Journal of Coastal Research 605-611 Fort Lauderdale, Florida Summer 1996

Morphological and Chemical Variations of a BuriedPalaeocatena in Late Holocene Beach-ridge Sands atMagilligan Foreland, Northern IrelandPeter Wilson

School of Environmental StudiesUniversity of Ulster at ColeraineCromore Road ColeraineCo. Londonderry BT52 lSA Northern Ireland, U.K.

,ttlllllll:.~tusss~-~ ",G+ &__

ABSTRACT _

WILSON, P., 1996. Morphological and chemical variations of a buried palaeocatena in Late Holocene beach-ridgesands at Magilligan Foreland, Northern Ireland. Journal ofCoastal Research, 12(3),605-611. Fort Lauderdale (Flor­ida), ISSN 0749-0208.

Vertical and horizontal variations in soil profile morphology and chemistry within a sequence of buried palaeosolsdeveloped in Late Holocene beach-ridge sands at Magilligan Foreland indicate the existence of a palaeocatena. Ahumus podzol occurs on the crest of the beach ridge and passes through a gleyic brown calcareous alluvial soil and atypical humic alluvial gley soil to a raw en-fibrous peat soil in the inter-ridge depression, This sequence reflectsdifferent slope positions and soil water regimes. Water-table fluctuations, possibly associated with Late Holocene sea­level oscillations. have comhined with pedogenic processes to produce the distinctive morphology and chemistry of thepalaeocatena. All palaeosols have developed in texturally uniform sands and have been decalcified to depths of be­tween at least O.SS m to 1.6 m. Ferruginous concentrations, present in the lower horizons of each palaeosol, formedafter the profiles were decalcified. Palaeocatena development was probably restricted to a time-span of c. 1,000 yearsto 1,SOO years. Two aeolian sand units hury the palaeocatena; the older of these has been 'welded', in part, to thepalaeocatena by pedogenic processes, Percolation of water through the younger calcareous aeolian sand unit canaccount for the present near-neutral pH values of the palaeocatena.

ADDITIONAL INDEX WORDS: Palaeocaiena, palaeosols, beach-ridpc-. pedogenesis, Holocene. Magilligan.

INTRODUCTION

VALENTINE and DALRYMPLE (1975) defined the term 'pa­laeocatena' as "a group of palaeosols on the same buried land­surface whose original soil properties differ owing to theirdifferent original landscape position and soil-water regimes";they also stated that "this is the one ubiquitous soil charac­teristic that sedimentation and diagenesis will not produce".Therefore. "the crucial test of a buried soil landscape must])E' the existence of a pnlacocatena' (VALENTINE and DAL­RYMPLE, 1976). However, as noted by FENWICK (1985) E'X­posures of buried palaeosols are rarely of sufficient lateralextent to demonstrate these palaeocatenary relationshipsand most interpretations have been based on restricted andisolated exposures.

At Magrlligan Foreland, Northern Ireland, WILSON andBATEMAN (1986) identified a buried palaeocatena developedin beach-ridge sands and presented field and laboratory datarelating to the Ireely-drnined end member (humus podzoll ofthe sequence located on the beach-ridge crests. ThE' poorly­drained end member (peat) of the sequence, situated in inter­ridge depressions. and the intermediate soil types were notstudied in any detail due to limited availability of exposuresin the unst.ahle sand cliffs,

As a result of erosion by winter storms in 1993 the entire

94083 receircd /() JUIIC 1994; accepted in rcrisiun 26 Fcbruarv 1995.

sequence of a beach-ridge palaeocatena was exposed in anarea of shoreline where buried palaeosols had not been re­corded previously. The palaeocatena extended for c. 50 m anddisplayed a series of soil types reflecting different originallandscape positions and soil-water regimes. This paper de­scribes some of the morphological and chemical variationswithin and between the palaeocatena soils, complements ear­lier work on palaeosols at Magilligan (WILSON and BATE­MAN, 1986; 1987), and adds to the already considerable bodyof information documenting the Holocene evolution of thiscoastal landform.

