Debris ridges along the southern Drakensberg escarpment as evidence for Quaternary glaciation in southern Africa

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Quaternary International 129 (2005) 61–73

Debris ridges along the southern Drakensberg escarpment as evidence for Quaternary glaciation in southern Africa Stephanie C. Mills*, Stefan W. Grab School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, P/Bag 3, WITS 2050, South Africa Available online 17 June 2004

Abstract Considerable geomorphological work has been undertaken on periglacial and possible glacial landforms found in the eastern Lesotho highlands. However, the origin of several of these landforms remains controversial and differences in opinion concerning their original process mechanisms and associated climatic implications is unresolved. Debris ridges were recently found on southfacing slopes along the Tsatsa-La-Mangaung and Leqooa Ranges in the southern Drakensberg. The objective is to use a variety of approaches to test a number of possible hypotheses to their origin. These include an analysis of topographic and spatial positioning, geomorphology, sedimentology, AMS dating and palaeo-climatological extrapolations. A variety of possible geomorphological process origins for particular deposits are tested against the results of the above-mentioned approaches. The possible origins considered include debris flows (water flow), pronival ramparts (seasonal/perennial snow patches), glacial moraine (permanent glacier/ice) and variations in landscape denudation (i.e. erosional remnant; e.g. valley fill). The results support the contention that the ridges are moraines, which relate to the former presence of small glaciers. r 2004 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction The high Drakensberg and eastern Lesotho highlands have received much interest and debate on their Quaternary environmental history. Observations on the occurrence of ‘cirque-shaped hollows’ located at altitudes above 2900 m led to the first suggestions for Quaternary glaciation in southern Africa (Sparrow, 1967; Harper, 1969; Marker and Whittington, 1971). Yet 75% of 577 slope hollows located in the highlands of Lesotho occur on the warmer north-facing slopes (Dyer and Marker, 1979; Marker, 1991), thus questioning the feasibility of a glacial origin. Steep east-facing riverheads (‘cutbacks’) along the main escarpment have also been attributed to glacial erosion (Hall, 1994). Subsequent findings of debris ridges emanating from steep, high altitude (>3200 m) south-facing exposures of cutbacks on the eastern flank of the Great Escarpment, apparently support the hypothesis of localized plateau, niche and cirque glaciation on higher summits during the Late Pleistocene (Grab, 1996). Grab (1996) postulated that the higher reaches of the plateau summits and escarpment edge, sidewalls and rock niches *Corresponding author.

were eroded by glacial ice during the last glacial maximum (LGM). A hypothesis of glacial dumping over the escarpment sidewalls to produce the morainelike ridges has been suggested (Grab, 1996), yet detailed sedimentological investigations of these deposits are still required to verify a glacial origin. Contemporary snow patterns further reinforce the glaciation theory. It has recently been demonstrated that contemporary snow cover is prolonged in heavily shadowed high escarpment localities where such ridges occur, thus providing an indication to the most likely snow accumulation sites during the LGM (Mulder and Grab, 2002). The occurrence of large relict sorted circles on high (ca. 3400 m a.s.l.) plateau interfluves in the Drakensberg is indicative of Late Quaternary permafrost (Grab, 2002). These patterns may either represent areas that have not been glaciated, or areas preserved beneath cold-based ice, similar to that reported by Rea et al. (1998). Similar Late Quaternary landforms in the Eastern Cape Province contain striated clasts and are said to be glacial moraine, despite the relatively low altitude of ca. 2100 m a.s.l. (Lewis and Illgner, 2001). It has been suggested that glacier development in the Eastern Cape Drakensberg may be the result of an extensive snowblow area, such that the equilibrium line altitude (ELA)

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reflects the combined effects of temperature, precipitation and snowblow, with palaeo-temperatures probably reduced by at least 10
C from those at present (Lewis and Illgner, 2001). This paper investigates recently discovered debris ridges on high altitude (>3100 m a.s.l.) south-facing plateau slopes, located immediately (1–2 km) to the west of the Great Escarpment. The objective is to provide a detailed morphological and sedimentological assessment of the ridges, so as to assist in identifying the processorigin of such ridges and verify the feasibility for Quaternary glaciation in southern Africa.

