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Geology of Australia's Blue Mountains

Posted Aug 17 2011 12:00am
The beautiful city of Sydney, Australia, sits on a thick slab of Triassic Hawkesbury sandstone, some 200 million years old. Below this bedrock are older Triassic sediments, shales, mudstones and conglomerates of the Narrabeen Group. All of these deposits, swept into the Sydney Basin by ancient rivers, lie atop Permian strata; containing seams of coal, these latter rocks were emplaced about 250 million years ago, when Earth's land masses had merged into the mega-continent of Pangea.

Just west of the Sydney metropolitan area are the scenic Blue Mountains, a broad, dissected plateau that rose during the Jurassic Period, some 170 million years ago, when Australia was still part of Gondwanaland. The geologic strata of the plateau is identical to the bedrock below Sydney; Triassic sandstone, capped at high points by Miocene basalt, forms massive cliffs which sit atop the older Narrabeen Group and underlying Permian deposits. Metamorphosed Paleozoic rock, from the Silurian and Devonian Periods, lies at the base of the Mountains.

As this elongated block of crust warped upward, vertical fractures developed in the sandstone cap, setting the stage for magnificent canyons to erode through the plateau as streams and rock falls gradually widened the gaps. Now home to a spectacular diversity of plant and animal life, much of the plateau, a component of the Great Dividing Range and named for the blue haze produced by its eucalytus forests, is protected as a World Heritage Area.
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