MAGILLIGAN FORELAND

Magilligan Foreland (Lat. 55'~ 7'-55° 12' N., Long. 6° 51'­6° 59' W.) is a triangular beach-ridge plain covering an areaof c. 32 km- on the eastern side of Lough Foyle and is prob­ably the most intensively researched coastal landform in Ire­land te.g. CARTER, 1972; 1975; 1979; 1986; CARTER and WIL­SON, 1990; CARTER et al., 1982; MALLORY and WOODMAN,1984; MALLORY and MCCORMICK, 1988; McBRIDE and WIL­SON, 1991; WILSON and BATEMAN, 1986; 1987; WILSON andFARRlN(;TON, 1989). The Foreland developed as a conse­quence of Holocene land/sea level changes and comprises over150 northeast-facing swash-aligned beach ridges that areoverlain by localised aeolian, lacustrine and alluvial sedi­merits (Figure 1),

606 Wilson

Figure 1. Holocene sediments of Magilligan Foreland (based on 1:50,000Northern Ireland Geological Survey map ). The horizontal arrow along theLough Foyle shore indicates the location of the palaeocatena. The mar­ginal grid is at intervals of 1 km.

FIELD AND lABORATORY METHODS

Five visually distinctive palaeosols were recognised in thepalaeocatena and each was described according to the systemoutlined by HODGSON (1976). Samples were collected fromall horizons of the palaeosols and from the overlying aeoliansand. The heights of each palaeosol and the modern groundsurface were surveyed with Electronic Distance Measuringequipment and related to Ordnance Datum (O.D.) Belfast.

In the laboratory, samples were air-dried and analyses per­formed on the fraction finer than - 14> (2 mm). All sampleswere tested for pH (1:2.5 w/v CaCI2 ) and dithionite-extract­able iron (Fed), and selected samples for pyrophosphate-ex­tractable iron (Fe.) and calcium carbonate equivalent(CaCOa) following AVERY and BASCOMB (1974). (Becausenone of the palaeosol horizons gave an observable reactionwhen a few drops of dilute hydrochloric acid were applied,they were not subjected to calcimeter determination ofCaCOa) Organic matter content was estimated by loss-on-ig­nition at 430°C (DAVIES, 1974). Thirteen sand samples (threefrom the overlying aeolian sands and ten from the palaeoca­tena) were analysed for particle size distribution by dry-siev­ing at 0.254> intervals; mean grain size and sorting were cal­culated using the measures suggested by MCCAMMON (1962).Classification of the palaeosols was achieved using the sys­tem of the Soil Survey of England and Wales (AVERY, 1980).

identify on the ground. Only on aerial photographs and incliff sections along the Lough Foyle shoreline, where the ridgesystem has been buried by aeolian dunes and subsequentlytruncated by wave erosion, can the original topography betraced. It was from exposures along the Lough Foyle shore­line that WILSON and BATEMAN (1986, 1987) identified bur­ied palaeocatenas developed in calcareous beach-ridge andaeolian-dune sands and where the present buried palaeoca­tena occurs (Figure 1).

Alluvium

Raised beachsand

Peat

~

ol2Za..

F/ BRITISHrrl- t: ISLES~~" ~Atlantic t~ f

ocean ~ (r"Yt'. "':S"

~~(?f- --r ,\

'1 !JO> \).. ~{~ .-

'7 !; .~ /}IJ \. . ~(' (zl ~4-:l- ) 1 )

d -1-.0$ , J~_./ ~ ./

,/ ---~-:'~~

•MAGILLIGAN POINT

ATLANTIC OCEAN

65

""-J

~

(;)

~

41

~

CoO

~

(;)

