QUATERNARY RESEARCH 10, 310-339 (1978)

Late Cenozoic Paleoclimates of the Gaap Escarpment, Kalahari Margin,

K. W. BUTZER, R. STUCKENRATH, A. J. BRUZEWICZ, AND D. M. HELGREN Departments ofAnthropology and Geography, University of Chicago, Chicago, Illinois 60637; Radiation Biology Laboratory, Smithsonian Institution, Rockville, Maryland 20852; University of Wisconsin Center, Manitowoc, Wisconsin 54220; and Department of Geography, University of California, Davis, California 95616 Received July 28, 1977

The Gaap Escarpment is a dolomite cuesta demarcating the southeast margin of the Kalahari. Since Miocene-Pliocene times, thick masses of lime tufa have repeatedly accumulated at several points along this escarpment, and four regional sequences are described. These allow discrimina- tion of six major depositional complexes, commonly characterized by basal cryoclastic breccias or coarse conglomerates that reflect frost shattering and torrential runoff, followed by sheets and lobes of tufa generated in an environment substantially wetter than today. A chronostratigraphy for the last 30,000 yr is provided by 14Cdating, with direct or indirect correlations to the sequence. The regional stratigraphy as well as faunal dating indicate an early Pleistocene age for Australopithecus africanus at Taung. Repeated episodes of protracted cold or wetter climate or both begin in terminal Miocene times, and the last Pleistocene cold-moist interval began after 35,000 yr B.P. and ended 14,000 yr B.P. Early and late Holocene times were mainly wetter, whereas the middle Holocene was drier than today. The paleoclimatic sequence differs from that of the southern and southwestern Cape or that of East Africa, but close parallels are evident throughout the lower Vaal Basin and the southern Kalahari. The tufa cycles provide a unique, 5,000,000-yr record of climatic variation in the Kalahari summer-rainfall belt that can be related to complex anomalies of the general atmospheric circulation.

INTRODUCTION been improved sophistication in dealing The last two decades have seen a revitali- with intraregional variability, e.g., the paly- zation of Quatemary studies in critical parts nological inferences of Maley (1977) in the of Africa, particularly in the Sahara and East Chad Basin. At the same time there has Africa. Such regional studies have been also been a disturbing tendency to attempt made possible by radiocarbon or potas- to fit pan-African data into a simple and sium-argon dating. The information gen- rigid formula and to question paleoclimatic erated has not supported traditional climato- anomalies from areas such as the Nile Basin stratigraphic paradigms. Although inter- or southern Africa that do not appear to regional patterns for the Pliocene and fit the new paradigm comfortably (e.g., earlier Pleistocene remain uncertain, a fresh Street and Grove, 1976). synoptic picture has emerged for the radio- The increasing body of information from metric range of 14C (Butzer, 1971, Chap. 20; southern Africa deserves consideration in Butzer et al., 1972). The last few years have its own right. It is sufficiently distinctive seen further radiocarbon-based surveys, to suggest renewed caution both in pan- providing ever stronger evidence that ter- African correlation and in substituting a minal Pleistocene and Holocene lake- new but equally sterile climatostratigraphic volume changes in the southern Sahara and scheme. A basic, emerging fact from East Africa varied in phase and that ex- modem climatological research is that the panded tropical lakes were not coincident African continent is not only organized in with high-latitude glacial stades. There has distinct latitudinal zones, but that within

0033-5894/78/0103-0310$02.00/0 310 Copyright Q 1978 by the University of Washington. AU rights of reproduction in any form reserved. PALEOCLIMATES OF THE GAAP ESCARPMENT 311 each belt there are major provinces that de- ward at 0.2 to 0.8” to cut across dolomitic rive their moisture from different sources. limestones and interbedded shales of the Precipitation in each zone is triggered by Transvaal System (Precambrian) that dip different interactions between multiscale NW at 2 to 15” (Stow, 1874; Du Toit, 1907. components of the atmosphere-ocean- 1908; Rogers, 1907; Toens, 1966; Visser and continent interface. Grobler. 1972: Helgren, 1978b, Chap. 2). One traditional area of research in Below the escarpment another planar sur- southern Africa has been the Vaal River face, in part stepped, dips 0.5” toward the valley (Helgren, 1978b, Chapter 1). Less Vaal River; here inclined Transvaal quartz- well known, despite the discovery of the ites and slates underlie a mantle of hori- first australopithecine here in 1924 (Young, zontal Karoo System rocks or Pleistocene 1926), is the great dolomite escarpment that calcretes. runs past Taung (Figs. 1 and 2). Situated on Drainage lines and watersheds are poorly the boundary between the Kalahari and the developed on top of the Gaap Plateau, interior plateau of South Africa, this escarp- crossed by strings of shallow (5- 10 m), low- ment, known as the Gaap (Ghaap, in gradient swales resembling coalescent, ) or Campbell Rand (Afrikaans for karstic sink holes or pockmarked by closed Campbell Escarpment), is of considerable depressions with ephemeral lakes (typical geomorphic interest. Peabody (1954) first relief, 3-5 m, and major axis, 100-500 m), demonstrated the existence of a long complex probably due to solution and periodic defla- record of Pleistocene events at Buxton (now tion (Butzer, 1974~). Below the escarpment, Norlim), near Taung. He identified four gen- drainage is better defined but essentially erations of freshwater limestone or tufa that restricted to subparallel master rills, carved argued for changing hydrologic conditions. 1 to 2 m into calcrete duricrusts. Visits to several sectors (by K. W. B. and The escarpment is a poorly articulated, D. M. H.) in 1970 and 1971 suggested that the tectono-erosional feature related to a com- Gaap Escarpment warranted an expanded plex of prominent lineaments, partly inter- study. This was continued by one or other of secting and mainly striking NNE; these in- the authors each year until 1977, as part of a clude minor faults, with throws rarely more continuing geomorphic and geoarcheologi- than a few meters, as well as master joints cal project in the Kimberley region. The and dikes of Precambrian andesite and mid- Gasp phase of this investigation led to Mesozoic diabase (Helgren, 1978b). There reinterpretation of the context of the is no single, major fault along the escarp- Australopithecus africanus holotype (But- ment face, which was generated by head- zer, 1974a, 1977) as well as to the es- ward erosion of coalescent pediment sur- tablishment of a regional paleoclimatic faces cutting back from the Vaal axis, prior sequence that is the topic of this paper. to burial under late Paleozoic (Dwyka) til- lites and shales still found on the scarp Piedmont or lapping over the rim. Maximum ENVIRONMENTAL CONTEXT OF THE pre-Dwyka relief at the southern end of the GAAP ESCARPMENT escarpment exceeds 400 m, suggesting pla- The Gaap Escarpment is a prominent nation on an impressive scale during the cuesta that runs NNE-SSW between 27”07’ late Precambrian or early Paleozoic (Hel- and 29”lO’S Lat., facing east and south gren, 1978b). There was further beveling toward the valleys of the Harts, Vaal, and when the Vaal-Harts valley was exhumed Orange rivers (Fig. 2). Relief averages 70 to during the Cenozoic; this erosion included 120 m and elevation, 1150 to 1250 m, along (a) removal of most of the Karoo infilling this 275km front. Above the rim a planar along the Vaal axis, (b) fresh scarp reces- surface, the Gasp Plateau, rises gently west- sion on a small scale, (c) gorge entrench- 312 BUTZER ET AL.

..- I.-. .-. .- r//l ARID _..‘“i / ...-... tzza SEMIARID ‘.._,._ a..j Mujo@ : & GAAP ESCARPMENT ‘1 \ l -***** BOUNDARIES OF “) RAINFALL PROVINCE! 1 Summer Rains 2 Summer & Winter Rain!

l Makwn

15. r FIG. 1. Climatic zonation in southwestern Africa. In the overlap zone of summer and winter rains, 45 to 75% of the rainy days occur during the summer half-year, April through September (based on Climate of South Africa, 1965b). PALEOCLIMATES OF THE GAAP ESCARPMENT 313

Gasp Escarpment W 1300m

OYKES 12oom

l Kl~plonwn

0 Doornlaaate

THE GAAP ESCARPMENT sfontein AND PLATEAU

I\y DOLOMITE SERIES

FIG. 2. The Gaap Escarpment and Plateau. Based in part on Landsat imagery 314 BUTZER ET AL. ment, and (d) the cutting of shallow, semi- creases from 300 mm in the south to 450 circular scallops (up to 5 km wide and 40 m mm or more in the northern part of the deep) just beyond the escarpment rim (Hel- escarpment. Winters are very dry (less than gren, 1978b). 5% of the rainfall during June, July, and At many points along the escarpment August), with a March maximum. The there are large or small accumulations of climatic type is semiarid mesothermal, with tufas (Butzer, 1974a): sheets or aprons, fan- no water surplus at any season (Schulze, like lobes, and carapaces or waterfalls. Most 1958). Away from the Vaal and Harts, sur- of these tufas at the foot of the scat-p or face water is rare except during unusually within gorges cut back into it are related wet years, such as 1974 through 1976, when to surface drainage lines; those that lack precipitation was 200 to 300% normal and substantial catchments are linked to the in- water persisted on swales of the plateau tersection of faults, master joints, or dikes throughout the dry season. Reflecting both with the scar-p face. Hydrogeological condi- the general water deficiency during most tions clearly are important in maintaining years and the shallow, weakly structured, minor karst springs or stream resurgences loamy soil mantle, the plateau vegetation today, but groundwater basins are relatively is a monotonous, deciduous bush savanna, small because the strata dip WNW and are with acacias prominent on the Piedmont; fragmented by dike networks. Nonetheless, grasses are tall but provide ground cover rainfalls 1.5 km and more from the escarp- of limited effectiveness against rainsplash ment can generate spring flow a few days and sheetwash. later, in part as water seeps through alluvial veneers, prior to entering karst fissure and LOCAL STRATIGRAPHIC EVIDENCE cavern systems. Records of more than 50 low-order cli- A particularly complex and informative matic stations (Climate of South Africa, 1954) tufa sequence is exposed at Gorrokop show that mean annual precipitation in- (28”22’3O”S, 24”lO’E Lat.; farm NW 61

