A Nyitting story, summarised by Moore, recounts how the islands were cut off from the mainland by the sea at Cockburn Sound.
The natives have a tradition that Rottnest, Carnac, and Garden Island, once formed part of the mainland, and that the intervening ground was thickly covered with trees; which took fire in some unaccountable way, and burned with such intensity that the ground split asunder with a great noise, and the sea rushed in between, cutting off these islands from the mainland.1
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1. Moore 1884b:8 cited in Hallam, Fire and Hearth, 112.