Morphology and origin of three bornhardt inselbergs near Lake Johnston, Western Australia

Abstract

Three domical inselbergs or bornhardts, Disappointment, McDermid and Bank Rocks, located near the Hyden-Norseman road in the south-eastern corner of the Yilgarn Craton, are described and analysed. All are basically similar in origin, being due to differential fracture-controlled subsurface weathering followed by the stripping of the weathered mantle. Disappointment Rock is older and topographically more subdued than either McDermid or Bank Rocks; it is located further from the Lake Johnston valley and has not been as greatly exposed. McDermid and Bank Rocks display spectacular flared slopes, whereas those developed at Disappointment Rock probably remain hidden by the lateritic regolith. All three Rocks display a similar range of minor forms, some due to subsurface weathering, others to weathering after exposure of the rock surface, yet others to contrasted rates of weathering on wet and dry surfaces. Some are due to protection against water attack, others reflect bedrock characteristics, and yet others the effects of gravity. Recent earth movements, as evidenced in neotectonic landforms, have affected all three residuals.