New evidence of past glacial and periglacial landforms in the Drakensberg, southern Africa

Geomorphological evidence of cold environments in the Drakensberg, southern Africa, has long been debated. Previous work, based on geomorphological, micromorphology and modelling techniques has suggested glacial conditions above ca. 3200 m a.s.l. However, this has been challenged due to the unclear glacial genesis of geomorphological features. This work aims to provide new evidence of past glacial and periglacial conditions in the Drakensberg.

Our aim is addressed by mapping a geomorphological sketch (1:25000) near the Sani Pass, Drakensberg, based on a 0.5 m resolution Digital Surface Model and an orthomosaic derived from Pléiades satellite imagery, and validated by field reconnaissance work. Specifically, the areas of the Sehonghong plateau, the upper reaches of the KwaNtuba cutback, and the Mangaung catchment, were investigated with several uncrewed aerial vehicle surveys for a detailed geomorphological mapping (1:5000). Finally, we applied Schmidt-Hammer exposure-age dating (SHD) and collected sixteen samples for Cosmic-Ray Exposure (CRE) dating from well-preserved features suggesting glacial and periglacial dynamics, which are currently being processed.

The Sehonghong plateau is located adjacent to the Great Escarpment, between 3400 and 3290 m a.s.l., and has gentle NE-facing slopes. The highest terrain is characterised by the widespread occurrence of weathered surfaces. The bedrock surfaces are littered with highly weathered and eroded ‘shattered’ debris, which provides the source for block fields and which have been reworked by periglacial processes, as evidenced by large sorted stone circles. At the plateau floor, degraded earth hummocky terrain and turf exfoliation areas are examples of wetland degradation. The different landforms suggest wind and seasonal frost conditions are the major geomorphic agents.

The KwaNtuba cutback faces ESE from 3350 to 2870 m a.s.l and encompasses steep slopes along the SW-NE-oriented Great Escarpment. The cutback headwall is shaped by an amphitheatric feature with 220 m high (3340-3120 m a.s.l) steep rock-plucked surfaces. From the foot of this rock face to the valley bottom, a 400 m wide and 200 m long deposit covers the slope. Here, five SE-facing elongated ridges developed along the deposit from 3120 to 2870 m a.s.l. The deposit is composed of subangular to angular metric-sized boulders embedded in a sandy matrix. The geomorphological context and sedimentological features suggest traces of ice flux dynamics, typically observed in debris-covered glaciers.

In the Mangaung catchment, two S-facing cirque-shaped basins were analysed. The eastern one has two well-developed parallel ridges at ca. 3210-3110 m a.s.l with subangular-subrounded metric-size boulders embedded in the sandy matrix that are considered lateral moraines. The centre of the basin has polished surfaces with striations suggesting former temperate subglacial abrasion. At the bottom of the western basin, a 1 km long and 60 m wide block stream developed between 3160 and 2980 m a.s.l. It has elongated ridges and is composed of matrix-free boulders whose central axis preserves alignment patterns downvalley.

The climatic conditions under which these features developed will be revealed in future results from CRE dating, palaeoclimate simulations and palaeoglacier modelling.