FIG. 3. The Gorrokop tufas and the Gaap Escarpment, looking southwest. (Photo by K.W.B., August 1971). PALEOCLIMATES OF THE GAAP ESCARPMENT 315

; a Umts 6-7 HOliCENE ! w U”,{ 5 : (0 Erecclasj LATER : m Unit 4 PLEISTOCE#E : (+ Isolated Outcrops) I : I m un,ts 1.3 EARLIER PLEISTOCENE

GAAP PLATEAU (doiomtte)

FIG. 4. Morphostratigraphic map of Gorrokop. See also Fig. 5. 316 BUTZER ET AL.

TABLE 1

MORPHOSTRATIGRAPHIC SEQUENCE AT GORROKOP

Gr-7 Soft porous tufa(O.5-5 m thick) in isolated outcrops or thin sheets on upper valley floors (1375,1205 yr B.P.) Gr-6 Porous tufa (l-5 m thick) in local rim carapaces; two generations (Gr-6A: 9550 yr B.P.; Gr-6B: 3155, 2520 yr B.P.); undifferentiated Gr-6 and Gr-5 sheets veneer Gr-3 and Gr-1 fans to east (Dissection) Gr-5 Porous tufa (5-12 m thick) forming rim carapaces or sheets, mantling Gr-3 (south sector) or dolomite (north sector); two generations (Gr-5A: 24,400 yr B.P.; Gr-SB: 19,950 yr B.P.) separated by weathering horizon; includes isolated breccias with porous, calcareous matrix, e.g., well-stratified angular tufa- rock rubble (25-40” dips) prograded over subhorizontal breccia (2-3” dips) and later cover by Gr-6 tufa (Stream downcutting, creating modem gorges) Gr-4 Steep-bedded, compact tufa carapaces representing two generations of waterfalls (Gr-4A, 4B) formerly filling gorges (36,800 yr B.P. minimum date) (Incision of deep, steep-sided concave valleys similar to modem drainage pattern) Gr-3 Compact tufa (30-40 m thick), including basal, crossbedded channel fills with local breccias or con- glomerates (20-45” dips), followed by progradation of high-relief carapaces (15-70” dips) (NE and S from head of modem Kloof C) or small lobes (Kloof A, south of G), with subhorizontal terminal beds (2-10” dips); related fans mantle Piedmont and interdigitate with Younger Gravels “A” at Vaal- Harts confluence (Erosion of concavoconvex valleys with 20” midslopes, perpendicular to escarpment) Gr-2 Compact tufa (8-18 m thick) with breccia or conglomerate lenses, forming large apron projecting SE (from head of Kloof B) (2-6” dips, but lo- 15” in central part) (Erosion of undulating swales) Gr-1 Well-stratified detritus and tufas resting on shales (? Dwyka) with relief of more than 40 m; forming fans extending E-SE (from Kloofs A and F) toward Vaal River; three subdivisions: Gr-1A (8 to 20-m-thick crude breccia); Gr-1B (30 to 35-m-thick tufa with thin breccia interbeds); and Gr-1C (15 to 18-m-thick breccia or conglomerate with tufa interbeds)

(1r4C dates are given in parentheses, with full data in Table 5.

“Rietfontein,” plat 234 Ba. W.Q.3.3., in- Company. At least seven units can presently correctly labeled Malony’s Kloof on the 1: be identified on geomorphic and lithologic 50,000 topographic sheet). Tufas are devel- criteria (Table 1, Figs. 4 and 5). oped along a frontage of 1.8 km in the form Further details are provided by the large of carapaces overlapping older dissected quarries of the Anglo-Alpha Cement Com- fans (Figs. 3 and 4). A resurgence of vari- pany at (29”19’3O”S, 24”12’E Lat.) and able but perennial flow is probably fed by a the short, deep gorge at Grootkloof, half- large part of the poorly defined 50-km2 way between Ulco and Gorrokop. Both sites catchment behind the escarpment; runoff are related to a major drainage line, the that does spill over the rim diverges into Steenkop River, with a catchment of 1000 seven major gorges, labeled Kloofs’ A to G, km2 that taps part of the Kuruman Hills (Fig. from north to south (Fig. 4). 2). The sporadic Steenkop cuts through a The tufa stratigraphy at Gorrokop has diabase dike 6 km behind the escarpment been resolved on the basis of field and air- and originally flowed due east to spill over photo mapping by K. W. B., 1973 and the rim at Ulco, before being captured by 1974, and A. J. B., 1974 through 1975, and the Grootkloof drainage. through interpretation of 54 deep cores The Ulco quarry sections were recorded (average length 29.5 m) logged in 1952 and by A. J. B. in 1975; completion of labora- 1953 by the Anglo-Transvaal Consolidated tory study should provide abundant data on Company for the Anglo-Alpha Cement facies and pedogenic trends. There are three major tufa bodies; geomorphic development ’ Afrikaans singular, kloof = cleft or ravine. and lithology suggest correlation with Gr-1 PALEOCLIMATES OF THE GAAP ESCARPMENT 317

MAZELSFONTEIN - comuosite cross-section -

GROOTKLOOF - composite section -

-nm Dolomite -* -wma~cm?-~wu Bedrock floor of gorge

GORROKOP E - generalized cross-section -

GORROKOP - idealized longitudinal section -

FIG. 5. Generalized morphostratigraphic sections at Gorrokop, Grootkloof. and Mazelsfontein. No horizontal tale. At Grootkloof unit (1) is developed on the escarpment face, units (2) to (5) within the gorge. 318 BUTZER ET AL.

TABLE 2

MORPHOSTRATIGRAPHIC SEQUENCE AT GROOTKLOOF

Gk-6 Minor cliff-face cascades (9310 yr B.P.); related carbonates cement breccia Gk-5 (now dissected) Gk-5 Gorge-bottom breccia (2 m thick) of dolomite and shale grit or rubble, due to headwall recession (Dissection of breccia, some bedrock erosion in plunge pool) Gk-4 Waterfall tufa plunging 50 m (40-70” dips); toeslope rests on, and related carbonates cement breccia Gk-3 (20,825 yr B.P.) Gk-3 Gorge-bottom breccia (2-3 m thick), with dolomite blocks and tufa debris, due to accelerated mechanical weathering and headwall recession Gk-2 Waterfall tufa plunging 35 m (40-70” dips) (30,760 and 26,130 yr B.P. minimum dates) (Deep gorge-cutting after pirating of Steenkop River) Gk-1 Massive tufa carapace plunging 75 m (45-65” dips), choking short ravine; with basal breccia

to Gr-3. The major Ulco tufas (Ul-1 , 2, 3)2 limited intervening erosion, at increasing are separated by thick horizons of reddish distance from the escarpment, each unit at sandy wash that rest on corroded surfaces; slightly lower levels. Coarse detritus is they are capped by a veneer of younger visible in some basal sections of the oldest tufa (Ul-4) similar to Gr-4. unit; absence of detritus and prominence of The morphostratigraphic sequence at bulbous bedding in the younger units sug- Grootkloof has already been published with gest low-energy deposition and multiple sedimentological data (Butzer, 1974a) and is springs. summarized in Table 2 (Fig. 5). It is note- The morphostratigraphic succession, worthy that the breccia units are not con- worked out by K. W. B. in 1973 and 1974 temporary with the tufas and are genetically and A. J. B. in 1975, is tentatively sum- incompatible with them. Gk-1 is lithologi- marized in Table 3 (Figs. 5 and 6). Further tally identical to Gr-3. resolution is expected when analyses of The tufa complex at Mazelsfontein (29” 1’ S , some 100 sediment samples are completed. 23”34’E Lat.; plat “Mazelsfontein 75”), first Nonetheless, broad lithological and geo- noted by Stow (1874)) has a minimal surface morphological similarities are apparent catchment of 4 km2 on the Gasp Plateau. with the tufas of the middle and northern However, the short but well-incised (30 m) sectors of the escarpment, e.g., develop- stream valleys that converge here include ment of Mz-1 compares well with that of entrenched meanders and coincide with a Gr-3, Ul-3, and Gk-1 . The 14C patterns prominent system of intersecting fractures are also comparable. (Fig. 8). One such fracture is intruded by The tufa deposits of Buxton-Norlim an andesite dike further west, suggesting a (27”37’S, 24”38’E Lat.), related to the Taung hydrogeological context favorable for sus- australopithecine site, are exposed in the tained spring activity. Unlike Gorrokop, quarries of the Northern Lime Company. successive tufa carapaces developed with First described by Wybergh (1920) and Young (1926), a semidetailed stratigraphy 2 Marker (1974), who did a reconnaissance here, was provided by Peabody (1954), with am- gives 10 unpublished Pretoria l*C dates: >46,700; plifications and sedimentological data by >44,700; 43,760; and >40,700 yr B.P. from Ul-1. ~44,700 Butzer (1974a). The plateau rim was deeply and 36,650 yr B.P. from Ul-2, and >46,500; >44,500; incised by a stream with a drainage basin >42,800; and 35,350 yr B.P. from Ul-3. Our regional of about 400 km2, prior to accumulation of information indicates that all these deposits should be of infinite age; the apparently finite assays provide a the first sedimentary units identified. The measure of potential contamination of inorganic car- setting was reconstructed by Peabody bonates by younger CO,. (1954, Pl. 5), and the stratigraphic sequence, PALEOCLIMATES OF THE GAAP ESCARPMENT 319