35

40

Based on previously published interpretations of buried pa­laeosols (WILSON and BATEMAN, 1986; 1987), the palaeo­ecology of peat beds (McMILLAN, 1957; McMoRRIS, 1979;BRADLEY, 1985), archaeological investigations (MALLORYand WOODMAN, 1984), and radiocarbon dating, WILSON andFARRINGTON (1989) were able to present a detailed recon­struction of the Late Holocene evolution of Magilligan. Theearliest preserved palaeotopography is the gently-undulatingbeach-ridge plain. Ridge widths range from 100 m to 300 m(CARTER, 1986) and ridge amplitudes are between 1 m and 2m. This sequence of beach ridges accreted diachronouslynor thwards between the local maximum of the Early Holo­cene eustatic transgression (c. 6,000 years B.P.) and the re­turn to near present mean sea level (c. 2,000-1,500 yearsB.P.) (CARTER, 1982). Pedogenesis affected the beach ridges,producing podzolic and brown soils on the ridge crests andflanks, and peat developed in the inter-ridge depressions. Asaccretion of the beach-ridge plain neared completion, promi­nent aeolian dunes formed along its seaward margins. Overmuch of Magilligan, agricultural activities during the last2,000 years (MALLORY and MCCORMICK, 1988) have resultedin land levelling and the beach ridges are now difficult to

RESULTS

Geomorphology

The morphology of the beach ridge / aeolian dune exposureand the spatial arrangement of the buried palaeosols are de­picted in Figure 2. Three sand units are present in the sec­tion, the lowermost of which is the beach-ridge sand contain­ing the palaeocatena. The ridge declines in height from 4.0m O.D. at its crest to 2.4 m O.D. at the lowest point of theinter-ridge depression, giving an amplitude of 1.6 m. Themaximum gradient on the ridge is 5°, between palaeosols 1and 3; elsewhere ridge gradients do not exceed 2°. The pa­laeocatena is overlain by two sand units which are consideredto be aeolian rather than marine (beach) sands because theypossess a strongly undulating topography adjacent to the ex­posure, contain abundant terrestrial snail shells, and lacklarge fragments of marine mollusc and gastropod shells,which are common in the beach sands. The aeolian sandsprobably originate from reworking of former beach-ridgesands in the area now occupied by the Lough Foyle inter-tidalsands. The older of these aeolian sand units is thin « 0.2 m)and is visually distinctive only between palaeosols 1 and 3

Journal of Coastal Research, Vol. 12, No. 3, 1996

Bu ried Palaeocaten a In Beach-RIdge Sand s 607

Metres

® ® CD~ , ~

Cu Lv ") ~ Cu'-' r' -.;.OAh bAhbCu bCubAh/ Ea bAhOAh bAhlEs

bBw bEs bEpbewl bBn

08w(g' 1bBw2

bBw

bBw(g l

bBw(g)

CD

1.1I;J;] 2 _ 3~ 4 c::=::::J S.c::=::::J 6.1IllIIIIllIllII 7 c::::=::J 8.~

180

South

Aeoliansand

North5 MOdem ground

eunece ' soil

_- - - -- Bea en .:» :: _ _~

2 BO~~~-----~ __

o 10 20 30 40

Figure 2 Th e bu r red beach-ridge palaeocatena at Mag-illiga n Fore landTh e dense s t ipple represent> the orgamc-rrch hori zon IbO and bA hori­zons I of th e pa laeocatena and the da shed hues mark horizon boundari esIn the sub-soil lndrvidua l pa laeo sols a re Indicat ed by nu mbered arrowsDeta iled p rofi les for each pa lae osol a re shown in Frgure 3.

where soil form at ion ha s occur red; subsequent burial has re­sulted in a pa ra llel sequence of pa laeosols (Figu re 2 J. Thissa nd unit also extends between palaeosols 3 a nd 5, bu t he repedogenic processes have 'welded' the sand to t he u nderlyingorganic hori zon (see below ). Th e younger aeo lian sand uni t isthicker 10.5-1.5 m 1and forms the undulating, modern gro undsui-face a t 4.2 m to 4.8 m O.D.

Particle Size Distribution

Th e dominant fraction 10 all samples is fine sand I+ 2<b to+ 3<b l with between 83.4% an d 95.2 Gi} by weigh t of the sizedistribu tions; this is followed by medium sand (+ 1<1> to + 2<\> ,2.3-1 4.7%) and very fine sand ( +3<1> to +4<b, 0.7- 5.8%). Verycoarse sa nd (- 1<b to Odi, < 0.1%) and coarse sa nd 10<1> to + 1<1>,< 0.2%) are negligible componen ts . Mean grain size rangesfrom +2.38<D to +2.6 1<1> and sorting va lues are 0.25<b to0.34<1> , ind icating very well sor te d fine san d. Thus, s t rong tex ­tural un iformity exists with in t he sample se t (ef WILSON andBATEMAN, 1986 , 1987); t here are no sign ificant par ticle sizevari a t ions that can account for the visua l contrasts betwee npalaeosol hori zons.

