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Comment on “the palaeoenvironmental interpretation of colluvial sediments and palaeosols of the late pleistocene hypothermal in Southern Africa”
Palaeogeography, Palaeoclimatology, Palaeoecology, 52 (1985): 159--163 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
159
Discussion
COMMENT ON " T H E P A L A E O E N V I R O N M E N T A L I N T E R P R E T A T I O N OF COLLUVIAL SEDIMENTS AND PALAEOSOLS OF THE LATE PLEISTOCENE H Y P O T H E R M A L IN SOUTHERN A F R I C A "
I. BAILLIE Department of Geography, The Marlborough Building, Polytechnic of North London, 383 Holloway Road, London N7 (Great Britain) (Received December 5, 1984) In a recent issue of Palaeogeography, Palaeoclimatology, Palaeoecology the Swaziland-centred archaeological and geomorphological group have provided a f u r t her interesting contribution to our understanding of the recent environmental history of southeastern Africa (Watson et al., 1984). Their emphasis on the previously neglected but very extensive colluvial deposits is particularly welcome. T h e y describe a num ber of sections in these deposits, located in f our countries and spread over a distance of more than 1200 kin. T h e y interpret an array o f sedimentological, archaeological and radio-isotope data as evidence that the younger and m ore widespread of the two main deposits {the Mphunga colluvium) was laid down gradually by low energy slope processes operating in a regime of severe but probably fluctuating aridity during t he period 30, 000--12, 000 B.P. This interpretation may eventually be validated for the deposits described and sampled. However, the range o f observations appears t o be somewhat restricted, and the data seem to be derived from only one part o f the full range of colluvia. It is therefore appropriate to sound a not e of caution before the group's conclusions are uncritically extrapolated on a broad regional scale. In an earlier a c c o u n t by the same group it was acknowledged that colluvial deposits in Swaziland have been reported in a range of physiographic regions, altitudes and climates. However, they decided to concentrate on the colluvia of the semi-arid Western Lowveld subregion because " t h e deposits below 400 m have proved to be of greater stratigraphic value" {Price Williams et al., 1982, p. 51). In addition to their wide extent, depth and completeness, one of the advantages of the colluvial deposits in this subregion is that they are extensively gullied, making examination easier than in the less dissected colluvia of higher and mo r e humid parts of Swaziland. However, since then, the conscious choice to co n cent r a t e attention on colluvial deposits in this restricted part of their range appears to have been transmuted into the idea that these sediments are of significance only in that particular environment. Thus the general description of the deposits in the more recent account states that:
160 "The whole sequence is capped by a modern soil of the Solonetz t y p e " {Watson et al., 1984, p.227) and the current climatic conditions are characterised by: "A noticeable feature of all the colluvial sites is a similarity in the present-day rainfall. The range is from 600 to 750 mm, falling in the six winter m o n t h s " (Watson et al., 1984, p.229). Apart from the elementary error of giving southeastern Africa a Mediterranean rather than summer rainfall climate, the effect of these and other statements is to create an impression that colluvial deposits are specific to a narrow range of physical environments. In fact colluvium, as broadly defined as: "poorly sorted mixtures of clay, silt, sand and gravel size particles" (Watson et al., 1984, p.226) is found in all physiographic regions of Swaziland. The deposits tend to be patchier, shallower, and less continuous in areas of high relief. They are therefore of limited extent in the more rugged Highveld and Lebombo regions, and are widespread in the gentler topographies of both the Western Middleveld and Western Lowveld regions. Because of their wide spatial distribution, the deposits are subject to a variety of current climates, with annual rainfalls ranging from over 1250 mm down to 500 mm. The deposits are capped with a range of m o d e m soils, according to local conditions of lithology, site stability and climate. As well as the Solonetzes described by Watson et al. (1984) softs of the Ferralsol, Acrisol, Luvisol, Cambisol, Vertisol, Planosol