TABLE 3

MORPHOSTRATIGRAPHIC SEQUENCE AT MAZELSFONTEIN

Mz-3 Two or more generations of minor tufa carapaces. cascades and valley-floor sheets (Mz-3A: 7715 yr B.P.: Mz-3B: 1070 and 515 yr BP.) Mz-2 Two or more tiered carapaces (Mz-2A: 26,840, 25,900, and 22,200 yr B.P.; Mz-2B: 15,150 yr B.P.); equivalent deposits (2 m thick) mantle Mz-IC on the Piedmont Mz-1 Three or more massive tufas (at least 80 m thick); Mz-IA forms fan with basal breccia and detrital interbeds (38,600 yr B.P. minimum date); Mz-1B is poorly exposed; Mz-lC, less recrystallized, con- tinues as 6 to g-km-long tufa sheet across Piedmont to interfinger with Younger Gravels “A” near Vaal River (Helgren, 1977, 1978a) as revised by K. W. B. in 1977, is sum- these samples were 770 t 50, 2910 2 50, marized in Table 4, retaining but modifying and 3300 ? 80 yr/B.P. These figures imply Peabody’s terminology. fossil waters or contamination by “dead” Buxton illustrates the systematic nature carbonates or both, but nuclear fallout con- of facies sequences within the depositional tamination has not seriously affected this phase of each geomorphic cycle better than groundwater reservoir. any of the other study areas do, thanks to Our dating strategy is based on the fact good exposures and development within a that fresh atmospheric CO, is continuously deep bedrock valley. A basic regional incorporated during the breakdown of dolo- morphostratigraphy can now be discerned mite into a bicarbonate and then through- on geomorphic, lithologic and radiometric out the period that these bicarbonates grounds and is discussed further below remain in solution. This is borne out by our (Table 6). reference samples. (a) A fresh lime mud accretion from a shal- RADIOCARBON DATING low pool of water in a cavity on the Mazels- Twenty-six samples, including two from fontein tufa surface (Fig. 6) was collected modern reference materials, were dated at in September 1974; it gave an apparent age the Smithsonian Institution (Table 1). These of 111% normal, i.e., some 1000 yr too tufa carbonates were removed from critical young. This material accumulated through stratigraphic units, selecting uncorroded surface runoff and sheetwash, with abun- specimens, with as little recrystallization as dant access to atmospheric COZ. Modern possible (13 samples by K. W. B. in 1972 to atmospheric pollution is prominent and 1973 and 1977, 13 by R. S. and K. W. B in “dead” carbonates are either absent or 1974). Samples were washed in 0.5 N HCl negligible. before CO, evolution in H,PO,. (b) Fresh tufa was deposited on living The critical question is to what degree plants between April and August 1974 by these apparent ages are of real chronometric major activation of the karst spring above value (for discussion of tufa and travertine Gorrokop, following exceptionally heavy dating see Geyh, 1970a, b; Geyh and Schil- rains during February through April of that lat, 1966). The original source of much of the year. These waters traveled rapidly along a CO, is Precambrian dolomite, and “dead” 0.7-km stretch, allowing for thorough aera- carbonates were and remain a major input tion; the simulated date of about 80 yr to the water-mineral-gas system produc- implies only a limited amount of contamina- ing tufas along the Gaap Escarpment. Bre- tion by “dead” carbonates. denkamp and Vogel (1970, Table 1) studied (c) A subrecent tufa adjacent to an active three water samples from a deep, dolomitic spring seep was dated 5 15 yr B.P. at Mazels- aquifer at Kuruman. Conventional ages on fontein, and a sheet tufa, with a weather- 320 BUTZER ET AL

GAAP PLATEAU

MAZELSFONTEIN

FIG. 6. Morphostratigraphic map of Mazelsfontein. ing rind and in process of dissection, 1070 of subrecent, nonfunctional tufas related to yr B .P. ; similarly, a small, inactive carapace groundwater discharge or surface runoff. at Gorrokop, suggesting a former low- Dates in a similar range were not obtained energy seep, gave dates of 1205 and 1375 from any deposits pertaining to older yr B.P. These examples indicate that 500 stratigraphic units. Each of these dates may through 1400-yr “ages” are representative be a few centuries too old, but the con- PALEOCLIMATES OF THE GAAP ESCARPMENT 321 tamination by groundwater-generated “dead” carbonates are free of such contamination, carbonates cannot possibly be more than and all compact, recrystallized tufas do in 500 yr. fact include a substantial component of Applied to the 14C framework presented younger CO,. Only 1% modem contamina- here, the empirical evidence shows that con- tion in a sample with a true age of 67,000 yr tamination by “dead” carbonates is not a will yield an apparent age of 37,000 yr; serious problem and only requires a cor- similarly, 1% modem calcium carbonate rection factor of a few centuries. equivalent for a sample of 23,000 yr will The reverse problem, contamination by decrease the apparent age by 5% or 1300 younger CO*, is more serious. Younger yr (Stuckenrath, 1977). Such contamination CO, is repeatedly or continuously supplied is serious for samples older than 20,000 yr to existing tufas by percolating rainfall and or so and renders any finite dates greater groundwater seepage; such CO, can be ex- than 30,000 yr highly suspect. changed with older, penecontemporaneous The potential value of our assemblage of CO, during repeated re-solution and recrys- 14C dates must be assessed in relation to the tallization or in the course of void filling stratigraphic framework. An isolated 14C by younger carbonates. No inorganic assay on carbonates is always suspect. but

TABLE 4 MORPHOSTRATICRAPHIC SEQUENCE AT BUXTON-N• RLIM

Bx-IO Several minor tufas in north valley downstream of Bx-8, as yet unstudied Bx-9 (Channel Alluvium) Sandy fill (5 m thick, three subdivisions) derived from residual soils and reworked eolian sand, resting on patinated surfaces in abandoned south valley downstream of Bx-6: partly contemporary with Bx-8 Bx-8 (Blue Pool Tufa II) Tufa apron (20 m thick) in north valley (base of lobe 15,980 yr B.P.: top outcrops 14,010 yr B.P.) (Cutting of 10-m gorge into north side of Bx-7: development of deep. reddish paleosol: widespread manganiferous patina [Peabody, 19541) Bx-I (Blue Pool Tufa 1) Porous tufa apron (9 m thick) at plateau level, choking new north valley (32,700 yr B.P. minimum date at 2 m below top of unit); fossiliferous beds with Middle Stone Age at Witkrans Cave (Fig. 2) have interbedded travertine (33,150 i 2500 yr B.P.; Clark, 1971) and are sealed by same black patina (Peabody, 1954); Eyuus Cave (Peabody. 1954, Fig. 7), recently excavated by P. B. Beaumont and M. L. Shackley, was dissolved within Bx-5 apron after deposition of Bx-6 and then filled prior to patination; Middle Stone Age therefore coeval with part of Bx-7 (Cutting or reexcavation of 55m-deep bedrock valley to north) Bx-6 (Oxland Tufa II) Tufa carapace (30 m thick, 35-75” dips) interbedded with detrital lenses (including l2- 16” small-scale crossbeds) (older than 43,000 yr B.P.), permanently sealing off south valley (Brown paleosol, preserved in fissures) Bx-5 (Oxland Tufa I) Crude, angular footslope talus (Bx-5A. 5 m thick) in south valley (Peabody, 1954. Fig. 6), then partly porous tufa aprons (3-15” dips) and carapace (25-75” dips) (Bx-5B. IO m thick) (Major valley cutting; some karst corrosion) Bx-4 (Norlim Tufa) Extensive, indurated tufa apron and carapace (60 m thick) blocking bedrock valley; Ausfralopithecus africanus comes from interbedded reddish clay and tufa (early Bx-4) over Taung “baboon sands” in karst fissure Bx-3 (Norlim Conglomerate) Pink sandy conglomerate of rounded, coarse- to cobble-grade dolomite (5 m thick. Fig. 8), preserved as indurated, eroded deposit disconformable under Bx-5; Taung “baboon sands“ deposited in fissures before or during Bx-3 phase (Partial dissection; major fissure corrosion opens Ausfralopirhecus cavern) Bx-2 (Thabaseek Tufa) Extensive, indurated tufa apron and carapace (40 m thick). in front of bedrock valley mouth Bx-1 (Thabaseek Breccia Complex) Massive, crude angular rubble with thick tufa interbeds on escarpment footslope (20 m thick, maximum dip 20”); shale and dolomite rubbles (Fig. 7) lack older tufa derivatives 322 BUTZER ET AL.

