikely.

5.6. Argon dating

Potassium–argon dating, the decay of radioactive40K to stable 40Ar, is a well-known technique in

geology and has long been used for measuring the

ages of igneous and metamorphic formations (Mon-

tigny, 1989). It has also been used to estimate

denudation rates of lavas (Ruxton and McDougall,

1967). Irradiating a sample with fast neutrons to

produce 39Ar from the main potassium isotope (39K)

forms the basis of the 40Ar/39Ar method (Malusky,

1989). The amount of 39Ar produced is proportional

to the concentration of potassium, so the age of the

sample can be determined simply by measuring the40Ar/39Ar ratio. Step-wise heating of samples and

measurement of the ratio by mass spectrometry can

produce reliable ages for landscape exhumation

(Eusden and Lux, 1994), tephra deposits (Ton-That

et al., 2001; Walter, 1994) and the onset of weathering

(Vasconcelos et al., 1994). In cases where argon has

been lost the ratio will be low, but measurement of a

plateau ratio value, particularly using a discrete min-

eral phase, should provide a reliable age (Hofmann et

al., 2000). The 40Ar/39Ar method has been crucial in

providing an absolute timeframe for many geological

problems and, whilst their applicability to geomor-

phology may be limited, their impact on geoscience

has been significant.

5.7. Electron spin resonance (ESR)

This method of spectroscopy essentially detects the

number of electrons trapped in paramagnetic centres

within crystals and results in the measurement of an

ESR signal, the intensity of which is proportional to

the natural radiation dose rate and the time of irradi-

ation (Grun, 1989). Radiation mainly comes from the

isotopes produced in the decay of uranium, thorium

and potassium, with minor contributions from rubi-

dium, radiocarbon and cosmic rays. An ESR age is

determined from the accumulated dose of radiation

generated by all the radioactive sources in the vicinity

of an in situ sample and by measuring a range of

parameters at the sampling site.

The accumulated dose is measured using an ESR

spectrometer in which the absorption of microwave

frequency is detected by linearly modifying the mag-

netic field strength surrounding a sample (Ikeya,

1978). At a sampling site, ideally, measurements

should be made of natural radiation from cosmic

and g-rays, and the actual concentrations and isotopic

ratios of radioactive elements. Additional information

should also be gathered about the attenuation of

radioactive particles by the sample, the water content

and the sample density. Major sources of errors in

ESR dating are systematic errors of calibration,

reproducibility of analytical measurements con-

strained by their detection limits and post-sedimentary

diagenetic effects.

ESR dating has been applied to a wide range of

materials in various geological and archaeological

situations. Foraminifera (Sato, 1982), archaeological

flints (Mozer, 1985), speleothems (Ikeya, 1975; Grun,

1985), travertines (Grun et al., 1988), mollusc shells

(Radtke et al., 1985), corals (Radtke and Grun, 1988),

tooth enamels (Grun et al., 1987) and quartz-rich

volcanic rocks (Shimokawa and Imai, 1987) have

been dated using ESR. Quartz found within intrafault


material has also been used to determine the timing of

the last faulting activity (Fukuchi et al., 1986). ESR

could be used to date loess and delta-sands, but further

research is needed into the nature of the defect centres

in quartz and establishing the radiation history of

samples. Limitations of the method depend on the

sensitivity of the material to radiation, the actual dose

rate and the capabilities of the ESR spectrometer, and

at low dose rates age estimates of 100 ka should be

possible (Grun, 1989).

5.8. Palaeomagnetism and oxygen isotope ratios

The migration paths of magnetic poles have been

dated by geologic and numerical methods, and com-

parison of palaeomagnetic determinations of mag-

netic components of regolithic profiles with ap-

parent polar wandering paths have provided age

estimates of those profiles. But in many instances

the relationship between time of magnetisation and

the time of weathering remains ambiguous. The ages

of formation of regolith, including laterites in Aus-

tralia (Schmidt and Embleton, 1976; Schmidt et al.,

1976, 1982; Idnurm and Senior, 1978) and in India

(Schmidt et al., 1983) have been successfully dated

using techniques that remove post-formational mag-

netic aberrations.