and Arenosol units (FAO, 1974) have been identified on colluvial parent materials in Swaziland (Murdoch, 1968). At a smaller scale, a similar kind of unjustified generalisation from a restricted range of observations appears to have been made in the chemical characterisation of the colluvia of the Western Lowveld. Thus "The ESP values of Swaziland donga colluvium average no less than 45%" (Watson et al., 1984, p.228) becomes: "The high exchangeable sodium content inherent in the c o l l u v i a . . . " (Watson et al., 1984, p.228). Rather than assuming that all of the colluvia are sodic, an equally plausible interpretation of the gully wall ESP data is that the more sodic colluvia are particularly susceptible to gullying. In fact a range of duplex soils with abrupt textural changes can be found on colluvia in the Western Lowveld. Some of these are Solonetzes of Zwide and Zikane series {using Swaziland rather than South African soil series names) in which subsoil ESP values can range from less than 10% to over 50%. There are also non-sodic Planosols of Habelo and related series (Murdoch, 1968). Other duplex soils on these deposits in this subregion have coarse textured upper horizons that may be deep enough to qualify the profiles as Arenosols of Enkulunyo series, Because of their lower subsoil ESP values, m a n y of these soils tend to be less prone to clay dispersion and extensive gullying, and will therefore be under-represented if gully sections are the main sites of observation. These comments, like the data of Watson et al. (1984), have concentrated on Swaziland. Limited field experience elsewhere in the southern Africa suggests that colluvial deposits occur in a wider range o f settings than Watson and his co-authors imply. Thus fieldwork in the Chimoio area of
161 central M o c a m b i q u e revealed colluvial d e p o s i t s in s u b h u m i d climates, w i t h m e a n a n n u a l rainfalls up t o 1100 m m , a n d c a p p e d w i t h a range of m o d e r n soil types. A m o r e general and flexible i n t e r p r e t a t i o n o f the colluvia is to view t h e m as the result o f a phase in w h i c h t r a n s p o r t processes o n slopes are m o r e active t h a n t h o s e in channels. T h e m a t e r i a l s e n t r a i n e d t e n d to a c c u m u l a t e m o r e or less w i t h i n the i m m e d i a t e vicinity o f their source, m a n t l i n g the valley f l o o r and l o w e r slopes. Dissection a n d stripping of colluvial d e p o s i t s are b r o u g h t a b o u t b y a change in the balance, in the f a v o u r o f channel t r a n s p o r t . This c h a n g e m a y well be caused b y regional changes in c l i m a t e o f t h e t y p e p o s t u l a t e d b y W a t s o n et al. H o w e v e r , t h e possible c o n t r i b u t i o n o f o t h e r f a c t o r s such as n o n ~ l i m a t i c changes in v e g e t a t i o n b r o u g h t a b o u t b y m i g r a t i o n , grazing pressure etc. m u s t also be c o n s i d e r e d . L o c a l changes in erosional base level due to m i g r a t i o n o f nick points or o t h e r causes (Baillie, 1970) s h o u l d also be t a k e n into a c c o u n t . T h e relative i m p o r t a n c e o f these and o t h e r f a c t o r s will b e c o m e clearer as m o r e e v i d e n c e a c c u m u l a t e s . H o w e v e r , in the m e a n t i m e , it is necessary to be as flexible and rigorous as possible w i t h the existing d a t a if t h e i r full value is to be realised. REFERENCES Baillie, I. C., 1970. Some Swaziland examples of localised control of base-levels: the Middle Ngwempsi basin. S. Afr. Geogr. J., 52: 77--86. FAO, 1974. FAO-UNESCO Soil Map of the World. Vol. I. Legend. F,A.O., Rome. Murdoch, G., 1968. Soils and land capability in Swaziland. Swaziland Ministry of Agriculture, Mbabane, 360 pp. Price Williams, D., Watson, A. and Goudie, A. S., 1982. Quaternary colluvial stratigraphy, archaeological sequences and palaeoenvironment in Swaziland, southern Africa. Geogr. J., 148: 50--67. Watson, A., Price Williams, D. and Goudie, A. S., 1984. The palaeoenvironmental interpretation of colluvial sediments and palaeosols of the late Pleistocene hypothermal in Southern Africa. Palaeogeogr., Palaeoclimatol., Palaeoecol., 45: 225--249.
R E P L Y T O C O M M E N T ON " T H E P A L A E O E N V I R O N M E N T A L INTERPRETATION OF COLLUVIAL SEDIMENTS AND PALAEOSOLS O F T H E L A T E P L E I S T O C E N E H Y P O T H E R M A L IN S O U T H E R N A F R I C A "