TABLE 5

APPARENT 14C AGES OF TUFAS FROM THE GAAP ESCARPMENT

Laboratory 14C Age C.C.E.O Stratigraphic unit no. (yr B.P.) (%I (location)

Gorrokop Sl-1953 80 2 65 97.3 1974 plant encrustation, Kloof G, site 1 Sl-2037 1,205 + 60 99.6 Gr-7, Kloof B, site 2 Sl-2038 1,375 2 65 99.8 Gr-7, Kloof B, site 3 51-1644 2,520 2 70 99.7 Gr-6B, above rim, site 4 Sl-2036 3,155 2 65 92.8 Gr-6B, Kloof B, site 5 Sl- 1645 9,550 f 115 99.7 Gr-6A, Kloof G, site 6 SI-2035 19,950 2 300 98.6 Gr-SB, Kloof B, site 7 Sl-2033 24,408 f 500 97.8 Gr-SA, above rim, site 8 Sl-2034 36,800 -c 1,800 95.6 Gr-4, Kloof B, site 9 Grootkloof Sl-1641 9,310 2 105 95.1 Gk-6 Sl-1640 20,825 2 230 99.0 Gk-4 Sl-1301 26,130 + 620 86.3 Gk-2 Sl-1639 30,760 k 1,035 97.2 Gk-2 Mazelsfontein Sl-2039 110.9% modem - Fresh marl, site 1 Sl-2045 515 + 80 98.1 Mz-3, site 2 Sl-2044 1,070 t 65 87.3 Mz-3, site 3 Sl- 1646 7,715 2 90 97.4 Mz-3, site 4 Sl-2043 15,150 2 165 99.4 Mz-2, site 5 SI-2041 22,200 + 300 95.1 Mz-2, site 6 Sl-2042 25,900 2 400 99.1 Mz-2, site 7 Sl- 1647 26,840 2 520 98.5 Mz-2, site 8 Sl-2040 38,600 k 2,350 88.5 Mz-IA, site 9 Buxton-Norlim Sl-1643 14,010 k 170 98.7 Bx-8, upper Sl- 1642 15,980 f 230 99.2 Bx-8, middle Sl-3379 32,700 r 1,190 97.9 Bx-7 Sl-3350 >43,000 99.2 Bx-6

a Calcium carbonate equivalent. in the case of multiple dates, contamination by “dead” CO,; apparent ages of 2000 to by younger as well as “dead” CO, would 5000 yr probably are reasonable approxima- not yield results reasonably consistent with tions, because “dead” and recent CO, con- a complex stratigraphic framework span- tamination will balance each other out at ning several tufa facies and four distinct some point; apparent ages of 5000 to 20,000 study areas along a DO-km N-S sector of yr probably represent minimum but finite the escarpment. Nonetheless, basic con- ages, due to cumulative contamination by sistency does not provide a license to con- younger COz, and may be on the order of sider our dates as true ages, quite apart from 10% too young; apparent ages of 20,000 the universal background problem of to 30,000 yr represent minimum ages, sub- atmospheric i4C flux. Instead we suggest the stantially too young (by 20-50% or more); following cautionary norms: Apparent ages apparent ages of 30,000 yr or more are likely of less than 2000 yr probably are several to be infinite and should be regarded with centuries too old, due to contamination appropriate caution. PALEOCLIMATES OF THE GAAP ESCARPMENT 323

A GENERAL STRATIGRAPHIC subdivisions. When formalized, this strati- FRAMEWORK FOR THE ESCARPMENT graphic scheme should be useful in dealing with the Quatemary record of the Gaap The preceding local stratigraphies can be Escarpment. synthesized into a gross regional stratig- Phase VI, subdivided into three episodes, raphy, as based on morphological context, spans Holocene time. The available record, facies sequences, tufa diagenesis, and 14C as presently deciphered, indicates that tufa dates. Such a macrostratigraphic frame- was forming on a small scale but at many work is presented in Table 6 with “phase” localities ca. 1400 to 400 yr B.P., ca. 3200 serving as an informal time-stratigraphic to 2400 yr B.P., and during the first two or term to represent the different morpho- three millenia of the Holocene. stratigraphic units at various localities along Phase V comprises two major generations the escarpment. The six major phases here of tufas at Gorrokop, Grootkloof, Buxton. recognized indicate the most convenient and Mazelsfontein. The younger of these. lithologic units for field mapping. Pending Vb, has bracketing dates of 20,825 and completion of the laboratory studies, Gor- 14,010 yr B.P.; the true age span may be rokop will provide an excellent type locality on the order of 24,000 to 15,000 yr B.P. for a formal lithostratigraphic entity with six “C Assays on the upper part of the older

TABLE 6

A MACROSTRATIGRAPHIC FRAMEWORK FOR THE GAAP ESCARPMENT

Deposi- Groot- Mazels- tional ‘T Age Vaal River Gorrokop kloof Ulco Buxton fontein phase (yr B.P.) correlations

Gr-7 - - Bx-IO Mz-3B VIC 1400-400 Gr-6B (Erosion) - VIb 3200-2400 Riverton V Gr-6A Gk-6 - Mz-3A Via 9700-7600 (Erosion) (Erosion) (Erosion) Gr-SB Gk-3,4,5 - Bx-8.9 Mz-2B Vb 21,000-14,000 Riverton IV (Soil) (Erosion) - (Soil and erosion) (Erosion) (Erosion) Gr-5A Gk-2 - Bx-7” Mz-2A Va 132.000 Riverton III”

Major drainage change (Erosion) Gr-4B Bx-6 IVb Riverton, I,IID (Erosion) Ul-4 (Soil) (Erosion) Gr-4A Bx-SA,SB 1Va Younger gravel C’

Major downcutting (Soil and erosion) Gr-3 Gk- 1 Ul-3 B~-3,~4 Mz-IC III Younger gravel A (Major erosion) (Soil) (Major erosion) (Erosion) Gr-2 Ul-2 Bx-2 Mz-IB I1 (Soil) (Erosion) (Soil) Gr-lA,B,C Ul-1 Bx-1 Mz-IA I Wedburg Terrace’

n Deposits with Middle Stone Age in siru. * Deposits with terminal Acheulian (Fauresmith) in situ. r Deposits with Acheulian in situ and middle Pleistocene faunas ca. 0.6 to 0.2 my old. d Australopithecus africanus. p Pliocene fauna ca. 4.2 to 3.2 my old. 324 BUTZER ET AL.

