The shape of glacial valleys and implications for southern African glaciation

Abstract

The classic U-shaped valley is a typical expression of glacial erosion, but situations can occur where the glacier effects little to no change in the landscape. Such an occurrence would be where the glacier is cold-based and remains so during its demisenever entering into a warm-based (erosional) phase. Here two present-day examples are provided where glaciers exist but valley form has remained unaltered despite multiple glacial events. The key to such a situation is suggested to be the altitudinal/latitudinal spatial location such that the ice has completely disappeared before, during the move towards an interglacial, there is the time for it to transform into warm-based ice. The argument is then made that perhaps the same is the situation for the reconstructed, small glaciers in the Lesotho-Drakensberg area. The ice was cold-based due to a combination of its thinness and the cooling effect of shading. Cold-based ice would explain the lack of striated clasts found in the moraines, the absence of any change to valley form, and the preservation of breaks in slope observed in the area of the former glacier.

Diagnostic criteria for pronival ramparts: site, morphological and sedimentological characteristics

“…) and incorporate characteristics which are not limited to ridge morphology but also focus on sedimentology and topographic setting of actively accumulating features. This is proposed as a starting point for the identification of pronival ramparts in the field and may also facilitate the reappraisal of questionable relict examples (see Shakesby ; Grab ; Sumner and de Villiers ; Lewis ; Hall ; Grab et al . ).…”

“…The debate surrounding relict pronival ramparts in southern Africa (e.g. Shakesby ; Grab ; Sumner and de Villiers ; Lewis ; Hall ; Grab et al . ) provides further examples.…”

“…The debate surrounding relict pronival ramparts in southern Africa (e.g. Shakesby 1997;Grab 2000;Sumner and de Villiers 2002;Lewis 2008;Hall 2010;Grab et al 2012) provides further examples. Shakesby (1997, p. 394) argues that 'only when further investigations on actively-accumulating ramparts have been carried out, will it be possible to compile a reliable list of criteria for distinguishing ramparts from moraines, protalus rock glaciers, and other bedrock cliff-foot depositional forms'.…”

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

“…Such glaciation is dated to ~19 500-17 350 calibrated years before present (cal yr BP) 6,12,13 , which coincides with the terminal stage of the LGM in southern Africa 2 . While the accuracy of the term 'niche glaciation' has been queried 29 , evidence based on these moraines for small-scale, spatially restricted glaciation during the LGM is undisputed to date 18 .…”

“…8,13,28 The most contentious palaeogeomorphic features in eastern Lesotho are those interpreted to be of glacial origin. 8,10,29 Many critiques of such glacial interpretations arise from the predominantly qualitative earlier approaches used, and difficulties in positive feature identification and age determinations 8,15 . However, more recent quantitative approaches, including detailed mapping, macro-and micro-sedimentology, and obtaining ages for small moraines (Figure 2a), have provided more substantive evidence for Quaternary glaciation in eastern Lesotho.…”