The Kimberley Plateau of Australia, which forms the northwest corner of that Continent, is composed of ancient Precambrian rock. During the Devonian Period (some 350 million years ago), when sea levels were significantly higher than today, the Plateau was ringed by a limestone barrier reef; lime-secreting bacteria and algae and primitive coral produced the reef as the first sharks and boney fish were evolving in Earth's oceans. As sea levels fell and erosional debris accumulated, sections of that reef were incorporated into the mainland of Australia, now outcropping as limestone ranges along the southern edge of the Kimberley Plateau.
Several streams, rising on the Plateau, have cut spectacular canyons through the Devonian Reef complex (the Napier, Oscar and smaller ranges) as they flow southward into the Canning Basin; the latter is a layer cake of Upper Paleozoic and Mesozoic sediments, eroded from adjacent highlands. Windjana Gorge (cut by the Lennard River), Gelkie Gorge (sculpted by the Fitzroy River) and Tunnel Creek are protected within National Parks and offer the best locations to explore the Devonian Reef.
Australia's Great Barrier Reef, the latest reincarnation of which has been forming over the past 8000 years, will eventually become part of the mainland as well. By then, Australia, which is slowly drifting to the north, will be much closer to (if not contiguous with) Southeast Asia.