generation, Va, vary between 24,400 and ing the late middle Pleistocene (as defined 32,700 yr B.P.; the last figure, on minimally in Butzer, 1974b). recrystallized tufa from the top of Bx-7, The volume of Phase III tufa exceeds all gives a terminal minimum date of 32,000 yr subsequent cumulative tufa accretion by B.P. In fact, Va probably lies well beyond several orders of magnitude, while display- the range of 14C dating, and such a place- ing a remarkable homogeneity of facies. The ment would be most reasonable in view of evident contrast of scale between these and the major phase of soil development that earlier tufas argues for geomorphic cycles separates Va and Vb at Buxton. of a different order and suggests a long Of particular interest are the breccias that intervening span with insignificant deposi- preceded and followed tufa deposition at tion and protracted valley cutting. Grootkloof during phase Vb. Local breccias The Australopithecus africanus fossil at Gorrokop are of similar age. Also, in from Bx-3 belongs at the beginning of Phase Wonderwerk Cave, Kuruman Hills (Fig. 2), III (Butzer, 1974a). The “baboon sands,” a major level of large roof blocks rests on a immediately predating the australopithe- deposit with abundant carbonized grass tine, provide a broad fauna1 age for the dated 38,600 + 1080 yr B.P. (SI-3562); the preceding period of protracted erosion; CO, evolved by acid application to a fine- these baboons have more in common with grained white ash overlying the block and those from Swartkrans (Member I), Krom- spa11 complex gave a date of 28,200 t 1000 draai (hominid site), and the Sterkfontein yr B.P. (SI-203 1); finally, two cultural hori- extension site (Member V) than those from zons resting on minor roof spa11 gave dates the older Makapansgat or, Sterkfontein of 12,130 ? 110 yr B.P. (SI-2032B) (lower) Member IV biostratigraphic horizons (Tobias, and 12,160 2 115 yr B.P. @I-3561) (upper) 1973; Butzer, 1974a). Vrba (1975) correlates on carbonaceous “soil” (Butzer and Stucken- the bovids of these younger fauna1 assem- rath, in preparation). The Wonderwerk blages with the upper Bed II fauna of chronostratigraphy can probably be applied Olduvai Gorge (Hay, 1976, Chap. 5), imply- to Grootkloof-Gorrokop, to suggest me- ing bracketing dates between 1.6 and 1.1 chanical weathering and local transport of m.y.a. for the beginning of Phase III. In crude detritus ca. 35,000 to 30,000 yr B.P. other words, the Phase III tufa probably and again ca. 15,000 yr B.P. spans much of the late early Pleistocene. The tufa-rubble complex representing Direct correlations are provided with the Phase Vb reflects a sequence of cold, then Vaal Younger Gravel A, which antedates moist, and finally cold climate during the ter- both the bulk of the middle Pleistocene minal Pleistocene. Phase Va, on the other fauna(s) and the Acheulian industry (Hel- hand, indicates a somewhat earlier moist gren, 1977, 1978a, b). period; the youngest phase of Middle Stone Since the Thabaseek Tufa is somewhat Age settlement in the coastal area of South earlier than the “baboon sands” at Buxton, Africa coincides with the beginning of deep- a late Pliocene age is probable for Phase II. sea oxygen-isotope stage 4 (Butzer, 1978). Phase I, a very long, complex sequence Phase IV is poorly recorded, except at of breccias, conglomerates, and tufas, is Gorrokop and Buxton. At least two genera- substantially older. Facies parallels suggest tions of deposits, each comparable in mass a broad correlation with the Mio-Pliocene to the total of Phase V tufas, were separated “Older Gravels” of the Vaal valley (Hel- by erosion and/or soil development. These gren, 1977,1978b; Cooke and Maglio, 1972), tufas were followed by bedrock incision and or more specifically with the Wedburg Ter- changes in drainage alignments and are race, which represents a long period of flu- therefore substantially older than phase Va. vial activity in the 4.2 through 3.2 my They record major geomorphic cycles dur- time range. Potential parallels with the early PALEOCLIMATES OF THE GAAP ESCARPMENT 325

FIG. 7. Thabaseek breccia (Bx-1) at Buxton-Norlim (Photo by K.W.B.. August 1974).

Pliocene Mammuthus sands (4.0 my) at Vir- matrix of calcrete, grit, and sand (Fig. 7), ginia, Orange Free State (Butzer, 1973c), are found as inclined sheets (4”), averaging can also be cited. over 5 m thick, in footslope contexts at Altogether, the escarpment tufas outlined distances of 250 m and more from the here range in age from early Pliocene or escarpment. Such deposits represent me- terminal Miocene to the present and pro- chanically weathered talus, partly reworked vide the longest consistent record of paleo- by water. In some instances, angular rubble climatic cycles available anywhere on the with chaotic bedding may grade laterally or African continent. vertically into better rounded or better stratified facies. Occasionally, as in Gor- FACIES DEVELOPMENT AND rokop Kloof E, a steep fan-like structure ENVIRONMENTAL INTERPRETATION (of Phase V) with 25” foresets may pro- grade over subhorizontal strata. The best-developed surficial deposits in Most of these crude detrital beds repre- the study area are tufas. Yet many strati- sent screes rearranged by powerful sheet- graphic profiles exhibit a fivefold facies wash across the escarpment footslope. Such sequence: (i) basal, angular rubble + (ii) an agency is not visible today, even after coarse, rounded gravel and gritty, calcreted major rainstorms, and any loose rubble is sand + (iii) subhorizontal, wavy-bedded to not only corroded in appearance but im- laminated tufas with some bulbous inter- mobilized by vegetation. The original ac- beds + (iv) increasingly steep-bedded, fore- cumulation of such midslope or knickpoint set tufa carapaces --+ (v) surface and fissure talus also calls for comment, since mechani- corrosion accompanied by fillings and cal weathering produces insignificant fresh veneers of reddish, sandy wash. Each of free-face rubble today, although the mean these facies requires brief discussion. monthly minimum temperature for July, the (i) Crude, angular rubbles, mainly with a coldest month at Kimberley , is -2.4”C. 326 BUTZER ET AL. with an overall record low of -7°C (Climate of the escarpment are those of Phases IA of South Africa, 1965a). Furthermore, the and IC, probably synchronous with one or Grootkloof plunge pool was sealed by 5 cm both of the late Miocene to early Pliocene of ice on August 4, 1974. Frost weather- cold intervals, However, Phases II, III, ing is therefore possible at present, and IVa, and Vb also include comparable facies lower Pleistocene temperatures (Butzer, wherever exposures in former scar-p proxim- 1973a; Talma et al., 1974) should have suf- ity are adequate. Basal, cryoclastic beds ficed for the deep frost riving implied by are the rule rather than the exception in the shattering of slates and block-by-block dis- standard depositional hemicycle of the Gaap integration in highly resistant, massive dolo- Escarpment. mite. In fact, no other agency of simi- (ii) Whenever crude, angular dolomite lar potential impact on dolomite can be sug- rubbles are well developed (e.g., Gr-1C in gested, following systematic examination Kloof E), it can be expected that better- of numerous such breccias along the rounded fanglomerates will form after ad- escarpment.3 ditional fluvial transport. Coarse-grade peb- The evidence for several episodes of bles with homogeneous rounding are fre- particularly intense cold during the late quent at both Gorrokop and Buxton- Tertiary (Phase I) is surprising at first sight. Norlim (Fig. S), in part overlying thick However, the influence of late Tertiary cold breccias, but more often as interbeds within intervals on the southern hemisphere con- horizontal tufas. Such conglomeratic lenses tinents has long been overlooked. So, for probably represent channel beds, mainly example, glaciers of the Southern Andes derived from older or penecontempo- expanded onto the Patagonian plateau ca. raneous cryoclastic rubble. Although seldom 3.5 my (K/Ar dating) and again repeatedly observed in thick or laterally extensive between 2.1 my and the end of the Pleisto- units, such fluvial facies demonstrate a cene; maximum glaciation on the plateau is genetic link between basal breccias and dated ca. 1.2 to 1.0 my (Mercer, 1976). The overlying tufa aprons. deep-sea isotopic records further show that The transport competence implied by a major ice sheet had accumulated and such conglomerates is astounding, even in stabilized on the East Antarctic continent the confined gorges of the escarpment. At by about 10 my (Shackleton and Kennett, Grootkloof several protracted flood surges 1975a), with major expansion of high- in March and April 1974 saw 4 to 5 m of latitude glaciers ca. 6 my (Berggren and water rushing through the lower gorge with Ul-Haq, 1976). Waters off New Zealand some 30 m of spray above the plunge pool may have been cooler ca. 3.5 my, then (H. L. Matter, personal communication, relatively warm until ca. 2.6 my (Shackleton 1974); appreciable bed-load transfer was re- and Kennett, 1975). Evidently, the southern stricted to coarse sand and fine- to medium- hemisphere was substantially cooler than grade pebbles (<2-cm major axis) that did today during parts of the Mio-Pliocene. not move beyond the escarpment front. The Gaap Escarpment provides the first The various conglomeratic units of Phases I direct indication that southern Africa was to V evidently required surges of hydrody- also affected by greater cold, and the po- namic energy exceeding that of the most- tential implications for late Tertiary geo- exceptional floods today. High-intensity morphic development are considerable. rains are 30 to 40% more common in the The best-developed cryoclastic breccias northern Transvaal than they are in the Kimberley region (Climate of South Africa, 3 Similar fossil frost rubbles have been described from 1965a, Fig. 155). This suggests that patterns karst terrain at 2500- to 2800-m elevation in Guatemala of high-intensity, sustained rainfall now and Mexico (Enjalbert, 1967). typical of central Africa must have been PALEOCLIMATES OF THE GAAP ESCARPMENT 327

FIG. 8. Conglomerates of basal Norlim unit (Bx-3) at Buxton-Norlim (Photo by K.W.B., August 1971). frequent on the Gaap Escarpment at times cumulations. Occasional bulbous interbeds when such coarse elastic sediments were suggest low mounds around heavily vege- mobilized. tated spring vents that emerged from be- (iii) Well-stratified, extensive sheets of neath the marshy ground of aggrading tufa subhorizontal tufas characterize Peabody’s sheets (Gardner, 1932; Butzer and Hansen, (1954) “aprons.” Sedimentary units may 1968, Chap. 7). Occasional ponding may range from thin-bedded, cellular beds to lead to accretion of massive, vertically laminated, dense, cryptocrystalline ac- oriented, “organ-pipe” tufas or well- 328 BUTZER ET AL.