Soil Profile Morphology and Chemistry

Figure 3 sh ows the profile morphology for each of the fivepa laeoso ls recogni sed in th e pa laeocatena and Table 1 lis tsselecte d properties of these profiles . Th e most st r iking fea­ture of the pa laeocatena is th e marked lateral va ria tion inthe altitude and thic kness of th e organic-r ich horizon (Fig­ures 2 and 3). Horizon altitude decreases and th ickness in­creases from less th an 0.1 m in pa laeosol 1 (bAh and bAh/Eahorizons) to 0.82 m in pal aeosol 5 (bOf/m hori zon ). Fu rth er­more, th e orga nic ma tter content increases , however , its de­gree of decomposit ion is not iceab ly less in the thicker hori­zons of the lower-altitude sites . In palaeosols 1 and 2 theorganic-rich horizo ns are underlai n respect ively by light greyand light brownish gre y (lOYR7/2, 6/2) sands tha t are defi­cient in both orga nic matter and iron; these qua lify as bE ahorizons . A thin , yellowish br own (10\ 'R 5/4) horizo n contain ­ing more organic mat ter a nd iron th an the hori zons imme-

F igure 3 Profi le mor phology of the five pa laeoso ls recognised In th ebeach-rtdge palaeocatena a nd palaeosols III t he older of th e aeo lian sa ndunits Expl a nation of sy mbols: 1 CaCO ,-rIch aeo lia n sa nd . 2 peat hort­zons, 3 mixed orga ruc a nd rnme ral hori zons . 4 deca lcified aeo lian sand.5. eluvia l hor izons, 6 illuvial hori zon . 7. decalcified beach-ridge sa nd . 8ferr ugm ou s de posit s .

dia tely above and below occurs beneath the bEa horizon inpalaeosol 1; thi s represe nts a blsh hori zon but is of very lim­ited lateral extent, being absent from pa laeo sol 2. The re­ma in ing horizons of palaeosols 1 an d 2 are si milar to thoselocated beneath th e organic-rich horizons of pal aeosols 3, 4and 5. They consist of br own, light bro wnish grey, palebrown, light yellowish brown , brownis h ye llow and very palebrown (10YR5/3, 6/2, 6/3 , 6/4, 6/6, 7/3) sa nds t hat lack CaC03

a nd a re deficient in organic matter. Some of these horizonspossess vertically-a ligned ferruginous deposits outlining thepositions of former root cha nne ls; consequently the Fed con­tent in these horizons is greater than in th ose without suchfeatures . All these zones qua lify as eithe r bBw or bBw(g) ho­rizons , depend ing on th e absence/presence of ferruginous con­cen trations.

Inte res ti ng ly, the CaC03-rich sand (bCu horizon ) of t hebeach ridges was lacking in a ll five pa laeoso ls sampled. WIL­SON an d BATEMAN (1986) showed that th e Magilligan beachridges contain c. 10% CaCO'l and the depth to CaC03 in thepalaeosols th ey examined was less th an 0.9 m. In palaeosols1 an d 5 of th e present seq uence th e profiles exte nd for dep thsof 1.6 m and 0.85 m respectively in to the beach rid ge wit houtreac hing CaC03-rich sand.

The palaeosols developed in the older of th e aeolian sanduni ts that bury the palaeoca tena in profiles 1 and 2 whichconsist of two hori zons only. Both soils possess a dark brown(10YR3/3, 4/3) bAh horizon , in which organic matter has ac­cumulat ed, underla in by eithe r a pale brown or light yellowishbrown (10YR6/3, 6/4) bCu horizon . CaCO'l is abse nt from eachprofile. In pro files 3, 4 an d 5, pedoge nic processes 'welded' th eaeolian sa nd to the palaeosol s of the beach -ridge palaeocatenasuch that it became an integral part of the palaeocatena priorto bur ia l by th e younger aeolian sa nd unit. Use of the term'welded' follows RUBE and OLSON (1980) and SCHAETZL and

.Iou rna) of Coasta l Rese arc h. Vol. 12, No. 3, 1996

608 Wilson

Tabl e 1. Propertie s of the palaeocatena soils.

