Embed Size (px)
Citation preview
This article was downloaded by: [New York University]On: 17 December 2014, At: 20:49Publisher: RoutledgeInforma Ltd Registered in England and Wales Registered Number:1072954 Registered office: Mortimer House, 37-41 Mortimer Street,London W1T 3JH, UK
South AfricanGeographical JournalPublication details, including instructionsfor authors and subscription information:http://www.tandfonline.com/loi/rsag20
On Geomorphic Units,and GeomorphicProvinces in South AfricaM. S. TaljaardPublished online: 13 Nov 2012.
To cite this article: M. S. Taljaard (1945) On Geomorphic Units, andGeomorphic Provinces in South Africa, South African Geographical Journal,27:1, 28-31, DOI: 10.1080/03736245.1945.10559239
To link to this article: http://dx.doi.org/10.1080/03736245.1945.10559239
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy ofall the information (the “Content”) contained in the publicationson our platform. However, Taylor & Francis, our agents, and ourlicensors make no representations or warranties whatsoever asto the accuracy, completeness, or suitability for any purpose ofthe Content. Any opinions and views expressed in this publicationare the opinions and views of the authors, and are not the viewsof or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verifiedwith primary sources of information. Taylor and Francis shall not beliable for any losses, actions, claims, proceedings, demands, costs,expenses, damages, and other liabilities whatsoever or howsoevercaused arising directly or indirectly in connection with, in relation toor arising out of the use of the Content.
This article may be used for research, teaching, and privatestudy purposes. Any substantial or systematic reproduction,redistribution, reselling, loan, sub-licensing, systematic supply,or distribution in any form to anyone is expressly forbidden.
Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions
Dow
nloa
ded
by [
New
Yor
k U
nive
rsity
] at
20:
49 1
7 D
ecem
ber
2014
28 SOUTII AFRICAS GEOGR.\I'IIICAL JOURS.\J.
ON GEO~10RPHIC UNITS, AND GEO~lORPHIC PROVINCES IN SOUTH AFRICA
M. S. TALJAARD
In the following discussion I present some definitions and views expressed in two recent textbooks on gcomorphology and attempt to apply such definitions and relevant descriptions to what has been called "Geomorphological Provinces" in South Africa. Being fully aware of the nature of evidence and the many forms in which it can be presented, I have thought it most fitting to make use of citations pertaining to definitions. . .
"By definition each such geomorphic unit will have uniforml~y of structure within its confines. The changes wrought by process WIll
be the same, or follow the same pattern, within the limits of any given unit. A geomorphic unit in its existin~ aspect . . . need not b~ encountered in its original structural state" (p. 58, O. D. von Engeln -"Geomorphology"). The latter part of the definition provides' an elasticity to the classification, which may in some cases overreach the bounds of definition. To put it more bluntly, an avenue of escape has been instituted, because as yet, no truly genetic basis has been established in the classification of land-forms and land-form groups. Such a basis has, however, been sought for. and is clearly essential, if geomorphologists wish to avoid the fate of petrologists. As has been demonstrated repeatedly a combination of precepts provides a working basis, whereas a school of thinking creates a bias.
Davis and the American school look to structure (very broadly defined and nearly all-inclusive) as forming the principal basis of classification, whereas A. Pcnck and others of the Continental school overstress the function of processes of development in "land-form"and "unit" -classification. \V. Penck goes a long step further and overstresses the .dynamic or diastrophic approach. "That Davis gave only cursory attention to process must be admitted" ... (op. cit. p. 70).
"Partition of Southern Africa into Geomorphological Provinces rests in the first instance upon geologic structure, for the nature of the bedrock undoubtedly stamps its mark upon the topography in su~h a way that no combination of other circumstances could produce qUIte the same effect" (p. 260, King). Before trying to fit what we have to any particular form of classification let us attempt an understanding of some words which repeatedly crop up in the foregoing statements, and in all texts of gllomorphology, viz .. "structure" and "process.' ,
Structure.
(a) Attitude of rocks underlying a particular area (horizontal or disordered).
(b) Constitution of the rock material (massive or thin-bedded). (c) Close- or wide-spaced jointed nature of the rocks. (d) Competent or incompetent. (e) Every inherent quality of the rock substance that may influence
the course of degradational dissection, e.g., lithology juxtaposition.
(f) Altitude of the rock exposure as a function of (a). (Fcnneman,)
Dow
nloa
ded
by [
New
Yor
k U
nive
rsity
] at
20:
49 1
7 D
ecem
ber
2014
ON GEO~IORI'HJC e:-n" .\ND GEO~IORPIlIt l'ROVINCES IN S.A. 29
On this broad definition two major structural classes are then recognised. (van Engeln.)
