The Hornsby Plateau is a sandstone plateau lying to the north of Sydney Harbour, which is cut by steep valleys. The North Shore and the Berowra Valley National Park are located on the Hornsby Plateau. Due to its hilly topography, it was slow to develop. It has two main sedimentary rock layers: Hawkesbury sandstone, consisting mostly of sandstone, but with some shale, and the overlying Ashfield shale.
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Hornsby Plateau Wikipedia(Text) CC BY-SA