Profile andSoil Type Horizon

Depth"(em) Dry Colour pH

Loss-on­ignit ion q

Fep

(ItFe,l~

csco,ck

Typica l sa nd-ra nker

Humus podzol

Cu

(bAhbCu

(

bAhbAhfEabEabBhbBwblswtg )

0-77- 17

17- 2020-2424- 3333-3636- 9999-1 79 +

lOYR5/610YR3/310YR6/4lOYR3/310YR5/310YR7/210YR5/410YR7/310YR6/4

6.97.06.77.06.76.56.66.46.4

073.9064.92.10.31.50.203

0.02005

0.01

0.0 2

0.260.250.110.100030.010.140080.18

3.3

2

Typica l sand-ran ker

Gleyic brow ncalca reou sall uvia l soil

Cu

(bAhbCu

(

bAhfE abAhbEabBw lbBw2bBw(g )

0- 88- 13

1:3-2020-2323- 2626-3939-8282- 112+

10YR6/610YR4/310YR6/3

10YR4/2IOYH312IOYR612IOYR5/310YIW1210YR6/3

7.67.36. 1

6.77. 16.86.86.66.5

0.55.20.62.6

12.20.40.40.20.3

0.08

0 .12

0.0 1

0 .260 .310.Q7

0.040.270.0 10.020.060.16

4.6

3

Typ ica l h umic­a lluvial gley sui l

Cu

(

bAh IbAh 2bOhbEwbBw(g) 1bBw(g )2

0- 55-11

11- 1717-3737- 6969- 1J9+

10YR6/410YR4/4lOYH5/310YR2/210YR6/2JOYR6/6IOYR6/4

6.9686.8676.:{6.562

0 35.52.5

20 .20.20.40.2

0.14

0:l2

005

0.270500.080.630 040.3 10.14

3.2

4

3.6

0.050.180 16

0.25

o.a0.30.3

0 512.6-29.4"

6.36.26.1

7.16.4 - 6.8"

lOYR6/410YR2/I ,311 & 3/210YH6/3IOYR6/410YR6/4

0-25

25-4949- 7979-99+

bBwbB wlgllbBw(g )2

Cu

j'0~Typ ical hu mic­all u via l gley soil

5

3 7

0.080 130. 15

0.18

0.40302

0.4:J2.HJl.9'

6.16.26.1

6.85.7-6 .4"

IOYR6/4IOYR2/1 ,2/2 & 3/3IOYR5/3lOYR6/4IOYR6/4

0- 82

82-107107- 127127- 167

bBwbBw(g )lbBw(gJ2

eu[ bOfim

Ra w eu -fibrouspeat soil

- not det ermin ed."Depths measured from bu r ied soil surface"Ra nge of val ues for five samples from different dept hs

SORENSON (987) and refers to situa tions where discernible Chori zon materi al no longer exists between the solum of agro und soil and a buried solum. Soils buried by a thin layer ofsediment are likely to become welded rapidly as the hor izonsof the ground soil develop and thicken downw ards towards thebu ried soil. In th e context of Magil Jigan , welding processes in­volved the accumulation and incorpora tion of organic matterand the leaching of CaC03 • In profile 3, welding has crea tedda rk yellowish brown an d brown 00YR4I4, 5/3 ) Ah hori zonsdirectly above the bOh horizo n of the pal aeocatena ; in profi les4 and 5 welding has been more se vere an d the sa nd has beentotally incorporated into th e bO horizons (Figu re 3). Analyses

of the relative proportions of sa nd and organic matter in th eupp ermost pa rts of the se orga nic-rich horizons have confirm edth e presence of signi ficant quantities of sand.