(A)-HoRIZONTAL, (B)-DISORDERED STRUCTURES
CLASS A: " perfect simplicity of attitude and arrangement of component parts; they are beds laid up horizontally to form a pile."
Group A: (weakly consolidated or unconsolidated bt.'<is). (1) Coa"tal Plains, (2) Piedmont Plains, (3) Tundra Plains, (4) Fluvial, Lacustrine and Deltaic Plains, (5) Erg, (6) Glacial Plains, (7) Loessial Plains.
Group B: (more or less firmly consolidated rocks of Sedimentary or igneous origin). The materials iike Group A have some simple uniform arrangement.
(8) Interior Plateaus, (9) Nested Saucer Basins, (10) Lava Flow Plains and Plateaus, (11) Volcanic Cones.
Group C: (geomorphic units made up wholly of calcareous rock; simple structures because of the homogeneity of the material).
(12) Karst Units, (13) Coral Islands.
CLASS. B: Disordered' Structttres .. Group D: Folded and Faulted units, made up of consolidated rocks
composed of sedimentaries or including or involving scdimcntaries. (14) Dome Uplifts-Ca) laccolithic, (b) deep-seated. (15) Fold Mountains-(a) Simple Folds, (b) Overfolded,
Nappes, thrusts. (16) Fault Block MOllntains ("hence are fdrms of process")
in a structural class (p. 64, von Engeln).
Group E: Shields or ancient rigid masses. TopographicaUy the Group E units have a plain, plateau or dome expression. "In general they are forms of process" (p. 65, von Engeln).
(17) Ancient Igneous and Metamorphic Shields. (18) Peneplaned Sediments and Metamorphics (forms of pro
cess) truncated by an erosion surface. (19) Continmtal Glaciers.
This is chiefly the structural basis for the institution of Geomorphic Units as put forward in the recent text by O. D. von Engeln.
L. C. King, also in a recent t<:xt, stresses the role of geologic structure, and as a control factor of major importance, climate is mentioned: "for this controls the type of erosion cycle which is in operation-humid, arid or glacial" . . . This author then passes on to a discussion of various geomorpnic provinces, and lists 26 such provinces on a sketch map (p. 266, King).
Process. The second term "process" leaves us with a still wider field
of definiti~n. "Climat.e" ?bviously includes the exogenetic processes of weathenng and erOSlon 111 all the known forms, but obviously does not cover the field of endogenetic forces. "Knowing the nature of
Dow
nloa
ded
by [
New
Yor
k U
nive
rsity
] at
20:
49 1
7 D
ecem
ber
2014
30 SOUTH AFIUCAN GEOGllAl'llICAL JOtJRNAL
the bedrock (structure?) and the type of erosion cycle operating, we only need to learn the stage within that cycle to which the landscape has attained, to form a very good mental picture of the prospect" (p. 261, King). •
Application: Taking a listed example, and subjecting it to tht? form-fitting scheme outlined by the cited authors \\ill serve to show how we stand.
The Highveld (1) is the first geomorphic province of King. It ~s constituted of horizontal, nearly horizontal, folded and faul.ted sedimentaries, metamorphics, and intrusive as well as volcanIC rocks, ranging in age, composition and juxtaposition, within very wide limits. It includes regions varying in rainfall from 45 inches {+ snow) at Harrismith to 15 inches at Christiana in the Western Transvaal. Apart fro~ any other climatic aspects, this variation i~ prccipitatio~ wo~ld on Its own produce markedly distinct effects, and mfluence the mteoslty and nature of exogenic processes. Within the confines of the "Highveld," there are exposed by recent denudation processes, stripped surfa~es o~ great geologic age, with present-day form, attributable to agenCIes dating to the Carboniferous. Superimposed on this we have th"c legacy of Jurassic, Cretaceous and Tertiary erosion cycles. Wc have also the undoubted topographic effects produced by linear warping along the Transvaal.Griqualand axis.