cover is thin on slopes, consisting mostly of residual and colluvial material. Woody species of Erica and Helichrysum, interspersed with grassland constitutes the alpine belt vegetation (Killick, 1963). The contemporary climate is characterized by wet, mild summers and cold, dry winters with an annual mean temperature of ca. 6
C (Grab, 1997). Precipitation along the escarpment amounts to over 1500 mm (Schulze, 1979), with 70% falling between November and March and less than 10% between May and August (Tyson et al., 1976).

3. Methodology 2. Study area The study sites are located on plateau slopes of the high Drakensberg, eastern Lesotho (Fig. 1). Site 1 is situated on the south-facing aspect of the Tsatsa-LaMangaung range, which reaches an altitude of 3275 m (Figs. 1–3). Detailed field surveys were undertaken for a debris ridge located 2 km west of the Escarpment. Further debris ridges were identified along the southern slopes of the Leqooa Range (site 2), which reaches an altitude of 3431 m (Figs. 4 and 5). The summit area falls within the Drakensberg Group Basalt, formed during Upper Triassic times. Regolith

A topographic survey of site 1 was undertaken in order to produce a topographic map of the main debris ridge using a Total Station 550. A Global Positioning System (GPS) was used in order to obtain coordinates for the stations. Three trenches were dug through the upper, middle and lower crest section of the Tsatsa-LaMangaung ridge (site 1) (Figs. 1 and 2). The trenches were approximately 1 m wide and 1.5 m deep. Sediment analysis was undertaken at 20 cm depth intervals and samples for radiocarbon dating taken at 1.5 m depth in each trench. Clast fabrics were assessed in the field with n ¼ 25 per sampling locality (Dowdeswell and Sharp, 1986), based on the presence of a distinctive a-axis

Fig. 1. The location of study sites in the Drakensberg.

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Fig. 2. Morphological map of site 1—Tsatsa-La-Mangaung.

Fig. 3. The Tsatsa-La-Mangaung ridge.

(a2b ratios of >1.5:1). Pebble shape was calculated through the measurement of a; b and c axes of n ¼ 25 clasts per trench. Five additional test pits were dug to 1 m depth on the adjacent slopes for comparative sedimentological analyses. Pebble fabric was evaluated according to Mark (1973). This three-dimensional method accounts for both pebble orientation and dip, as opposed to twodimensional methods such as rose diagrams, which consider only pebble orientation. This technique treats

individual observations of pebble orientation and dip as unit vectors. In order to extract the eigenvectors and eigenvalues of a 3  3 matrix, three-dimensional vector analysis was used. Eigenvectors (V1 ; V2 and V3 ) and normalized eigenvalues (S1 ; S2 and S3 ) of the matrix are then computed. Eigenvector V1 refers to the direction of maximum clustering and V3 to that of minimum clustering. The eigenvalues summarize fabric strength or the degree of clustering. Eigenvalue S1 measures the strength of clustering about the mean axis V1 ; whereas S3 represents fabric strengths about the axis of minimum clustering (V3 ). Samples were sieved at 1/4 phi intervals to 63 Nm. A sedigraph 5100 was used for the finer fractions. Graphic mean, inclusive graphic standard deviation, inclusive graphic skewness and graphic kurtosis (Folk and Ward, 1957) were calculated in order to summarise the data obtained.