stratified marls, depending on the propor- and Bosnia). Each of these environments tion of suspended, detrital sediment. Im- is characterized by strong seasonal varia- pressions of plant stems, rootlets, and tions of water flow and consequently of leaves are preserved in pockets and deserve calcium carbonate concentration; each is a study by a qualified specialist; results of one high-insolation environment, during the dry such study near Buxton-Norlim (Camp, season, favoring diurnal fluctuations of 1948; Peabody, 1954: 694) remain unpub- saturation levels. Mean precipitation ranges lished, but impressions of Olea, Zizyphus, between 650 and 1000 mm. and an unidentified broadleaved tree, now At the microenvironmental level, carbon- absent from the escarpment, were provi- ates are selectively precipitated at and just sionally identified by Richard Liversidge downstream of convex channel irregu- (personal communication 1971). Several po- larities, in response to improved mixing tential samples were tested for pollen by and aeration, with CO, loss to the Louis Scott (personal communication, 1974) atmosphere and reduction of the calcium but unfortunately proved sterile. bicarbonate to a less soluble calcium carbon- Minor tufas were actively deposited along ate. Particularly effective in a dry environ- the spring-fed brook at Gorrokop in 1974. ment such as that of the Gaap Escarpment These formed narrow, sloping stairways (es- is the rapid evaporation or percolation of caliers), typically 2 cm thick and a few tens waters diverging across tufa aprons or ul- of centimeters long; they aggraded rapidly, timately finding their way across the pied- probably as a result of aeration and CO, mont to the Vaal valley. This is confirmed loss as water moved over the lips of the by the increase in MgC03, from an average escaliers. Accretion took place as an aggrad- of about 1.5% in tufa samples from the ing front. Encrustation of plants and other escarpment to some 20% near the Vaal protruding objects in the path of these (H. L. Matter, personal communication, waters proceeded simultaneously. Even at 1974). For a geochemical discussion of car- this small scale, such tufa accretion along bonate solution and precipitation, see the Gaap Escarpment was only initiated by Barnes (1965) and Rocques (1969). exceptional rains: Some 850 mm of precipi- Tufa formation today is generally inac- tation was measured at Ulco between tive in the dolomite country of the Trans- February and April 1974, with rains of up to vaal that receives 600 to 750 mm of pre- 30 mm/hr (H. L. Matter, personal communi- cipitation; dissolved calcium increases cation, 1974). rather than decreases downstream, al- On a large scale the Gaap sheet tufas though fossil tufas are well developed most closely resemble those described in ac- (Marker, 1971, 1973); few substantial tufa tive formation from the seasonally wet and bodies exist in areas receiving more than dry, semiarid environment of northeastern 750 mm of precipitation. Marker (1973) Brazil by Branner (1911). Prograding tufa argues that lithology, catchment area, as sheets also form sets of steps at regular well as amount of and seasonal concen- intervals along streams of the subhumid tration of precipitation are the critical vari- Arbuckle Mountains, Oklahoma, where tufa ables in tufa accretion. On and below the carapaces develop at major breaks of Gaap Plateau, the limited nature of tufa gradient (K. W. Butzer, unpublished); ac- development prior to 1974 can be attributed cretion is balanced by flood surge corrosion. to the small amounts of rainfall received The more porous, cellular varieties are per- since the early 1890s (A. Lahoud, personal haps better represented in karst regions of communication, 1974; see also the long- the Mediterranean Basin (eastern Mallorca; term records from , Hopetown, the “travertine” holotype at Tivoli, near , and Newlands, Climate of Rome; or in the stream valleys of Slovenia South Africa, 1954). Limited rainfall implies PALEOCLIMATES OF THE GAAP ESCARPMENT 329 little dolomite solution as well as poorly By the late winter of 1974 the waterfall calc- integrated surface and subterranean catch- mats along the Gaap Escarpment had de- ments, with the result that any dissolved hydrated and began to flake off as the algae carbonates are precipitated locally, rather died. It seems that persistent, if reduced, than channeled together into restricted spring-fed discharge during the dry season zones of the escarpment. If rainfall were is essential to maintain algal mats and so to sustained at 150% its pre-1974 average ensure net tufa accretion. over a few decades, large-scale accretion Regardless of whether inorganic or organic would probably take place in the wake of precipitation is dominant in carapace tufa periodic surface runoff as well as in rela- accretion, large-scale accumulation in the tion to seasonal or perennial (but fluctuat- study area presupposes a substantial in- ing) karst spring discharge. crease of available water. Tufa aprons and (iv) Tufa “carapaces” (Peabody, 1954) and carapaces both imply a subhumid climate constricted tufa waterfalls probably account on the Gaap Escarpment, compared with the for the bulk of the Gaap freshwater lime- semiarid climate of today. stones. Foreset beds inclined at 25 to 90” are (v) Surficial corrosion and stream incision dominant, although deposits with interme- probably begin as soon as tufa attrition out- diate dips of 8 to 15” commonly link apron weighs accretion, except near spring seeps. and carapace facies. Such carapaces ap- As the water table falls, joint networks are parently developed when subhorizontal liable to corrosion by intermittent, percolat- aprons continued to increase their con- ing waters; deep fissures are created and vexity, until slow area1 accretion was re- local underground caverns form where major placed by accelerated vertical buildup. Other fissures intersect. Such processes would be carapaces have also been generated by favored by an excess of water, thereby re- waterfall tufas prograding within a gorge or ducing carbonate concentrations, or by a projecting beyond a free face. drastic diminution of water, eliminating Algal growths and mosses are prominent lateral transfer and convergence of solubles. on intermittent waterfall surfaces, both Along the Gaap Escarpment such corrosion dolomite and tufa. In fact, conspicuous white generally appears to have followed tufa ag- sheets of fresh tufa were present on flow gradation, but it preceded accumulation of surfaces after the rains and spillovers of sandy fissure fills that include eolian de- 1974. Up to 2 mm of silty tufa were found rivatives. This argues that slow corrosion adhering to discontinuous algal mats, com- was commonly associated with drier condi- pared with little or no fresh carbonate on tions. On the other hand, deep incision of clean rock surfaces. Such organisms extract the Gorrokop or Buxton gorges would only CO, from the water and provide convenient, be possible with abundant water over pro- irregular microsurfaces for precipitation. tracted periods. Similar relationships between algal mats and The reddish, sandy fissure, cave, and sur- carapace or waterfall tufas can be observed face colluvia have already been discussed in the Arbuckle Mountains and have been in some detail (Butzer, 1974a) and are not described from Afghanistan (Jux and Kempf, central to the present paper. In general they 1971). Where such organic growth is abun- appear to be the product of intermittent, dant, as at Grootkloof, there may be pro- effective sheetwash in an environment sub- liferations of snails (Xerocerastus Cf. stantially similar to that of today. At least psammophilus, identified by A. Zilch, per- part of the eolian components (such as the sonal communication, 1972); so far, how- subrounded, macropitted, oxide-coated ever, such snails have been found in only “Kalahari Sands”) was originally introduced one fossil deposit (Gr-5). The net rate of to the study area by long-distance stream algal tufa accretion appears to be limited: transport, rather than by wind. 330 BLJTZER ET AL.

The preceding discussion outlines the tinuous record of half of a glacial and most basic arguments for a sequential, explanatory of an interglacial. Consequently, the average model that describes the geomorphic cycles cycle probably includes both a cold and a of the Gaap Escarpment. warm segment; deposition was not only dis- Stage 1. Deposition begins with colluvial continuous but also interrupted by longer reworking of sandy surficial sediments and intervals of erosion. In other words the in- soils while the energy of geomorphic activ- formal phases identified here to provide a ity increases. provisional stratigraphic framework for the Stage 2. Accelerated frost weathering pro- escarpment are not synonomous with geo- motes scarp recession in areas with favor- morphic cycles. These cycles instead serve able jointing and adequate rock moisture, to synthesize broader trends of geomorphic accumulating crude talus or even thick, evolution and to explain the processes re- cryoclastic rubble aprons. sponsible. Stage 3. Increased fluvial competence, re- An apparent defect of the model is that it lated to exceptionally intensive flood surges does not provide an adequate explanation of sufficient duration and frequency, eventu- for the several episodes of effective bed- ally reworks talus deposits into fanglom- rock or tufa incision. However, protracted erates. downcutting in compact rock can be viewed Stage 4. As the hydrographic system re- as a special case of stage 5, with a long- integrates, both above and below ground, term excess of runoff and correspondingly appreciable masses of calcium carbonate are low bicarbonate levels. Frost-weathering selectively deposited on aprons below the may also accelerate headward erosion. In escarpment, thus gradually replacing elastic general, therefore, the model provides rea- sedimentation. sonable paleoenvironmental interpretations Stage 5. As perennial discharge (sub- for the specific, depositional facies and ject to seasonal fluctuation) becomes charac- general geomorphic trends identified in the teristic, flood peaks and stream competence study area. are reduced; tufa carapaces and waterfalls develop on an increasing scale, prograding PALEOCLIMATIC PATTERNS AND over earlier aprons; the moisture balance EXTERNAL CORRELATIONS is probably optimal and ground cover at a maximum. Episodes of excessive runoff re- This paper has presented stratigraphic duce bicarbonate levels, leading to tempo- evidence and geomorphic arguments for a rary erosion. sequence of Pliocene to Holocene paleo- Stage 6. Reduction of moisture and dis- climates on the Gaap Escarpment, along the integration of the drainage network termi- southwestern margins of the Kalahari. This nate tufa accretion, except in the vicinity of succession provides the best overall strati- spring seeps; rainwater corrosion attacks graphic framework currently available for tufa surfaces and enlarges jointing systems, the interior of southern Africa. It is as amen- favoring slow but cumulative surface cavita- able to differentiation as the alluvial de- tion, fissure corrosion, and linear incision. posits of the Vaal River (Helgren, 1977, This model (Fig. 9) fits an average cycle 1978a, b) and far more complete than the as presently understood. It does not claim lacustrine record of the Kimberley region to match each facies sequence, and some (Butzer, 1974~; Butzer, et al., 1973) and the cycles, such as those of Phases Va and VI, Botswana Kalahari (Grove, 1969; Lancaster, lack evidence of stages 2 and 3. However, 1974; Grey and Cooke, 1977), on the one when telescoped in the geological record, hand, or the cave breccias of the Transvaal Phases Vb and VI would appear as a single (Partridge, 1975; Butzer, 1976b), on the complex. In reality they represent a discon- other. The particular contribution of the es- PALEOCLIMATES OF THE GAAP ESCARPMENT 331