The adjacent province (2) called the "Upper K~rroo" presents a dissimilanty of "structure" to an almost equal degree, including surface and form composed of flat· lying Karroo System rocks, ancient folded metamorphics of the Primitive System, and wide expanses of granite-gneiss. Climatic extremes are again encountered from Aliwal North in the east to Kenhardt in the west. Regions 21 and 22. viz., the Great Karroo and the Southern Folded Belt, lie adjacent; composed of rocks of the Pre-Cape-, Cape-, Karroo- and Cretaceous Systems, with patches of younger less consolidated Tertiary deposits. Dissimilar structure is widespread, climate varies extremely. and
I erosion surfaces of Jurassic, Cretaceous and Tertiary age are present within the confines of either region. The regions listed as 24, 8, 9, 4a and 4b, 18," 7a and 7b present similar discrepancies of "structure," "process" and "stage" and the question goes begging, on what basis such delineation into "provinces" has been made. The only region of relatively uniform "structure" and "climate" is 3, namely. the "Basutoland Highland," and even within the confines of that region marked differences in lithologic reaction to exogenic processes are noticeable. The author is not blind to Brazil for all the nuts which come from there, nor is he walking into trees on a lion hunt, but he is seriously in doubt as to the validity of those gcomorphic boundaries which have been proposed for this southern portion of Africa, and not improbably for other parts of that Continent.
He is asking himself whether, in the present state of our knowledge, we are in a position to postulate gcomorphicprovinces, as yet, and if it would not be advisable to begin by mapping the occurrence of groups of landforms of similar genealogy? Knowing thc history of forms of the second order would enablc. one to group the types and at a later stage to delineate more extensive "regions."
In this' connection, I would suggest that the working basis in tracing the genealogy of forms, or groups of forms be, firstly, the
Dow
nloa
ded
by [
New
Yor
k U
nive
rsity
] at
20:
49 1
7 D
ecem
ber
2014
ON GEOMOIU'IllC U:-;If" AND l;EOMORPlIlc l'ROVINCES IN S.A. 31
dominant process which gave birth and shape to the second and third order forms in our landscape. To distinguish further, let the structural attitude and composition of the component surface rocks be a guide to more specific sub-division. As an illustration I will take the region flanking our fold mountain chains bordering the south coastal belt. From the Langeberg-Outeniqua watershed there extends southwards to the present-day coast, an undulating surface, partly the product of stream planation, partly of marine depositional processes. The boundary between these regions of fluvial planation and marine construction is sharp; the age of birth is coincidental, being early to Middle Tertiary. Wc are dealing therefore with a composite geomorphic region, composed (a) landward, of truncated, folded Cape-sedimentaries and infolded Creataceous sedimentaries, (b) seaward, of partly consolidated Tertiary sedimentaries. The processes were co-active, and even though either product was subsequently affected by more recent diastrophic agencies such as linear warping and marginal coastal faulting of the normal type, those effects were merely third order detail.
Shall we call this form a composite coastal plain and, if this composite feature is traceable in part, or as a whole, right around the periphery of the Union, shall we institute a geomorphic unit or province apart from present-day climatic differences, which tend merely to influence the shape of forms of the third order?
This instance is but a random sample taken froIp a mOte comprehensive scheme. Quite logically our first order forms are the continental surfaces and ocean basins. Within the confines of these there are larger detail forms of varying structure and mode of origin. Firstly, wc recognise within the continental and continental-island confines those forms resulting from constructional agencies; weathering, river-action, glacial action, \vind-action, marine-action, lacustrine, terraquatic, volcanic and diastrophic action. If it is self-evident that tlle place of occurrence, form, extent and internal .structure of a plain, plateau, dome-mountain, fold-mountain, cone or esker, to mention only a few, is due primarily to a single or composite constructive action then the specific land-form and its environs is listed as a second order form.
If, on the other hand, such attributes as those mentioned, are attributable to the denudational effects of weathering, fluvial action, and the other processes, excepting volcanic and diastrophic activity, then the land-torm and its environs logically become third order types. Quite logically this latter group includes forms either in a state of reduction or as relics, enabling us to delineate in greater detail. On this basis the Great Karroo, from LoeriesfonteiI1 to the Kamdeboo watershed, bounded on the west and south by Fold ranges, presents a genealogy similar to that of the Little Karroo, even though the latter is fenced in among Fold ranges. As far as is traceable, they began developing their present-day surface fOflQ as far back as the Middle Cretaceous, through the agency of stream work aided by weathering .
. They both remained unaffected by warping, folding or faulting of later date than the Mid-Cretaceous and are therefore genealogically closely related. _ . 1 feel sure t~at ~etailcd tra.cing ?f genealogy will bring the picture mto focus, and if thIS short dISCUSSIon lets loose a flood ·of criticism the ball will start rolling in the right direction.
Dow
nloa
ded
by [
New
Yor
k U
nive
rsity
] at
20:
49 1
7 D
ecem
ber
2014