4. Results 4.1. Morphology The Tsatsa-La-Mangaung ridge (site 1) is located at 29
330 3500 S and 29
170 3900 E. It is the only ridge-like feature along the Tsatsa-La-Mangaung mountain range

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Fig. 4. Morphological map of site 2—Leqooa Valley.

and occupies the base of a shallow southeast-facing depression in the mountainside, approximately 2 km west of the Great Escarpment (Figs. 2 and 3). The ridge is 214 m in length and varies between 14–30 m in width (Figs. 2 and 3). The altitudinal range of the ridge varies from 3005–3088 m a.s.l. The ridge is steepest on its eastern side, where it reaches a maximum height of 16 m and a side-gradient of 26
at section C–C (Fig. 2). The side gradients on the western flank vary from 10–16
(cross section B–B, C–C and D–D). The ridge surface is dominated by tussock grass and a large concentration of boulders ranging from 0.15 to 1.80 m in length. The Leqooa Valley (site 2) hosts a pair of ridges that are located below a south-facing slope hollow, approximately 1.5 km west of the Great Escarpment (Figs. 1, 4 and 5). The hollow-shaped basin above the ridges extends to over 1100 m across and reaches an altitude of 3431 m a.s.l. The slope directly above the ridges consists of fragmented bedrock with little regolith cover, whilst the adjacent slopes at similar altitudes host a deeper regolith cover with solifluction lobes (Fig. 4). The eastern ridge is located at 29
440 3500 S, 29
070 0400 E, whilst the western ridge is positioned at 29
440 3500 S, 29
060 5100 E. The two ridges terminate at a similar

altitude of 3054 and 3057 m, where the distance between them is 122 m (Fig. 4). The ridges extend upslope to 3112 and 3108 m a.s.l., where they are 195 m apart. The western ridge is 308 m in length, up to 67 m in width and 12 m in height. The eastern ridge is 230 m in length, up to 64 m wide and 10 m high. The ridges predominantly consist of boulders and stones varying from 0.05 to 1.6 m in a-axis length. 4.2. Sedimentology of the Tsatsa-La-Mangaung deposit The lithofacies present at the three trenches consist of a present-day soil underlain by an unconsolidated diamict within which a variety of different sized boulders are present, held together by a sandy matrix (Fig. 6). All three trenches indicate an increased dominance of coarser fractions below 0.6 m (Fig. 7). At 1.2 m depth at trench 3, gravel constitutes 90% of the sample, whilst percentages for the pits indicate a much greater proportion of sand and silt at this depth (Fig. 7). Statistical grain size parameters were calculated according to the formulas after Folk and Ward (1957) (Table 1). Graphic mean size ranges from 1.11 to 1 phi for samples T1 40–140, T2 40–120 and T3 40–140.

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Fig. 5. The western (a) and eastern (b) ridges at site 2—Leqooa Valley.

Sorting varies from 3.29 to 0.07, which represents very well sorted to poorly sorted components according to Folk and Ward (1957). Skewness ranges from –0.98 to – 1 for the trenches, indicating that they are all very negatively skewed, whilst the data for the test pits show a much wider range of skewness. Kurtosis for the trenches ranges from 0.83 to X3, meaning mesokurtic to leptokurtic, with the largest concentration of the data falling within the leptokurtic ‘‘peakedness’’. The pit samples range from platykurtic to leptokurtic. The particle size analysis and associated statistical results indicate that the material sampled in the three trenches is significantly different to the material sampled in the pits away from the ridge. Clast orientation appears to be in the direction of the ridge crest in the cases of trenches 1 and 3 (Fig. 8). Clasts in trench 1 are predominantly oriented downslope, whereas the clast orientation for trench 3 is less significant but falls within 180–270
. The clast orientation in trench 2 is somewhat more transverse to the ridge itself. Dip varies widely across the three trenches. In the case of trenches 1 and 3, clasts tend to dip away from the slope whilst in trench 2 they dip into the slope (Fig. 8). All sediments within the Tsatsa-La-Mangaung deposit have a low isotropy value (I ¼ 0:0520:14)

Fig. 6. The vertical profile of Trench 3.

and a relatively high elongation fabric (E ¼ 0:5720:77) (Fig. 9). The clasts within trenches 1 and 3 display maximum clustering (V1 ) in the direction of the ridge deposit (Fig. 8). The low isotropy and high elongation values indicate strong preferred orientation (Benn, 1994; Benn and Evans, 1998). Particle shape is primarily bladed to very bladed for the three trenches (Fig. 10), with a number also falling within the elongate category in the case of trenches 1 and 3 (Figs. 10b and d). Most clasts are very angular to angular, with a very small percentage falling within the rounded categories. RA values range between 84 and 88 and the C40 ratio (after Benn and Ballantyne, 1993) ranges from 52 to 60, indicating that the clasts are highly angular and lack compactibility.