Net IDeposition Net Erosion

i 6. Limtted geomorphic change

FIG. 9. Idealized geomorphic cycle for the Gaap Escarpment. Increasing periodicity (stages l-3) is char- acterized by greater frequencies of high-intensity rainfall, producing greater peak runoff, the trend to more mesic conditions (stages 3 and 4) is accompanied by relatively heavy, seasonal rains, with sustained stream and spring flow but reduced flood peaks; the xeric trend (stages 4-6) is marked by declining precipitation totals and number of rainy days, as well as disintegration of the surface and subterranean drainage network. The talus and gravel phases are absent in some cycles, and protracted valley deepening probably requires long periods of moist climate with reduced carbonate concentrations. carpment cycles is that they allow greater members (I-V), provides a stratigraphic paleoclimatic discrimination, since the tufas framework for the alluvial fills of the drain- provide an incontrovertible record of hydro- age net (Butzeret al., 1973; Helgren, 1978b: logical cycles as well as fragmentary in- Butzer et al., 1979; K. W. Butzer, in prep- formation on thermal anomalies. aration). As now understood, the Gaap sequence Member V represents a suite of flood silts has been resolved at an intermediate level capped by a vertisol (Soil Survey Staff, 1975. of detail. Upon completion of the extensive p. 375) that generally records seasonally wet sedimentological study still in progress, floodplain surfaces with meandering streams 14C dating of a further selection of appro- ca. 4500 to 1300 yr B.P.; however, in some priate samples, and new field investigation, valleys a temporary phase of downcutting phases V and VI will be resolved in micro- can be identified ca. 2600 to 2250 yr B.P.. stratigraphic detail. while in others the floodplain remained un- The escarpment sequence in the 14C time dissected from the middle Holocene until a range can be compared with that from other century or two ago. Probably Gaap Phases parts of the lower Vaal Basin, where dif- VIb and VIc, with their inherent problems ferent facies have different geomorphic of absolute dating, are broadly equivalent thresholds. Some key aspects can be out- to Member V (Table 2). lined. Member IV records a sandier flood silt (a) The Riverton Formation, with five regime, with greater periodic&y of discharge. 332 BUTZER ETAL. with a terminal date of 14.670 + 270 (SI- (c) Two earlier lacustrine sequences have 3563) on carbonaceous soil, i.e., coeval with been recognized in other basins west of Kim- late Phase Vb. berley; the first is linked to Acheulian, the Member III normally is a thick body of second to Fauresmith settlement, the latter sandy alluvium, derived in part from eroded older than 200,000 yr B.P. on the basis of eolian sands and similar to sediments gen- uranium-series dating (Butzer, 1974~; Szabo erated today by unusually vigorous floods; and Butzer, 1979); a correlation with Phases an increased frequency of torrential dis- IVa and IVb can be suggested. In addition, charge is suggested. The only useful 14C there are younger lacustrine beds, as at date is 38,500 ? 1150 yr B.P. (SI-3381), on Bushman’s Fountain, farm Klipfontein (Fig. fluvial shell in position of growth, from the 2). Here two generations of marly sands col- uppermost subunit of Member 111; the real lected in a small ponded basin; the first has age is probably beyond the range of effec- an apparent 14C carbonate date of 20,500 tive 14C dating. Member III includes vestiges k 900 yr B.P. (SI-2858), the second, 6820 of Middle Stone Age settlement and appears 2 115 yr B.P. (SI-2292); correlations with to correlate with Phase Va. Postdepositional Phases Vb and Via are implied. soil carbonates range in apparent age from (d) Finally, the incomplete pollen se- 18,500 to 8000 yr B .P., and proliferations of quences from Florisbad and Aliwal North land snails at the base of this paleosol date (Van Zinderen Bakker and Butzer, 1973) de- 17,000 yr B.P., equivalent to Phase Vb. serve mention, although east of the study Members I and II, including a range of area. Those segments of the late Pleistocene clayey to sandy flood deposits and fresh with pollen preservation suggest a relatively Fauresmith (terminal Acheulian) artifacts, cold and moist climate contemporary with are well beyond the range of 14C. They prob- episodes of more abundant water along the ably relate to Phase IVb. Gaap and at Alexandersfontein. (b) Accelerated spring activity in the Alex- These regional comparisons are adequate andersfontein Basin (Dwyka shale and re- to show that the Gaap paleoclimatic record, lated Karoo intrusives), near Kimberley, pre- as summarized in Table 7, is representative dates ca. 15,000 yr B.P.; this episode was of the Province and the west- followed by conspicuous soil formation ca. ern Orange Free State (Fig. 2). 15,000 to 13,500 yr B.P. (K. W. Butzer et Broad paleoclimatic trends across the al., 1979; Butzer, in preparation). Colluvially southern and central Kalahari appear to have reworked eolian sands were washed into the been similar as well. It is still premature dehydration cracks of now-buried spring to compare 14C dated lacustrine sequences vertisols prior to 11,500 yr B.P. After 4600 in Botswana (Grove, 1969; Street and Grove, yr B.P. temporary, shallow lakes, soil for- 1976; Lancaster, 1974; Grey and Cooke, mation, active springs, and polleniferous 1977; D. M. Helgren, in preparation; K. local organic fills all serve to indicate that Heine, in preparation), although high lakes much of the later Holocene was slightly appear to coincide with Phases Vb and Via. moister than today. This Alexandersfontein Alluvial fills and related paleosols (Leser, record closely parallels that of Phases Vb 1972) or spring tufas (K. W. Butzer and J. C. and VIb/c. An older lacustrine phase, with Vogel, in preparation) in eastern Namibia middle Stone Age settlement on foreshore are also suggestive, but dates are still un- dunes and at spring locales cannot be prop- available. Of particular interest is Cooke’s erly dated, but suggests correlation with (1975; Grey and Cooke, 1977) sequence in Rivet-ton Member III and Phase Va; a re- and adjacent to Drotzky’s Cave, western constructed paleohydrological budget Ngamiland, at 20”s Lat. Four major tufas (Butzer er al., 1973) argues for twice the (“calcretes”) and cave travertines (“sinter”) precipitation of today. are identified; the youngest has 14C dates of PALEOCLIMATES OF THE GAAP ESCARPMENT 333

TABLE 7

GEOMORPHIC AND PALEOCLIMATIC EVENTS ALONG THE GAAP ESCARPMENT

Phase VIb and WC tufa. Mainly subhumid 4500 to 400 yr B.P.. with drier condi- Holocene tions ca. 2400 yr B.P. Phase Via tufa. Mainly subhumid 9700 to 6500 yr. B.P., then semiarid. -- 10,000 yr B.P. ------______Erosion. Mainly semiarid, in part arid 13,500 to 11.500 yr. B.P. Phase Vb tufa and breccias. Subhumid climate 2 21,000 to 14,000 yr B.P., preceded Upper Pleistocene and followed by greater cold ca. 30,000 yr B.P. and ca. 14,000 yr B.P. Protracted soil development, under subhumid conditions, as well as erosion, mainly semiarid. Phase Va tufa. Subhumid climate. -- 130.000 yr B.P. _---_--_------__-_------Erosion, including dissection. Mainly semiarid, but major valley cutting indicates times of humid climate. Middle Pleistocene” Phase IV tufas and breccias. Two main intervals of subhumid climate, separated by erosion under semiarid conditions and preceded by greater cold. Erosion. including major downcutting. Partly semiarid, partly humid. --7700,oCJOyrB.P.------~---.----..------Phase III tufa and conglomerate. Long interval of subhumid climate; includes Aus- tralopithecus africanus, and preceded by greater cold, Taung “baboon sands.” Lower Pleistocene Erosion, including downcutting. as well as soil formation. Partly semiarid, partly subhumid to humid. --]1.8my----~~-~~--~--~-~~------.- Phase II tufa. Long interval of subhumid climate. Pliocene Erosion, as well as soil formation. Mainly semiarid.

--S.Omy-- Phase IA, B, and C breccias and tufas. Includes protracted cold periods (IA, 10. with long intervening span of subhumid climate. Upper Miocene

I’ Middle Pleistocene chronometric definition follow is Butzer (1974b).