Oxygen isotope ratios can provide both checks on

palaeomagnetic age estimates and independent age

determinations. Fractionation of oxygen isotopes

(16O/18O) in oxygen-bearing geologic materials is a

function of temperature, as is oxygen isotope enrich-

ment: thus in both processes the oxygen isotope ratio

acts as a geological thermometer. As such, the ratio is

useful in palaeoclimatology, but because of the behav-

iour of the Australian plate since the rifting of

Gondwana and the migration of its several parts it

has been used also as a means of dating regoliths and

the surfaces on which they developed.

For the past 100 Ma or so, Australia has been

migrating towards the equator across a strong latitu-

dinal gradient. Though such factors as climatic

change, and altitude of the land surface must also be

taken into account, latitude is a major control of

surface temperature. As palaeomagnetism has pro-

vided a chronology of the migration of the continent,

temperature can be linked to time. The theory behind

the isotopic age dating of the Australian regolith is

succinctly summarised by Bird and Chivas (1989, p.

3240):

As mean air temperature is one of the major

factors controlling the isotopic composition of

meteoric waters . . . and therefore of the isotopic

minerals formed in equilibrium with them . . . theisotopic composition of regolith minerals can be

expected to have become increasingly enriched in

deuterium and 18O since the initiation of north-

ward drift . . . By choosing samples for isotopic

analysis from localities where there is some

independent age control... it should be possible

to calibrate the change in the isotopic composition

of regolith materials with time. Profiles of an

unknown age can then be fitted to this ‘‘palae-

oisotopic curve’’ in the same way that palae-

omagnetic studies fit the apparent pole position of

a sample to an apparent polar wander path to

obtain an estimate of age . . .

Bird and Chivas (1988, 1989, 1993) have, in this

way, analysed clays from a wide geographical range

and identified materials predating the late Mesozoic.

6. Discussion of absolute dating methods and ap-

plications

Promising as absolute methods are, their applica-

tion calls for caution and consideration. The incom-

patible results obtained when dating various surfaces

using different methods suggests that greater care is

needed in applying the methods.

6.1. Potential sampling problems

Appropriate sites with well-documented samples

arising from well-constrained geomorphological his-

tories are preferable. Thus, in using TL or OSL for

dating dune sands it is preferable to sample from an

exposed surface so that structure (cross-bedding)

form is visible and samples can be placed in relative

sequence with confidence. It is useful to be able to

identify the base of the dune so that the beginning of

dune deposition can be dated (see e.g., Twidale et

al., 2001).

While TL, OSL and ESR dating methods can be

used to estimate the age of a sedimentary formation,


where the energy trapping centres in quartz grains

have been zeroed, comparison of the results should

be cautiously considered. The interdependence of

these methods may not be obvious, but each method

relies on knowing the sources and levels of natural

radiation at the sampling site. Inhomogeneities of

radionuclide distribution in sediments, Ra and Rn

diffusion, and the preferential uptake of uranium by

mollusc shells, bone and corals over time are some

factors that affect assumptions about the dose rate.

Where groundwater flow has affected the abundance

of U decay products in sediments the measured in

situ modern radiation levels will not accurately

represent the past dose rate, leading to serious

uncertainty in age calculations. Similarly, results

obtained using the U-series dating method should

only be compared with those of TL, OSL and ESR

when the radionuclide history is certain. The degree

of uranium mobilisation can be estimated from a

plot of 234U/232Th vs. 238U/232Th and an indication

provided of the early, continuous or delayed uptake,

or recent loss of uranium. A similar plot using230Th/232Th vs. 234U/232Th will reveal whether detri-

tal 230Th is a problem resulting either from the

washing in of physical particles or deposition as

dust. If 230Th was absent from a sample initially,

then the 230Th/234U activity ratio and age are both

virtually independent of the 234U/238U activity ratio

(Ivanovich et al., 1992, p. 71). Estimating the effect

of radiation history as a source of error and problems

associated with saturation levels, thermal stability

and diagenetic processes remain significant chal-

lenges for luminescence and ESR chronologists.