5. Possible process origins Slope deposits reported from the high mountains of southern Africa include lacustrine deposits (Hanvey and

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Fig. 7. Cumulative percentage graphs for sediment from three trenches along the Tsasta-La-Manuang deposit and from pits adjacent to the deposit.

Table 1 Numerical description of particle size distribution Trench 1 Mean Sorting Skewness Kurtosis

T1 0 2.01 2.78 0.18 0.86

T1 20 1.50 2.96 0.72 0.78

T1 40 1.11 2.78 0.83 0.83

T1 60 1.10 3.04 0.98 0.89

T1 80 0.79 2.71 1 0.87

T1 100 0.46 2.32 1 X3

T1 120 0.39 2.15 1 1.97

T1 140 0.15 1.69 1 X3

Trench 2 Mean Sorting Skewness Kurtosis

T2 0 3.34 3.31 0.26 0.74

T2 20 1.33 2.60 0.48 0.86

T2 40 0.36 1.38 1 X3

T2 60 0.50 2.30 1 1.05

T2 80 0.49 2.23 1 2.70

T2 100 0.44 2.21 1 1.16

T2 120 0.41 0.98 1 X3

Trench 3 Mean Sorting Skewness Kurtosis

T3 0 3.18 2.84 0.01 0.94

T3 20 3.05 3.39 0.02 0.57

T3 40 0.85 2.62 1 0.85

T3 60 0.46 2.38 1 1.31

T3 80 0.89 2.80 1 0.88

T3 100 0.88 0.70 1 X3

T3 120 1 0.07 1 X3

T3 140 0.5 1.27 1 X3

Pits Mean Sorting Skewness Kurtosis

Pit 1 1.86 3.02 0.58 0.59

Pit 2 0.97 2.75 1 0.81

Pit 3A 0.16 1.92 1 5.08

Pit 3B 0.38 2.23 1 1.35

Pit 3C 3.65 2.90 0.05 1.17

Pit 4A 2.15 2.97 0.26 0.62

Pit 4B 2.65 3.09 0.06 0.62

Pit 5 1.98 3.29 0.68 0.61

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Fig. 8. Clast orientation, dip and contouring at trenches 1 (a–c), 2 (d–f) and 3 (g–i).

Lewis, 1990), block deposits (Boelhouwers, 1999; Grab, 1999; Boelhouwers et al., 2002), slope wash and solifluction deposits (Hanvey and Lewis, 1991), debris flow deposits (Illgner, 1995; Lewis, 1996; Boelhouwers et al., 1998; Grab, 1999), protalus ramparts (Lewis, 1994), rock glaciers (Lewis and Hanvey, 1993) and glacial moraines (Grab, 1996; Lewis and Illgner, 2001). Some of these suggested deposit types have been questioned and may have been misinterpreted (Grab, 2000; Boelhouwers and Meiklejohn, 2002). It is thus necessary to consider a variety of different process origins for the formation of the ridge deposits located at sites 1 and 2, which are discussed in the context of the results obtained.