All pal aeosols a re overlain by either ye llowish brown ,browni sh yellow or ligh t yell owish brown 00YR5/6, 6/6, 6/4)sa nd conta in ing between 3.2% and 4.6% Ca CO'J and between0.18'k' and 0.27 % Fed' The surface zone of this sand uni t ischaracter ised by Ah horiz ons .

Throughout the pa laeosols and modern soils pH va lues a rewithin a limited ran ge from sligh tly acidic to sligh t ly a lka line15.7-7.6 ) a nd, where measured , th e Fer content is of neg ligi ­ble pr oportions .

Jou rn al of Coasta l Research , Vol. 12, No. :J. 1996

Buried Palaeocatena in Beach-Ridge Sands 609

Soil Classification

Palaeosol 1, on the beach-ridge crest, has the characteris­tics required for designation as a humus podzol (AVEI{Y,1980). It has an albic E horizon of high colour value and lowchroma and an iron-deficient Bh horizon containing translo­cated organic matter. This palaeosol is identical to the humuspodzols described by WILSON and BATEMAN (1986) from sim­ilar locations at Magilligan. A thin albic E horizon is alsopresent in palaeosol 2 but is underlain by Bw and Bw(gJ ho­rizons rather than a podzolic B horizon. The soil may be anintergrade between a gleyic brown calcareous alluvial soiland a humus podzol but is best accommodated by the formersub-group because of the presence of weathered B horizonsand gleyic features, (for the purposes of soil classification ma­rine beach sand is regarded as a type of alluvium by AVERY(1980)): although calcareous material was not encountered inthe profile it is believed to exist at greater depth. Soil profilemorphology and chemistry were sufficiently similar in pa­laeosols 3 and 4 for them to be allocated to the same classi­fication sub-group. They are typical humic alluvial gley soilspossessing hurnose or peaty topsoils and gleyed sub-surfacehorizons. The bOf/m horizon in palaeosol 5 exceeds the thick­ness requirement of 0.4 m for classification as a peat soil. Atthe sub-group level it qualifies as a raw eu-fibrous peat soilconsisting predominantly of fibrous and semi-fibrous peatthat accumulated in a poorly-drained site (t.he inter-ridge de­pression I under fen vegetation.

The two palaeosols that overlie the palaeocatena in Profiles1 and 2 are designated as typical sand rankers and the mod­ern ground soils are typical sand pararendzinas. These areIithomorphic soils associated with young ground surfaces orrelatively short soil-forming intervals (AVEI{Y. 1980 I.

DISCUSSION

The four buried palaeosols constitute a logical sequencedownslope from the beach-ridge crest to the inter-ridge de­pression and comprise a palneocat.ona (sel/sll VALENTINE andDAU{YMI'LE, 1975 I. TIl(' vertical and horizontal variations insoil profile morphology and chemistry reflect different slopepositions and soil-water regimos. With the except.ion of pH(see bclow i, th« chemical analyses support the field data inthat the palaeosols have the expected cbnrnctet-ist.ics of thegenetic soil types developed in very wel l sorted fine sand.Moreover. the purt.icle size distributions show that the parentmaterials are texturally uniform both within and betweensites: therefore. different.iul sedimentation cannot account forthe visual and chemical contrasts hot ween horizons.

Although the existence of a pnlneocntr-na at Magilligan wasfirst reported by WII.SON and BATEMAN (1986) only the free­ly-drained end member (humus podzol : of the sequence wasdescribed in detail. Now. as a result of examining ali palaeo­sols comprising the palaeocatena some refinements can bemade to the Late Holocene evolution, stabilisation and pedo­genesis of the Mai,rilligan beach-ridge topography, originallyoutlined by WILSON and BATEMAN (1986) and WILSON andFAI{I{IN( i']'ON (1989l.