When attempting cosmogenic nuclide analysis,

cover by till, regolith (grus) and snow (which may

well be transitory) reduces production of isotopes and

shifts the chronology to younger ages. The problem

is the impossibility of quantifying this effect given

that environmental conditions over the tens of thou-

sands of years of exposure are unknown. Nishiizumi

et al. (1986) selected samples from sloping surfaces

in order to minimise the possibility of seasonal snow

cover, but at the expense of incurring the need for

geometric correction factors to the integrated cosmic

ray flux. Neglect of possible erosion also results in

younger ages. Conditions in which the present

exposed boulder or surface slowly emerges above

the horizon due to the lowering of the weathering

front compared to an assumption of instantaneous

exposure will lead to an overestimate of the true

exposure age (Zreda et al., 1994). However, estima-

tion of long-term rates of denudation of a surface

may be possible where the duration of exposure is

long and known from independent means (Summer-

field et al., 1999).

6.2. Methodological issues

The choice of dating method for land surfaces

is restricted to cosmogenic isotopes (stable and

radionuclide) because cation-ratio dating of rock

varnishes has been shown to be invalid, 14C is re-

stricted to young surfaces and luminescence, and U-

series dating require wider application and greater

testing. Argon dating, whilst an extremely useful

technique in geology, is limited because datable

materials are generally absent in the geomorpholo-

gical domain.

Radiometric techniques, for example the radioac-

tivity of 14C, and deductions based on accumulations

of radionuclides such as 10Be, 26Al and 36Cl are

increasingly used for dating surfaces. The theory is

sound, but complications related to the sources of

datable components and problems of their direct

relationship to formation of a surface have emerged.

Though many problems relating to cosmogenic nu-

clide dating have been addressed and largely re-

solved, other difficulties remain, concerning inherited

nuclides, duration and thickness of regolith cover, the

possibility of solution and translocation of nuclides,

and present low confidence levels. The main restric-

tions derive from the short half-lives of the radioac-

tive isotopes, resulting in specific dates usually

much less than 3.0 Ma. In practice reliable ‘absolute’

dates using in situ cosmogenic nuclides can be

obtained only in ideally controlled and simple con-

ditions, for instance a fault scarp or other recently

exposed bedrock surfaces which have neither in-

herited nuclides nor have ever carried a regolithic

veneer that may have masked the target rock. The

cosmogenic method is appropriate to the dating of

catastrophic events such as faulting; or a mass move-

ment, during which a scarp, or tension scars or blocks

incorporated in debris flows, slides or falls, are

instantly exposed. Such well-defined, sudden geo-

morphic events, and their associated forms can also


be used as sources for accurately defining cosmogenic

nuclide production rates (Kubik et al., 1998), that can

be applied regionally.

Both relative dating and cosmogenic nuclide anal-

ysis carry inferences concerning ages of landforms

and surfaces. The putative age assigned to a landform

refers to the time of initiation of the landform, which

although it has maintained its essential morphology

and remains part of the contemporary landscape, may

have evolved continuously, albeit very slowly. How-

ever, a problem arises because ‘surface’ and ‘age’ are

used differently in the two dating approaches result-

ing in confusion. Geological/geomorphological meth-

ods attempt to determine the ages of initiation of

landforms; subsequent changes are identified and

dated but the form is labelled according to the age

of origin. The present landforms are the weathered

remnants of the originals. Rock varnish dating meth-

ods (cation ratio and AMS 14C) attempt to determine

the age since a surface reached equilibrium. Lumi-

nescence and U-series methods describe the period of

radiation in stabilised deposits and assume no deple-

tion or influx of radioactive components. Cosmo-

genic in situ analyses address the relatively recent

chronology of contemporary surfaces or surfaces ex-

posed by recent catastrophic events within the range

of the method. This last approach seems to offer the

best strategy for dating land surfaces under ideal

circumstances, but nature does not always provide

those conditions.