5.1. Debris flows Debris flows are known to develop distinctive features, which consist of an erosional scar and two major depositional features including double debris flow levees and bouldery frontal lobes. The levees occur on the sides of a channel and are formed by snout materials that move laterally and finer-grained material that produces waves (Rapp and Nyberg, 1981; Johnson and Rodine, 1984). However, the ridges at sites 1 and 2 are not associated with typical debris flow erosional scars or depositional lobes. The topographic setting at site 1 may be conducive for the occurrence of debris flows, as these are most likely to occur on slopes steeper

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than 27
that exceed the angle of internal friction (Lewin and Warburton, 1994). However, given the small catchment area above the deposit, debris flow initiation is most unlikely. The slope gradient at site 2 is between 10 and 14
which is considerably lower than that required for the initiation of a debris flow (Lewin and Warburton, 1994). Typical debris flow attributes, which are represented within the Tsatsa-La-Mangaung deposit, include angu-

lar material that is poorly sorted (Owen, 1991). No sorting of grain sizes occurs in debris flow deposits because they are mixed during flow. They also contain a wide variety of grain size distribution ranging from large boulders to clay (Coussot and Meunier, 1996). Debris flows are slightly skewed towards the fines and mesokurtic (Owen, 1991). The importance of clay to enhance mobility of debris flows has been highlighted by Johnson and Rodine (1984). However, particle size analysis at site 1 revealed that the material was skewed towards the coarser fraction. Other attributes that are typical of debris flow deposits, yet not adequately represented within the Tsatsa-La-Mangaung deposit include variability for both orientation and imbrication, and orientations that range from parallel to transverse or oblique to flow direction (Lawson, 1979; Mills, 1984; Gale and Hoare, 1991; Owen, 1991; Benn, 1994). Very low isotropy values (0.05–0.14) and high elongation values (0.57–0.77) were obtained for the Tsatsa-La Mangaung deposit, whilst values of low to moderate isotropy (0.100–0.450) and variable elongation (0.210– 0.740) have been recorded for debris flows (Lawson, 1979; Mills, 1984; Benn, 1994). 5.2. Pronival ramparts

Fig. 9. Till fabric using eigenvalue data. The range of orthorhombic shapes and fabric data are for trenches 1, 2 and 3 (after Benn, 1994).

Pronival ramparts have been morphologically defined as either arcuate, where the rampart curves upslope at one or both ends, or linear, where the rampart crest is effectively straight (Ballantyne and Kirkbride, 1986;

Fig. 10. Triangular diagrams after Sneed and Folk (1958): (a) the descriptive classes according to Sneed and Folk (1958) where C = compact, P = platy, B = Bladed, E = elongate, V = very; (b–d) triangular diagrams and roundness histograms for trenches 1, 2 and 3 respectively. C40 is the percentage of sample with c : a [0.4 (Benn and Ballantyne, 1993)]; RA is the percentage of sample in very angular and angular categories (Benn and Ballantyne, 1993). Roundness categories: VA = very angular, A = angular, SA = sub-angular, SR = sub-rounded, R = rounded.

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Shakesby, 1997). Pronival ramparts can form ridges that are 300–1800 m long, 50–350 m wide and 10–50 m high (Linder and Marks, 1984). Although pronival ramparts are morphologically similar to the feature at site 1, they usually form parallel to the mountain slopes (Linder and Marks, 1984) rather than linearly downslope, as is the case at the Drakensberg sites. Pronival ramparts generally consist of coarse clastic sediments with an absence of fines even at depth and usually host poorly sorted but highly angular clasts (Washburn, 1979; Linder and Marks, 1984; Ballantyne and Kirkbride, 1986; Hall and Meiklejohn, 1997), as is the case at the Tsatsa-La-Mangaung deposit. Although pronival ramparts usually develop below debris producing rock walls (Ballantyne and Kirkbride, 1986; Shakesby, 1997), scarp faces are absent at the Tsatsa-La-Mangaung site and we thus question whether supranival debris transport processes (c.f. Ono and Watanabe, 1986; Shakesby et al., 1995) or snow-push processes (c.f. Shakesby et al., 1999) would have produced such a single debris ridge. 5.3. Valley-fill erosion remnants Valley-fill erosion remnants occur as a result of preferential fluvial erosion through a deep regolith cover. Several of the cross-sections indicate that the features are upraised above the level of the surrounding regolith covered slopes (Figs. 2 and 4) which would be expected from a valley-fill erosion remnant, as the surrounding slope would have been eroded preferentially, thus making these features more prominent. However, rills or fluvial channels do not always extend upslope above the ridges, or occur adjacent to the ridges, thus discounting a hypothesis that these are remnants of fluvially incised debris mantles. In addition, the particle size analysis and statistical grain size analysis for the three trenches and pits adjacent to the ridge indicate that the ridge consists of coarse sediment whilst the pits contain much finer sediment. 5.4. Glacial moraines The different types of moraines are not always morphologically distinct and it is often difficult to distinguish between tectonic, dump and ablation type moraines in the field (Bennett and Glasser, 1996). Push moraines are usually produced by minor glacial advances and are typically less than 10 m in height (although they can attain greater heights) and are frequently lobate or arcuate in plan but are often meandering when looked at in detail (Bennett and Glasser, 1996; Benn and Evans, 1998). In some cases they may display a single crest orientated parallel to the ice margin (Bennett, 2001). Push moraines may consist of subglacial till, mass movement deposits, water-sorted sediments or large boulders, depending on the nature of

