Well to poorly preserved sandstone surfaces with glacial grooves, longitudinal ridges, bulbous bedforms and large lodged clasts occur sporadically at the base of the Dwyka Formation along the western margin of the Karoo Basin. The bedforms developed when ice overrode a thin (0·1–2·0m thick) subaqueous icemarginal apron formed primarily during periods of ice front retreat. Bergstone mud and rain-out diamicton blanketed the glacial bedforms.
The subglacial bedforms formed by (i) the lateral movement of water-saturated sediment into low-pressure zones, caused by crevasses and cavities at the base of the ice; (ii) the presence of areas of higher strength substrate, due to variations in bed lithology and porewater dissipation; and (iii) sediment flowage into low-pressure zones on the leeside of obstacles formed in areas of higher strength substrate due to dissipation of pore-water pressures and sediment compaction. The preservation of the bedforms, with their delicate slump fans, is attributed to separation of the glacier sole from the substrate during a sudden rise in sea-level. A series of dynamic ice-marginal events, including feedback relationships between sea-level oscillations, isostatic responses, ice-margin fluctuations, ice-margin type and the type of substrate, controlled the deposition of the basal sedimentary sequence and the formation of the associated glacial bedforms. The presence of a complex combination of glacier-related formative and depositional processes may have consequences for past interpretations of basal ‘tillites’.