Australia: The Land Where Time Began

A biography of the Australian continent 

Eyre Peninsula

The Eyre Peninsula is situated south of the Gawler Ranges and the Corrobinnie Depression, and is a region of mainly plains with old dune fields and a crust of calcrete that is prominent in northern Areas. Remnants of a higher ground occur in the east and south, in the Lincoln Uplands and in the Cleve Hills where there are exposures of very old gneisses and other igneous and metamorphic rocks (Johns, 1961; Parker, 1993). In the Early Mesozoic they have been weathered and laterised, probably at the same time as the Mt Lofty Ranges and Kangaroo Island were duricrusted, though in the Cleve Hills there are only pockets of pisolitic iron oxide that are associated with deeply kaolinised and mottled bedrock beneath the summit surface. The laterised surface appears to dip beneath the strata of Eocene age of the Cummins Basin (Johns, 1958).

Granite terrain

Most of the north and northwest of the Peninsula are underlain by granitic rocks, most of these old igneous rocks which being weathered and eroded, which resulted in the formation of rolling pains, but there are especially massive components of rocks that are resistant to weathering, that formed inselbergs or bornhardts, prominent rounded hills (Twidale, 1964, 1982a, 1982b). E. J. Eyre noted them in 1939 on his exploration of the northern Eyre Peninsula and the Northern Gawler Ranges (Eyre, 1845, pp. 198-205). Eyre was west of the Baxter Range on the 24 th of September 1839 in a landscape of many isolated hills he described as resembling 'resembling so many islands in the level waste around them' (Eyre, 1845, p 203). Giles remarked of some hills he encountered in central Australia: 'The mount, and all the others connected with it, rose simply like islands out of a vast sea of scrub' (Giles, 1889, p. 158). These are only 2 of the use of the 'island' analogy by early explorers to describe what are now known by the German equivalent, inselbergs.

The process of subsurface differential weathering, followed by the exposure of the compartments of still-fresh rock, formed the inselbergs. They are etch forms as is the case with the bornhardts of the Gawler Ranges. Some are bevelled and stepped with breaks of slope, and there are flared sectors where pauses in weathering and erosion are marked. The author¹ suggests possible Mesozoic age and possible correlation with the summit surface of the Gawler Ranges, for the crests and high shoulders of major inselbergs such as Mt Wudinna, Carappee Hill and Ucontitchie Hill.

Sources & Further reading

1. Twidale, C.R., 2007, Ancient Australian Landscapes, Rosenberg Publishing Pty. Ltd. , NSW