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the sediment on the glacier foreland (Benn and Evans, 1998). Push moraines commonly display an asymmetric cross-section, with gentle proximal slopes and steep distal slopes, and are thus similar to the cross section presented in Fig. 2 (cross-section C–C) for the TsatsaLa-Mangaung ridge. Lateral moraines form by the deposition of glacial debris, which may be supraglacially and subglacially transported and therefore highly variable. Moraine morphology depends on where the debris is predominantly derived from. If debris is primarily derived from the valley sides, the moraine will have a ‘bench-like’ form (Bennett and Glasser, 1996, p. 231) or may be absent. However, if the debris supply from the valley sides is less important, a distinct ridge may develop (Bennet and Glasser, 1996). Ice-marginal meltwater may also alter the morphology of the lateral moraine by depositing material between the lateral moraine and the valley wall. Cross-glacier asymmetry is typical of lateral moraines, with the lateral moraine on one side of the valley being larger than the other, which reflects debris distribution within the glacier (Bennett and Glasser, 1996). According to Boulton (1976), supraglacially derived debris has a silt-clay content of below 15% owing to the lack of a tractive phase. This is further reinforced by Benn and Evans (1998), who state that supraglacial debris tends to be predominantly coarse grained, lacking in fine sand and smaller fractions. Given that the TsatsaLa-Mangaung ridge contains less than 7% clay, it could conform to a supraglacial depositional process. Tills host a wider range of particle sizes than most other sediments, thus making them extremely poorly sorted (Gale and Hoare, 1991). The clasts measured in our study tend to be elongate and platy, and angular to very angular, which corresponds with results for supraglacial till, for which it is said that angular shapes reflect particle fracture during weathering and transport onto the glacier (Benn and Ballantyne, 1993; Benn and Evans, 1998). The wide range of fabric orientations displayed in Fig. 8 may reflect varying spatio-temporal mechanisms of material deposition and/or subsequent re-working of sediments. A further suggestion for this variability in fabric results may be owing to particle shape and size differences, which are said to influence particle orientation (e.g. Kjc! r and Kruger, . 1998; Bennett et al., 1999; Carr and Rose, 2003). Prolate (rod-shaped) grains tend to roll along their axes, resulting in a transverse fabric, whereas oblate (blade-shaped) grains are more likely to remain parallel to the stress field orientation (Boulton, 1970). Similarly, under conditions of a single stress type, smaller particles usually align transverse to the stress orientation whilst larger grains align parallel (Carr and Rose, 2003). Assuming a south-eastward stress field for the Tsatsa-La-Mangaung ridge, clasts in trenches 1 and 3 display a transverse orientation, which could result