A. WATSON 9 Hartley Terrace Boston MA 02134 (U.S.A.)
(Received February 20, 1985) B e f o r e r e s p o n d i n g to the specific p o i n t s r e i t e r a t e t h a t the d a t a o n t h e d i s t r i b u t i o n s e d i m e n t s p r e s e n t e d b y W a t s o n et al. ( 1 9 8 4 ) We are grateful, t h e r e f o r e , to I a n Baillie f o r
raised above, we w o u l d like to and c h e m i s t r y of t h e colluvial are all f r o m gully ( d o n g a ) sites. providing this a d d i t i o n a l infor-
159
Discussion
COMMENT ON " T H E P A L A E O E N V I R O N M E N T A L I N T E R P R E T A T I O N OF COLLUVIAL SEDIMENTS AND PALAEOSOLS OF THE LATE PLEISTOCENE H Y P O T H E R M A L IN SOUTHERN A F R I C A "
I. BAILLIE Department of Geography, The Marlborough Building, Polytechnic of North London, 383 Holloway Road, London N7 (Great Britain) (Received December 5, 1984) In a recent issue of Palaeogeography, Palaeoclimatology, Palaeoecology the Swaziland-centred archaeological and geomorphological group have provided a f u r t her interesting contribution to our understanding of the recent environmental history of southeastern Africa (Watson et al., 1984). Their emphasis on the previously neglected but very extensive colluvial deposits is particularly welcome. T h e y describe a num ber of sections in these deposits, located in f our countries and spread over a distance of more than 1200 kin. T h e y interpret an array o f sedimentological, archaeological and radio-isotope data as evidence that the younger and m ore widespread of the two main deposits {the Mphunga colluvium) was laid down gradually by low energy slope processes operating in a regime of severe but probably fluctuating aridity during t he period 30, 000--12, 000 B.P. This interpretation may eventually be validated for the deposits described and sampled. However, the range o f observations appears t o be somewhat restricted, and the data seem to be derived from only one part o f the full range of colluvia. It is therefore appropriate to sound a not e of caution before the group's conclusions are uncritically extrapolated on a broad regional scale. In an earlier a c c o u n t by the same group it was acknowledged that colluvial deposits in Swaziland have been reported in a range of physiographic regions, altitudes and climates. However, they decided to concentrate on the colluvia of the semi-arid Western Lowveld subregion because " t h e deposits below 400 m have proved to be of greater stratigraphic value" {Price Williams et al., 1982, p. 51). In addition to their wide extent, depth and completeness, one of the advantages of the colluvial deposits in this subregion is that they are extensively gullied, making examination easier than in the less dissected colluvia of higher and mo r e humid parts of Swaziland. However, since then, the conscious choice to co n cent r a t e attention on colluvial deposits in this restricted part of their range appears to have been transmuted into the idea that these sediments are of significance only in that particular environment. Thus the general description of the deposits in the more recent account states that:
160 "The whole sequence is capped by a modern soil of the Solonetz t y p e " {Watson et al., 1984, p.227) and the current climatic conditions are characterised by: "A noticeable feature of all the colluvial sites is a similarity in the present-day rainfall. The range is from 600 to 750 mm, falling in the six winter m o n t h s " (Watson et al., 1984, p.229). Apart from the elementary error of giving southeastern Africa a Mediterranean rather than summer rainfall climate, the effect of these and other statements is to create an impression that colluvial deposits are specific to a narrow range of physical environments. In fact colluvium, as broadly defined as: "poorly sorted mixtures of clay, silt, sand and gravel size particles" (Watson et al., 1984, p.226) is found in all physiographic regions of Swaziland. The deposits tend to be patchier, shallower, and less continuous in areas of high relief. They are therefore of limited extent in the more rugged Highveld and Lebombo regions, and are widespread in the gentler topographies of both the Western Middleveld and Western Lowveld regions. Because of their wide spatial distribution, the deposits are subject to a variety of current climates, with annual rainfalls ranging from over 1250 mm down to 500 mm. The deposits are capped with a range of m o d e m soils, according to local conditions of lithology, site stability and climate. As well as the Solonetzes described by Watson et al. (1984) softs of the Ferralsol, Acrisol, Luvisol, Cambisol, Vertisol, Planosol and Arenosol units (FAO, 1974) have been identified on colluvial parent materials in Swaziland (Murdoch, 1968). At a smaller scale, a similar kind of