6.3. Some cosmogenic nuclide problems

Pre-exposure, shielding, erosion, temporal varia-

tions in production all affect cosmogenic concentra-

tions and their omission will generate ages both higher

and lower than the real age of a surface (Zreda and

Phillips, 1994). Generally, model in situ ages of older

surfaces will be more susceptible to such influences

and will typically show larger age distributions.

Measurements of large sample populations from a

defined site or formation (and even multiple samples

from a single surface or rock) are essential to map the

apparent age distribution and extract a realistic esti-

mate of the true age.

There is here a circular argument: how is the

appropriate regime of exposure history of the meas-

ured sample identified if it is the age itself that

determines the choice? Are measured cosmogenic

abundances average erosion rates or ages that fit the

constructed model for the erosional surface? Two

points can be made. First, the measure of additional

in situ radionuclides of different half-lives better

constrains the exposure history between model

extremes (Lal, 1991; Liu et al., 1994a). Second, the

geomorphological and geological settings of sites can

be used to evaluate the regimes of the surfaces to be

sampled. Once the exposure history has been com-

petently assessed, the in situ model can be quantita-

tively applied.

6.4. Limitations of the cosmogenic method

Nishiizumi et al. (1993a) have written:

. . . cosmogenic nuclides are not simply a one-

number surface exposure dating method, but rather

a set of tools to study geomorphic processes on

time scales truly appropriate to the development of

most landscapes. The challenge is to see the

potential of cosmogenic nuclides as being more

than just a dating method.

The general tenor of this comment is admirable but

in detail calls for comment. The age limitations

inherent in the radionuclide methods are real and

critical, for it is increasingly recognised that signifi-

cant areas of the continents are older than encom-

passed by even the most optimistic evaluation of

cosmogenic nuclides (for reviews, see King, 1962;

Twidale, 1976, 1994; Twidale and Campbell, 1988,

1993; Twidale and Vidal Romani, 1994; Young, 1983;

Bishop and Goldrick, 1998). With an appreciation of

the great antiquity of many land surfaces throughout

the world and acknowledging the difficulties of

applying the exposure history model and cosmogenic

isotope dating method it seems unrealistic to assume

that the approach can always produce valid age esti-

mates or erosion rates. Even under optimum circum-

stances cosmogenic methods can only offer an

estimate of geologically recent rates of erosion and

it is unreliable to extrapolate these back over geo-

logical time because climate change and tectonism

can have had profound effects.

Considering this inherent limitation, radionuclide

dating ought for the present to be concentrated on


Fig. 14. (a) Thick displaced slabs in granite, King Rocks, near Hyden, Western Australia. (b) Displaced slabs on slope of Kokerbin Hill,

south of Kellerberrin,Western Australia. (c) Sloping zone of concavity, on the flank of Chilpuddie Hill, northwestern Eyre Peninsula, South

Australia. (d) Etch surface in sandstone, southern Drakensberg of South Africa.


ideal situations, on those forms genetically related to

events rather than surfaces per se. Thus, the effects of

possible tsunamis are being studied on the New South

Wales south coast (Bryant et al., 1996; Young et al.,

1997) and palaeoseismic events and what are inter-

preted as catastrophic palaeometeorological impacts

(rock avalanches) in South and Western Australia.

These sites are characterised by new exposures of

fresh rock (fault scarps, slipped slabs, displaced

blocks) which have not been covered by a regolith

and that have taken place over the past few millions,

and certainly the last half millions of years.