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from a high number of elongate/prolate grains present in the sample (Figs. 8a, d, g and 10). In contrast, the greater number of bladed/oblate grains present in trench 2 would explain the more parallel clast orientation to such a stress field. It has also been suggested that samples displaying transverse orientations have been deposited under higher stress than those displaying parallel orientations (Carr and Rose, 2003), which could indicate that samples from trenches 1 and 3 reflect a higher stress situation than those from trench 2. As stated previously, C40 ratios for sediment from the three trenches (Figs. 10b–d) range between 52 and 60, which is similar to values obtained for supraglacially deposited material (64–78) recorded by Benn and Ballantyne (1993) and that of actively transported scree (50–78) obtained by Benn and Evans (1998). Roundness and angularity (RA) ranges from 84–88 (Fig. 10b–d), which is once again similar to findings by Benn and Ballantyne (1993), where RA values ranged between 82 and 90 for supraglacial samples. It is proposed that the deposit at the Tsatsa-LaMangaung site is a latero-push or dump moraine, given that the morphological attributes are typical of such a moraine type. In addition, particle size and shape results would support a mechanism of supraglacialy transported debris. The Leqooa Valley ridges display morphological and topographic-positional characteristics that strongly resemble those of lateral moraines.

whilst precipitation along the Drakensberg was reduced by ca. 30% (Partridge et al., 1999; Tyson and Partridge, 2000). Elsewhere in the southern hemisphere, LGM temperatures were depressed by 6–8
C (e.g. Barnola et al., 1987; Denton et al., 1999), and thus the suggested southern African decreases are not exceptional. Ohmura et al. (1992) produced a best-fit climate curve for the ELA of 70 glaciers, which offers opportunities to make deductions on the likely climatic changes required for glaciation at the Drakensberg study sites. Given the palaeoclimatic extrapolations for the Drakensberg, it is possible to plot the likely LGM summer temperature and annual precipitation for study sites 1 and 3 on the ELA curves produced by Ohmura et al. (1992) (Fig. 11). Assuming a likely palaeoprecipitation of 1100 mm/annum, both sites would fall within the best-fit curves for glacier ELA’s, even with the most conservative temperature reduction of 5
C (Fig. 11).

6. Discussion and implications 6.1. Climate Palaeoclimatic extrapolations are difficult to make for the Drakensberg region, given the absence of contemporary climate data sets. Nevertheless, the mean summer temperature recorded during 2000/2001 for the Sani Valley (2878 m a.s.l.) was 10
C. It should be noted that this was a particularly warm summer throughout southern Africa. Assuming an average global environmental lapse rate of 0.55
C/100 m1 (Meyer, 1992), the slope above the Tsatsa-La-Mangaung ridge at 3100 m a.s.l., would have had an average summer temperature of 8.8
C, whilst the slope above the Leqooa Valley deposit at 3200 m a.s.l. is expected to have had a mean summer temperature of 8.2
C during 2000/2001. Contemporary precipitation amounts to 1500–1600 mm/annum along the escarpment (Killick, 1963; Schulze, 1979). Southern African temperature decreases during the LGM are thought to have been between 5
and 7
C (Heaton et al., 1986; Talma and Vogel, 1992; Partridge et al., 1999; Tyson and Partridge, 2000; Grab and Simpson, 2000),

Fig. 11. Plotting of the high Drakensberg LGM palaeoclimates for study sites 1 and 3 within the best-fit climate curve for ELA of 70 glaciers, after Ohmura et al. (1992).

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Table 2 AMS dates for the Tsatsa-La-Mangaung deposit (sampling depths: T1 ¼ 1:5 m, T2 ¼ 1:35 m, T3 ¼ 1:45 m) Anal. No. 1Gr–A

Sample designation

13

Radiocarbon age yrs BP

21456 21459 21457

Sani Top T1 Sani Top T2 Sani Top T3

20.8 21.3 16.1

13,7907110 17,3007100 11,700770

C (% PDB)

Table 3 Suitability of geomorphic processes and landform types for the various debris ridge attributes