4350,2250, and 1690 yr B.P.; the next young- different relative degrees of development. est, has eight dates ranging from 17,300 to Nevertheless a very large area has been in 14,300 yr B.P.; while the third youngest has phase, climatically speaking, during at least doubtful dates ranging from 30,000 to the late Pleistocene and Holocene, much >45,000 yr B.P. The parallels with Phases Vb as this same area today responds to similar and VIb/c are striking. A number of moister variations of the summer rainfall belt. intervals can also be recognized in western A comparative review of the Cape coastal Namibia (M. J. Wilkinson, in preparation), regions, which now have winter or winter where the last major episode of increased and summer rains, is in order. The Holocene precipitation dates from the early Holocene of the southern Cape (34”s Lat.) exhibits (Seely and Sandelowsky, 1974; Van Zinderen a threefold paleoclimatic subdivision: Bakker, 1975). (i) The early Holocene, until ca. 4200 yr These examples suggest that broadly com- B.P., was relatively dry, with major forma- parable paleoclimatic trends will eventually tion of coastal dunes, active slope denuda- be established for the southern and central tion, and a more open vegetation than to- Kalahari, an area spanning 9” of latitude. day, except for a brief period of soil for- It is not implied that all paleoclimatic de- mation and reduced eolian activity during tails were identical. Indeed, the various the eighth millennium B.P. (Martin, 1968; tufas of the southern, central, and north- Butzer and Helgren, 1972; Schalke, 1973; ern sectors of the Gaap escarpment show Helgren and Butzer, 1977). The fauna1 evi- 334 BUTZER ET AL. dence from widely scattered sites indicates lished and eolian activity became prominent a substantial increase in woodland from (Schalke, 1973). These contrasts between the Pleistocene/Holocene transition until the winter rainfall belt of the southwestern well after 8000 yr B.P. (Klein, 1977). This Cape and the summer and winter rainfall coastal pattern of intermediate moisture con- zone of the southern and southeastern coasts ditions contrasts with the Gaap sequence, are significant. They suggest that early last where the period 9700 to 6500 yr B.P. was Glacial climatic anomalies were different relatively wet, but matches the pattern of a from those of the late last Glacial, when relatively dry early Holocene in the Med- semiarid conditions were universal in the iterranean (Butzer, 1975a, b). coastal zone. A drier climate was charac- (ii) The middle to late Holocene, from teristic of cold Pleistocene phases in the 4200 to 1000 yr B.P., was mesic in the south- Mediterranean Basin (Butzer, 1975a, 1975b). ern Cape, with an expansion of bush and A dry, late last Glacial along the Cape coasts forest within the vegetational mosaic, slope contrasts with the relatively moist condi- and dune stability, soil formation outpacing tions here documented for the interior. denudation, and local peaty accumulations (2) The time span corresponding to the in valley bottoms. last Interglacial, ca. 130,000 to 70,000 yr (iii) About 1000 yr B.P. increased runoff B.P., includes this temporal sequence: (a) periodicity and channel instability led to high beaches (+7 m) with thermophile mol- stream dissection, followed by sandy al- lusca, intergrading with estuarine and al- luviation on valley floor fans. There has luvial terraces inland, and characterized by been renewed gullying and dune reactiva- a fauna suggesting relatively closed vegeta- tion in response to rapidly increasing human tion; (b) a record of accelerated denuda- interference during the last two centuries tion and slope scree aggradation, paralleled (Butzer and Helgren, 1972). The coastal by an increase of open-country fauna; and patterns of the last four millenia compare (c) a major interval of soil development well with those of the interior. (Butzer and Helgren, 1972; Klein, 1976, The late Pleistocene of the southern Cape 1977; Helgren and Butzer, 1977; Butzer, can be characterized within two broad cate- 1978). The Mediterranean parallels (Butzer, gories. 1975a, 1975b) are striking. Unfortunately the (1) The time span corresponding to the contemporaneous record from the Gaap- last GlaciaL4 i.e., ca. 70,000 to 10,000 yr Kimberley region is not dated and cannot B.P., was essentially xeric in what is now be directly compared. the coastal forest belt, with a grassland This paleoclimatic review of the Cape fauna and limited soil formation; in addition, coastal regions and the interior of southern major eolian activity began 16,000 yr B.P. in Africa shows that these climatic provinces response to marine transgression (Butzer were often, although not always out of phase. and Helgren, 1972; Klein, 1977; Helgren and In particular, the late last Glacial was xeric Butzer, 1977; Butzer, 1978). However, in in the southern Cape but, in large part (com- what is now the mediterranean shrub zone paratively), mesic in the interior of South of the southwestern Cape, a moist forest Africa. Paleoclimatic parallels for the south- was established well before 40,000 yr B.P. em Cape appear to be limited to comparable and persisted until a little after 28,500 yr latitudes in the Mediterranean Basin. B.P., when shrub vegetation was reestab- The only long geomorphic sequence from the northern fringe of the Kalahari Basin

4 Last Glacial is here used in an informal time-strati- has been studied along the Angola-Zaire graphic sense for the approximate span of deep-sea border, near Mufo. The related 14Cdates are isotope stages 2, 3, and 4; last Interglacial is similarly as difficult to interpret as those of the Gaap used with reference to stage 5. Escarpment, but the Mufo paleoclimatic PALEOCLIMATES OF THE GAAP ESCARPMENT 335

succession (Clark, 1963; also, summary in oceanic-atmospheric interface ca. 18,000 yr Butzer, 1971, pp. 342-345) appears to be B.P. are as follows. (a) During the maximum basically comparable. On the other hand, cold of the last glacial, the boundary be- the East African lakes were generally low tween subantarctic and subtropical water during most or all of the last Glacial but masses (subtropical convergence) off South high during the early and middle Holocene Africa was about the same as today, indicat- (Butzer, Isaac, ef al., 1972; Livingstone, ing an intensified thermal gradient near 45% 1978). These very brief external comparisons Lat. (Hays et al., 1976). A further im- appear to provide a gross definition for the plication is that the axis of the southern hemi- paleoclimatic province represented by the sphere westerly belt retained its present lati- Gaap- Kimberley sequence. tudinal position. (b) The warm Agulhas cur- Climatic variations with an amplitude of rent, which today washes the southern Cape several millennia were broadly synchronous coast, was weak, with surface waters off through much of the Kalahari, an area now Natal at least 5°C colder (Be and Duplessy , characterized by summer, “monsoonal” 1976). Since the subtropical convergence did rainfall and by the cold Benguela current not shift at this longitude, changes in the and upwelling waters offshore. Modern rain- strength or location of the subtropical gyre fall anomalies in this climatic province are in the South Indian Ocean must be postu- directly associated with development of the lated. (c) The cold Benguela current was summer easterly circulation and the strength weaker off the coast of Namibia and Angola of the subtropical high-pressure cell over the (CLIMAP, 1976). with increased upwelling South Atlantic Ocean. So, for example, the of cold waters in the central, equatorial At- abnormally heavy rains of early 1974 that af- lantic. Here, at the equator, August surface fected most of the Kalahari and the South waters were 4 to 8°C colder in August, but African interior were a result of a weakened only 2 to 3°C in February (Gardner and Hays, high pressure over the South Atlantic and 1976). This argues for considerable strength- an unusual southwestward penetration of ening of the South Atlantic gyre in winter. maritime tropical air masses from the These glacial anomalies of the general cir- Mozambique Channel and Indian Ocean. culation apply to the last 200,000 yr with Winter rainfall anomalies affect the Gaap the sole exception of the Holocene and iso- Escarpment in an insignificant way only. tope stage 5e, ca. 125,000 yr B.P. The im- These modem patterns lend special sig- plications for continental climates in south- nificance to the poor paleoclimatic paral- ern Africa are momentous. lelism between the Gaap and the southern Long-term Pleistocene aridity along the Cape. It can therefore be proposed that southern and southeastern Cape Coast was late Cenozoic moist intervals in the south- evidently related to the failure of the Agulhas em Kalahari were primarily, although not current, which today favors surface instabil- exclusively, a response to variations in the ity and provides a critical source of atmos- penetration and persistence of summer, rather pheric moisture. This current plays no role than winter rains. This should not imply that off the southwestern Cape, where climatic the southern Kalahari was unaffected by up- anomalies differed substantially. North of per air impulses related to the steering of the Orange River mouth, less coastal upwel- the southern hemisphere jet stream. But the ling and slightly warmer waters, juxtaposed available deep-sea core data from the with generally cooler air masses, would have oceans surrounding southern Africa suggest reduced the prevalent stability, favoring a that the complexity of Pleistocene general greater number of subtropical disturbances circulation anomalies seems to be widely over the Kalahari (see the simulation of Wil- unappreciated. liams et al., 1974). In this way the Namib, The more pertinent facts concerning the the southern Kalahari, and Kimberley region 336 BUTZER ET AL could have experienced greater summer REFERENCES rains, at the same time that contraction of Acocks, J. P. H. (1975). Veld types of South Africa. the monsoonal rainfall belt and increased Botanical Survey of South Africa, Memoir 4&l- 128. atmospheric stability over the tropics Barnes, Ivan (1965). 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