6.5. Successful applications

Some meteorite impact craters, fault scarps and

surfaces exposed by the dislodgment of slabs (as a

result of earth tremors, directly or indirectly) have

chronologies which can be described as having con-

tinual exposure following a rapid formation event.

Fig. 14 (continued ).


The in situ cosmogenic model can date such events,

provided, of course, that they fall within the age

range of the method, and particularly if they are

young (10–100 ka). Dated samples of young vol-

canic flows, volcanic bombs and cinder cones at

Lanthrop Wells, Nevada (Zreda et al., 1993) suggest,

but do not prove, the absence of multiple eruptions.

Meteor Crater, Arizona, provides an example of

cross-checking for the 50-ka impact age from in situ10Be, 26Al and 36Cl is corroborated by TL dating

(Nishiizumi et al., 1991b; Phillips et al., 1991).

Dating features like rock avalanches, landslides, cliff

collapses, and tsunamis, putatively related to seismic

events, result in new and fresh surfaces, though

geometric corrections between surfaces of different

orientations may be required.

Glaciated surfaces and deposits similarly pose

many problems because the exposure histories of

erratics and till fragments are not known. Sampling

strategies can alleviate such difficulties but concerns

remain. Nevertheless, recent in situ dating of samples

related to alpine glacial cycles in the Sierra Nevada of

California over the past 50 ka (Phillips et al., 1996b;

see also Phillips et al., 1990) compare well with the

oxygen isotope marine record. Morainic sequences

spanning the last glacial transition (Gosse et al., 1995)

show strong supporting evidence for a Younger Dryas

readvance in northern America. Neon dating has

allowed clarification of the glacial sequence of the

northwestern Iberian Peninsula, with multiple glacia-

tions now identified within the previously recognised

stratigraphic framework and correlated with 14C dat-

ing (Vidal Romani et al., 1999).

Even when armed with production rates for cos-

mogenic nuclides, which are assumed to be reliable,

results from simple geomorphological settings fre-

quently indicate a much more complex chronology

than anticipated. Antarctic Dry Valleys and nunataks

supplied a promising environment for in situ studies.

In the Arena Valley (Brown et al., 1991; Brook et al.,

1993), the Sor Rondane Mountains (Nishiizumi et

al., 1990) and the Trans Antarctic Mountains (Brook

et al., 1995) surfaces have 10Be concentrations at or

near full saturation. Only prolonged exposure over

an age range of a few million years could produce

such high in situ concentrations. Moreover, model

erosion rates must necessarily be low in order to

support such accumulations. Although arguments

relating to non-constant production rates over this

age scale, sample pre-exposure and differential

weathering can be invoked, there is no escaping

the basic conclusion that these Antarctic formations

have stood as they are seen today for the last few

million years. In some locations the ice volume has

changed by only a hundred or so metres over this

time span. The difficulty here—and elsewhere—is

that great age can be invoked, but is the determi-

nation of this ancient age accurate? How can the age

determination be tested?

6.6. Testing

Studies of surfaces developed on features such as

alluvial fans (Liu et al., 1996), tufa and pedogenic

deposits (Liu et al., 1994b), laterites (Brook et al.,

1995), desert sands, palaeo-beach ridges (Caffee et

al., 1994; Trull et al., 1995) call for extreme caution,

if only because of the strong possibility of materials

containing inherited nuclides being present. At sites

where regional geomorphological evidence points to

weathering, transportation and phases of deposition

and burial, the application of the in situ model is

limited. Cosmogenic abundances may offer relative

‘ages’ (though this is frequently apparent in the

landscape anyway) and at best, may indicate general

chronologies or erosion regimes (e.g., Bierman and

Turner, 1995). How are these chronologies reliably to

be fitted into an absolute or even a closely approx-

imate time scale? Although these may be of interest,

the measured cosmogenic abundances are integrating

exposures belonging to different environments and

geometries. Measurement of as many in situ radio-

isotopes as possible offers the best avenue to differ-

entiate the phases of the combined processes. This is

because each in situ radioisotope has a half-life which

dictates the time scale for its sensitivity as a chro-

nometer, or put otherwise, each in situ radioisotope

has its own time window into the past. Identifying the

optimum samples to reduce the complexities requires

a competent knowledge of local stratigraphy and

geomorphology.