Geomorphic process Debris flow Nival Water erosion Glacial

Topographic positioning

Morphology

Particle size

Fabric

Sorting

Shape

X X X YES

X YES X YES

YES YES X YES

X X ? YES

YES YES ? YES

YES YES YES YES

6.2. Age of the Tsatsa-La-Mangaung ridge Three AMS dates were obtained for the upper, mid and lower portions of the Tsatsa-La-Mangaung ridge and indicate ages that are all pre-Holocene (Table 2). The dates were obtained for sediment sampled at a depth of 1.5 m (trench 1), 1.35 m (trench 2) and 1.45 m (trench 3). These sampling depths represent the maximum depth of each trench. Material tested was soil organic matter (SOM), which is generally introduced into the soil as either root material or organic detritus at the surface. There were very small amounts of SOM in the samples, suggesting slow biological turnover. It is likely that carbon which post-dates deposition, was introduced into the sediment, and would thus provide younger ages than expected. The oldest date of 17 3007100 Radiocarbon yrs BP indicates that the deposit originated when glaciers were at their maximum during the Late Pleistocene (21–18 kyr), which is also known to have been a cold period throughout southern Africa (Partridge et al., 1999). The two younger dates (13 7907110 and 11 700770 yrs BP) may reflect the intrusion of slightly younger SOM at the depths sampled (Table 2). Alternatively, the younger dates obtained at the upper and lower parts of the ridge may indicate a final phase of deposition, prior to deglaciation. The dates obtained correspond well to documented glacial advances in equatorial and southern hemisphere regions such as the Venezuelan Andes (Schubert, 1979; Mahaney et al., 2000), Chilean Andes (Denton et al., 1999), Mt. Kenya (Mahaney et al., 1989) and New Zealand Alps (Benn and Evans, 1998; Kirkbridge and Brazier, 1998). Deglaciation in the northern Venezuelan Andes began after 12 000 yrs BP (Mahaney et al., 2000),

Geomorphic landform Debris flow deposits Pronival rampart Incised valley fill Moraine

whilst moraines dating back to 15 000 yrs BP have been identified to 3200 m a.s.l. on Mt. Kenya, East Africa (Mahaney et al., 1989).

7. Conclusions The elongate debris ridges discussed in this paper have thus far only been identified on high altitude (>3000 m) south-facing slopes along the southern Drakensberg escarpment. These sites coincide with areas of greatest snow accumulation during contemporary winters and would have a bearing on snow cover during the LGM when there was a high frequency of cold fronts (Mulder and Grab, 2002). From a climatological perspective, the deposits were formed during a time that was conducive to niche/cirque glaciation on slopes above the deposits. The morphological and sedimentological characteristics of the deposits most suitably typify attributes of glacial moraine (Table 3). Based on the findings obtained thus far, it is proposed that the Tsatsa-La-Mangaung ridge is a latero-push or dump moraine, whilst the two ridges in the Leqooa Valley are most characteristic of lateral moraines. The results offer strong supporting evidence for niche/ cirque glaciation during the LGM in the high Drakensberg of southern Africa. According to Lewis and Illgner (2001), moraine ridges in the Eastern Cape Drakensberg (about 150 km south of the high Drakensberg sites) indicate an ELA of ca. 2100 m for the LGM. It was further suggested that these small glaciers were capable of forming at such low altitudes owing to an extensive snowblow area and a palaeo-temperature drop of ca. 10
C (Lewis and Illgner, 2001). In contrast, the restricted geographical distribution of the high Drakensberg

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moraine ridges to the central and southern regions, south-facing slopes above 3000 m a.s.l., and areas along the Great Escarpment, suggests that conditions conducive to the formation of small niche/cirque glaciers was climatologically restricted to a few sites and that these glaciers did not survive long enough to produce more substantial erosional and depositional remnants.

Acknowledgements Many thanks to Dr. S. Woodborne and staff at QUADRU, CSIR, for discussions and providing the AMS dates. Our appreciation goes to Dr. Simon Carr and Dr. Wishart Mitchell, who provided detailed and constructive suggestions for improving the paper, however, any remaining flaws are solely ours. We acknowledge the financial support provided through the Wits University Research Committee Grant. We thank Wendy Job for improving the maps.

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