The major soil forming processes to have operated in thepalaeocatena are organic matter accumulation and incorpo-

ration, decalcification, gleying and podzolisation, Further­more, the first three of these are common to all palaeosols invarious degrees, while podzolisation is restricted to palaeosol1. WILSON and BATEMAN (1986) explained podzolisation onthe beach-ridge crests in terms of a longer soil-forming inter­val resulting from their elevated and freely-drained locationsrelative to the inter-ridge depressions. They believed that theridge crests would have been exposed first following relativesea-level fall and that vegetation colonisation and pedogen­esis would have started here while the depressions were stillsubject to either permanent or tidal marine incursions. Anequally likely scenario is that shoreline progradation led toisolation of the beach ridges, without any change in relativesea level. Whatever the cause of beach ridge stranding, thedepressions remained wet due to either a high ground-watertable or the establishment of shallow freshwater ponds; thisis suggested by the occurrence of thin Chara-marl beds, con­taining freshwater shells, both below and within some inter­ridge peats, and basal peat layers that are rich in macrofos­sils of mosses such as Drepanocladus aduncus. Calliergoncuspidatum, C. giganteum and Leptodictyum riparium, andmacro-and/or microfossils of Phragmites, Filipendula, Carex,Menyanthes and Typha, indicative of a brackish or freshwaternutrient-rich environment (McMILLAN, 1957; McMoRHIS,1979; BRADLEY, 1985; GODDARD, 1988). These species gaverise to fen and reedswamp peats. Later, conditions becameless wet and fen woodland developed with Alnus, Salix, Bet­ula. Corylus and Taxus all present. Drier conditions may havebeen a response to a fall in the water table during peat ac­cumulation.

As peat formed in the depression. pedogenic processes onthe flanks and crest of the ridge were dominated by organicmatter accumulation and its incorporation into the sand. de­calcification and podzolisation. These two latter processes in­dicate free-drainage, the development of relatively acidic soilconditions and the establishment of calcifuge vegetation com­munities. in contrast to the waterlogged and nutrient-richconditions of the depression where calcicole species flour­ished. However, the presence of decalcified sand below thepeat in palaeosol 5 is evidence for both acidic drainage atsome stage during peat development and a fall in the watertable to facilitate such drainage. This cannot have OCCUlTedafter the palaeocatena was buried by the calcareous aeoliansand as percolation of water through this sand is likely tohave raised pH to the present near-neutral values. Rather, itmay correlate with the phase of drier conditions identifiedfrom pollen and macro-fossil analyses of the peat (GODDARD,19881. A possible cause of water-table fall is a slight reductionin relative sea level. Such a fall and the consequent reductionin tide-controlled high water levels in Lough Foyle wouldhave probably lowered the water table in the adjacent raisedbeach ridges, permitting decalcification to penetrate previ­ously saturated sands. However, as yet there is no other ev­idence for minor changes of sea level, The sea-level curve forthe north coast of Northern Ireland is acknowledged by CAR­TEl{(1982) as being based on very few index points, especiallyfor the Late Holocene period, and is not sufficiently refinedto show other than the general trend.

Decalcification in palaeosol 1 (humus podzol J extends for

.Iournal or Coastal Research. Vol. 12. No. :3. 1996

at least 1.6 m into the beach ridge and is significantly deeperthan that reported by WILSON and BATEMAN 11986) for otherbeach-ridge humus podzols «D.9 m l. This anomaly cannotbe explained by differences in the altitudes of the podzols;the humus podzol of the palaeocatena is at 4.0 m O.D. whilethose examined by WILSON and BATEMAN (1986) rangedfrom 3.75 m to 4.23 m O.D. A lower initial CaCO, content inthe palaeocatena is also unlikely to account for the greaterdecalcification depth; analyses by WILSON and BATEMAN(19861 and WILSON (unpublished data) demonstrate a near­uniform CaCO, content of c. 10'/r throughout the beach-ridgesystem. In addition, there is no evidence of a significantlylonger soil-forming interval for the humus podzol (see below I.

This depth of decalcification most probably exceeds that dis­cussed by WILSON and BATEMAN (1987) for a buried aeolian­dune palaeocatena from elsewhere along the Lough Foyleshore of Magilligan; here the decalcification front was locatedD.6 m below dune-slack peat and a maximum of 2.1 m belowthe slopes of the palaeodune. In palaeosol 5 of the palaeoca­tena, decalcification has penetrated for at least D.85 m belowthe peat; horizontal projection of this decalcification level topalaeosol 1 suggests that at least 2.5 m of decalcified sandoccurs below the crest of the beach-ridge palaeocatena. Thepatterns and depths of decalcification in both beach-ridge andaeolian-dune sands at Magilligan are unusual and demandfurther investigation.