Some situations appear, in theory at any rate, to

offer good possibilities of testing. Features associated

with catastrophic events such as earth tremors or

mass movements of debris are suitable because they

formed instantly. Thus, displaced slabs of sufficient


Fig. 15. The stripping of the regolith here at Elkington Rock, north of Minnipa on northwestern Eyre Peninsula, South Australia, has revealed that the granite mass, naturally

exposed in a platform flush with the surrounding plain and about 10 m diameter, is a large-radius dome already shaped beneath the land surface.

A.L.Watch

man,C.R.Twidale

/Earth

-Scien

ceReview

s58(2002)1–49

37

thickness (Fig. 14a and b) allow ages of the initial

and the new surface to be dated using cosmogenic

nuclides. There is no possibility of there having been

a regolithic cover and their age is a moment in time.

Fault scarps of sufficient throw also are suitable and

there is the added possibility of providing a cross

check by dating debris accumulated at the base of the

scarp by using other methods such as TL or OSL

(e.g., Hutton et al., 1994; M. Machette, personal com-

munication, April 2000).

Stepped inselbergs with multiple well-developed

flared scarps may offer good possibilities for nuclide

dating, for the flight of steps and treads are in known

relative age sequence (Fig. 5). In some instances the

removal of regolith must have been effectively simul-

taneous and probably instantaneous on any given

level of exposure. However, zones of flared slopes

are not everywhere horizontal but rather quite steeply

inclined and here the concavities are partly exposed

and partly still covered (Fig. 14c). Had this occurred

in the past, exposure regimes would have varied

along the same slope. Finally, where a bedrock sur-

face has been exposed gradually as a thin regolith has

been gradually worn away, there ought to be a

progression of increasing age from the edge of the

regolith to the stream channel located in the valley

axis (Fig. 14d).

6.7. Salutary tales

Interpretations of all land surface dating results

must at all times be consistent with local and regional

stratigraphic and geomorphological chronology, con-

cepts and processes. Problems have arisen because of

overenthusiastic and consequently uncritical applica-

tion of new techniques. For instance, the inversion of

the well established and apparent stratigraphy (Sharp

and Birman, 1963) implied by the 36Cl datings at

Bloody Canyon, in the eastern piedmont of the Sierra

Nevada in California (Phillips et al., 1990) ought to

have been supported either by independent corrobo-

ration of the dates, or by a mechanism whereby valley

glaciers could flow under earlier deposited materials,

or both. Similarly, the cosmogenic dating of granite

inselberg surfaces on Eyre Peninsula, South Australia,

led Bierman and Turner (1995), to claim that the

residuals had not been covered by a mantle of

weathered granite. This is at odds with both local

and overseas evidence (Fig. 15) concerning the origin

of such bornhardts (e.g., Falconer, 1911; Twidale,

1982a,b).

Another illustration is the unrestrained enthusi-

asm concerning the geological nature of sand grains

used in the thermoluminescence dating of the

archaeological deposit at the Jinmium rock shelter,

Northern Territory (Fullagar et al., 1996; Roberts et

al., 1998; Spooner, 1998). Ancient rock shelter sand

deposits associated with human occupation, as ini-

tially determined by TL, embarrassingly became

much more recent when radiocarbon and OSL dat-

ing of small aliquots and single quartz grains high-

lighted the problem of in situ degradation of sand-

stone saprolite.

7. Conclusion

Absolute dating methods have provided the essen-

tial temporal framework for the stratigraphic column

and even in that indirect way have contributed cru-

cially to denudation chronology, the unravelling of

landform and landscape development through time,

for rock dating by various radiometric means has

provided essential benchmarks for landform and land-

scape interpretation.

