Yarwondutta Rock, South Australia



Yarwondutta Rock is an isolated granite inselberg (or island mountain) standing abruptly from the surrounding cleared plains. The rock is cut by several vertical or near-vertical fractures, three trending north-east to south-west, two trending west-north-west to east-south-east and one aligned north-west to south-east, so that the outcrop is subdivided into several roughly quadrangular blocks. Preferential weathering has taken place along the major fractures giving rise to clefts or slots. One of the major clefts is bounded by flared slopes like those formed around the margins of the inselberg and on some of the upper slopes. Clear evidence of the subsurface origin of the flares can be seen in the reservoir (in summer, when it is dry). The upper flattish slope is poked and grooved due to the development of many rock basins (or gnammas), linked by bedrock channels. Most are of the flat-floored pan variety but those near the edges of The Summit plain take the form of the armchair-shaped hollows. The large residual boulder standing on the north-western step displays an active tafone or hollow. The entire north-western slope is stepped, which flared slopes linking flattish platforms. Good exposures of the fresh hiltaba granite can be seen in the quarry. There is also a good exposure of a regolith in granite and with calcrete (pedogenic limestone) superimposed on the weathered bedrock.

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