Periods of temporary saturation due to water-table fluctu­ations are indicated by the vertically-aligned ferruginous con­centrations outlining root channels in the lower horizons ofall palaeosols. These channels, and, therefore, the ferrugi­nous deposits, must have formed after decalcification as theinternal reorganisation of sand grains following removal ofCaCO" would have destroyed them (ct: KEMP, 1985 I. The fer­ruginous concentration features are considered to result fromthe establishment of anaerobic conditions following waterlog­ging and the reduction of ferric compounds to their more sol­uble and mobile ferrous forms. As the water table fell thelarger soil voids (e.g. root channels) would have drained firstcausing the soil solution to diffuse towards these aeratedzones where the ferrous ions were oxidised and precipitatedas ferric oxide. Root channels have been stabilized and pre­served by this process. The water-table fluctuations respon­sible for initiating these features may be associated with therecovery of sea level from slightly below present O.D. to itscurrent level and the consequent increase in tide-controlledhigh water levels (presently 1-2 m O.D. during spring tides)in Lough Foyle (CARTER, 1982).

As no 14C dates are available for the organic-rich horizonof the palaeocatena the time-span over which it developed isnot known with certainty. '"C dates from the base and top ofother inter-ridge peats and from within peats, given by WIL­SON and BATEMAN (1986), MALLORY and MCCORMICK (1988)and WILSON and FARRINGTON (1989 I, reveal that peatgrowth was initiated at different times between c. 2,500 yearsand c. 1,000 years B.P. and that growth was terminated atdifferent times between c. 1,100 years and c. 600 years B.P.The maximum age for an individual peat bed is 1,400 years.Although the peat in palaeosol 5 is slightly thicker 10.82 m i

than the maximum thickness previously reported (().79 m:

there is no reason to suspect a significantly longer peat-form­ing interval at this site than elsewhere. This also is supportedby the morphologicul and chemical properties (except for de­calcification depth 1 of thp humus podzol which an' not anymore strongly developed than in podzols at other sites. Thus,the palacocat.onu is considerod to have acquired its charac­teristics over a time-span similar to that for other peat-podzolsequences.

CONCLUSIONS

Detailed investigations of all gerwtic soil types present inthe beach-ridgo palaeocatena have provided a clearer under­standing of the relationship between the factors and process­es of soil formation at Magilligan prior to burial of this land­scape by aeolian sands. The morphological and chemicalcharacteristics of the palaeosols reflect their different originallandscape positions and soil-water regimr-s. More elevatedand freely-drained location" display relatively thin organic­rich horizons, deep decalcification and podzolisation. Low-ly­ing sites are characterised by thicker organic accumulations,indicative of a high water table and/or the existence of a shal­low pond. Water-table tluctuations (perhaps related to oscil­lations of relative sea level I allowed the decalcification ofsand below low-lying peat soils and also caused ferruginousconcentrations to develop in the lower horizons of all palaeo­so],;,

The morphological and chemical properties recorded in thepalaeocatena demonstrate that pedogenesis on low-amplituderaised beach ridges is capable of producing marked small­scale spatial variations in soil charucteristics that are suffi­cient to create distinctive (genetic) soil types. Beach-ridge soilsequences arc a neglected component of both coastal and pe­dological research; the results reported above suggest thatsimilar studies on buried and unburied beach ridges else­where may yield valuable' inforrnat.ion concerning several as­pects of coastal landform evolution.

ACKNOWLEDGEMENTS

This paper is dedicated to the memory of the late Bill Car­ter whose encouragement over many years for my work oncoastal dunes and soils, and his friendship, are very muchappreciated. Magilligan was the location of Bill's doctoralwork (1968-72) and one of the locations on which he pub­lished many papers.

In connection with this paper my thanks are due to PeterDevlin and Robert Stewart for assistance with field samplingand surveying, Dehhie Rainey for assistance with laboratoryanalyses, and Kilian McDaid and Mark Millar for preparingthe diagrams for publication.

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