In addition, the last half century has seen the

development of physical methods that offer the pos-

sibility of direct dating of surfaces. Even the earliest

developed of these methods still pose severe prob-

lems, and there is the danger of placing too much

reliance on what are still tentative age determinations.

Nevertheless, and despite the many potential pitfalls,

they are worth rigorous and tenacious pursuit. Phys-

ical methods are widely applicable in a geographical

sense, and have the attraction of giving numerical

approximations. But they are approximations, no

more and no less, and moreover, many land surfaces

have evolved over time and are more appropriately

allocated an age range rather than a specific age.

Nominations of specific moments of time are unreal-

istic save for catastrophic features such as fault scarps.

All physical methods suffer from the major disadvant-

age of being temporally restricted—they can provide

dates for features and events originating during the

last few millions of years, equivalent to the last few

seconds of Earth history.


Relative methods have no such limitation. To take

one example, exhumed granitic inselbergs, no matter

whether Archaean or earliest Proterozoic (Twidale,

1986b), sub-Cambrian (e.g., du Toit, 1937, p. 230),

sub-Cretaceous (Twidale, 1986b), sub-Miocene (Twi-

dale et al., 1978), or sub-Middle Pleistocene (Twi-

dale and Campbell, 1984) in age, are susceptible to

dating by the same stratigraphic reasoning, namely

that a minimum age for the forms is provided by the

age of the oldest cover material. The methods are

basically understood and sound. But good fortune is

needed for the required evidence to be both pre-

served and exposed.

Inconsistencies and incongruities in and between

ages of surfaces derived from different procedures,

and even when the same method is used, suggest that

it is desirable to make use of different dating methods

at the same site where it is possible to do so, or at

least to have a sampling strategy that involves

internal checks. Cosmogenic nuclide dating, for

example, is not yet the new revolution in geomor-

phology, as has been claimed. Absolute methods

promise invaluable data, but at this time some still

require rigorous testing, and all must needs be used,

and the results interpreted, with care, and with geo-

logical data and perspectives in mind. Thus, the

chronologists whose cosmogenic nuclide age esti-

mates of some inselbergs on northwestern Eyre

Peninsula led them to announce the ‘‘likely existence

of pre-Pleistocene landscapes’’ in the area, were

unaware, or chose to ignore as inadmissable, the

earlier stratigraphic identification of Mesozoic land-

scapes there and in adjacent areas (Twidale, 1976,

1991b, 1994, 2000; Twidale et al., 1976; Campbell

and Twidale, 1991), as well as in other parts of

Australia (e.g., Hossfeld, 1926; Craft, 1932; Hills,

1934; Miles, 1952; Campana, 1958; Young, 1983;

see also Dixey, 1938; King, 1962; Partridge and

Maud, 1987). Though the absolute procedures based

in physical science may appear universally applica-

ble, in reality there are, everywhere and always, local

circumstances which introduce strong special case

elements, so that critical consideration of the working

environment is as crucial in the application of abso-

lute, as well as of relative, methods. The physical

sciences are not exact, only more precise than some

others. Subjective correction factors may be applied

to results. Absolute age estimates provide results that

appear precise. Whether precision is appropriate to

the dating of many, perhaps most land surfaces, is

debatable.

Clearly, the best outcomes will be obtained from

absolute dating when there is genuine collaboration

between physicists and earth scientists and when in

consequence the numerical results are interpreted in

terms of geological principles and local and regional

realities. In particular, physically derived data must be

compatible with relative ages, which are mainly

derived by the application of common sense to geo-

logical problems.

Acknowledgements

The authors thank Dr. Jennie Bourne for critical

readings of various drafts of this lengthy review, for

constructive comments, and especially for painstak-

ingly formatting our final manuscript.

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Documents

Relative and ‘absolute’ dating of land surfaces