Observations on a blockstream in the vicinity of Sani Pass, Lesotho highlands, Southern Africa

Abstract

Coarse slope deposits, frequently grading into blockstreams, are common throughout the Lesotho highlands, southern Africa. This paper describes one such blockstream. It is interpreted as a typical lag deposit derived from valley‐wide colluvial mantles, which contains superimposed and incorporated Late‐Pleistocene blocky material. Blocks are largely derived from local scarps but may contain a component of corestones from mobilized regolith. The widespread mantle argues against landscape‐scale glaciation in the Lesotho highlands in the Late Pleistocene. Rather, the environment appears to have been conducive to deep seasonal frost during the period of block production. No unequivocal evidence for permafrost is found. Copyright © 2002 John Wiley & Sons, Ltd.

Comment on “Lightning as a geomorphic agent on mountain summits: Evidence from southern Africa” by Knight and Grab (2014)

“…It is written that "Previous investigations have largely attributed mountain top weathering in this region to frost shattering (e.g. Boelhouwers et al, 2002;Sumner, 2004[a]; Sumner et al, 2009)" (p. 64) and their final statements specify that "to make uncritical climatic inferences based on the presence of 'frost shattered debris' on mountain summits is wholly erroneous" (Abstract, p. 61 and see also p. 68). The weathering context is clearly stated but none of the references cited in support of their contention mention 'frost shattering' in their reports.…”

“…Lightning strike sites are identified by debris, rock surfaces that are 'freshly-fractured' or 'unweathered' (p. 65) and from evidence for recent remagnetisation, although 'older' strikes are noted on page 67 to be more difficult to identify by this latter method alone. In contrast, their critique (on pages 67 and 68 in particular) is directed against studies interpreting relict valley-slope and -floor openwork block accumulations (Boelhouwers et al, 2002;Sumner, 2004a) set in the Last Glacial (maximum approximately 21-15 ka BP). Clearly this was a period of enhanced block production and slope mobility that is not comparable to today (see Sumner, 2004a p. 300), but when encompassing the areas below the summits, as Knight and Grab intrinsically do with their critique, a point that is evidently missed is the uneven geographic distribution of relict blocks in eastern Lesotho.…”

multi-disciplinary review of late Quaternary palaeoclimates and environments for Lesotho

“…However, palaeoenvironmental interpretations of associated precipitation levels during these cold periods are conflicting. 6,8,15,31 Boelhouwers and Meiklejohn 8 suggested that some periglacial features, including large patterned ground, solifluction mantles and blockstreams ( Figure 2c) were indicative of dry, cold conditions at their time of formation, whereas typically erosional features, including nivation hollows, cirques and cryoplanation terraces are likely indicative of wetter conditions during cold periods 36 . Other scholars 11,26 have argued that similar periglacial features, including large sorted patterned ground near Mafadi Summit ( Figure 2b) and at Thabana Ntlenyana, and pronival ramparts at Thabana Ntlenyana, imply relatively high moisture availability during their formation.…”

“…This knowledge has improved the potential for palaeoenvironmental inferences to be drawn from such features. 6,12,13 Accurate inferences could be achieved through the provision of corroborating environmental evidence, and by filling in the large temporal gaps between cold periods represented by these features 6,8,18,36 .…”

“…The restricted climatic requirements for such landform genesis can facilitate improved quantification of climatic change during these cold periods. 11,18 Continued debates concerning these cold region landforms highlight many unresolved uncertainties as to their identification, processes and environmental drivers, oftentimes with an over-reliance on northern hemisphere analogues 28,36,39,40 . The identification of features may become increasingly difficult to address as landscape disturbance increases in response to greater human and livestock pressure in the region 27 .…”