unjustified generalisation from a restricted range of observations appears to have been made in the chemical characterisation of the colluvia of the Western Lowveld. Thus "The ESP values of Swaziland donga colluvium average no less than 45%" (Watson et al., 1984, p.228) becomes: "The high exchangeable sodium content inherent in the c o l l u v i a . . . " (Watson et al., 1984, p.228). Rather than assuming that all of the colluvia are sodic, an equally plausible interpretation of the gully wall ESP data is that the more sodic colluvia are particularly susceptible to gullying. In fact a range of duplex soils with abrupt textural changes can be found on colluvia in the Western Lowveld. Some of these are Solonetzes of Zwide and Zikane series {using Swaziland rather than South African soil series names) in which subsoil ESP values can range from less than 10% to over 50%. There are also non-sodic Planosols of Habelo and related series (Murdoch, 1968). Other duplex soils on these deposits in this subregion have coarse textured upper horizons that may be deep enough to qualify the profiles as Arenosols of Enkulunyo series, Because of their lower subsoil ESP values, m a n y of these soils tend to be less prone to clay dispersion and extensive gullying, and will therefore be under-represented if gully sections are the main sites of observation. These comments, like the data of Watson et al. (1984), have concentrated on Swaziland. Limited field experience elsewhere in the southern Africa suggests that colluvial deposits occur in a wider range o f settings than Watson and his co-authors imply. Thus fieldwork in the Chimoio area of
161 central M o c a m b i q u e revealed colluvial d e p o s i t s in s u b h u m i d climates, w i t h m e a n a n n u a l rainfalls up t o 1100 m m , a n d c a p p e d w i t h a range of m o d e r n soil types. A m o r e general and flexible i n t e r p r e t a t i o n o f the colluvia is to view t h e m as the result o f a phase in w h i c h t r a n s p o r t processes o n slopes are m o r e active t h a n t h o s e in channels. T h e m a t e r i a l s e n t r a i n e d t e n d to a c c u m u l a t e m o r e or less w i t h i n the i m m e d i a t e vicinity o f their source, m a n t l i n g the valley f l o o r and l o w e r slopes. Dissection a n d stripping of colluvial d e p o s i t s are b r o u g h t a b o u t b y a change in the balance, in the f a v o u r o f channel t r a n s p o r t . This c h a n g e m a y well be caused b y regional changes in c l i m a t e o f t h e t y p e p o s t u l a t e d b y W a t s o n et al. H o w e v e r , t h e possible c o n t r i b u t i o n o f o t h e r f a c t o r s such as n o n ~ l i m a t i c changes in v e g e t a t i o n b r o u g h t a b o u t b y m i g r a t i o n , grazing pressure etc. m u s t also be c o n s i d e r e d . L o c a l changes in erosional base level due to m i g r a t i o n o f nick points or o t h e r causes (Baillie, 1970) s h o u l d also be t a k e n into a c c o u n t . T h e relative i m p o r t a n c e o f these and o t h e r f a c t o r s will b e c o m e clearer as m o r e e v i d e n c e a c c u m u l a t e s . H o w e v e r , in the m e a n t i m e , it is necessary to be as flexible and rigorous as possible w i t h the existing d a t a if t h e i r full value is to be realised. REFERENCES Baillie, I. C., 1970. Some Swaziland examples of localised control of base-levels: the Middle Ngwempsi basin. S. Afr. Geogr. J., 52: 77--86. FAO, 1974. FAO-UNESCO Soil Map of the World. Vol. I. Legend. F,A.O., Rome. Murdoch, G., 1968. Soils and land capability in Swaziland. Swaziland Ministry of Agriculture, Mbabane, 360 pp. Price Williams, D., Watson, A. and Goudie, A. S., 1982. Quaternary colluvial stratigraphy, archaeological sequences and palaeoenvironment in Swaziland, southern Africa. Geogr. J., 148: 50--67. Watson, A., Price Williams, D. and Goudie, A. S., 1984. The palaeoenvironmental interpretation of colluvial sediments and palaeosols of the late Pleistocene hypothermal in Southern Africa. Palaeogeogr., Palaeoclimatol., Palaeoecol., 45: 225--249.
R E P L Y T O C O M M E N T ON " T H E P A L A E O E N V I R O N M E N T A L INTERPRETATION OF COLLUVIAL SEDIMENTS AND PALAEOSOLS O F T H E L A T E P L E I S T O C E N E H Y P O T H E R M A L IN S O U T H E R N A F R I C A "
A. WATSON 9 Hartley Terrace Boston MA 02134 (U.S.A.)
(Received February 20, 1985) B e f o r e r e s p o n d i n g to the specific p o i n t s r e i t e r a t e t h a t the d a t a o n t h e d i s t r i b u t i o n s e d i m e n t s p r e s e n t e d b y W a t s o n et al. ( 1 9 8 4 ) We are grateful, t h e r e f o r e , to I a n Baillie f o r
raised above, we w o u l d like to and c h e m i s t r y of t h e colluvial are all f r o m gully ( d o n g a ) sites. providing this a d d i t i o n a l infor-