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The geology of the SMCMA catchment area is relatively simple, however the bedrock and resultant soils have influenced the landform, hydrology, biology and human history of the area. The majority of the surface rock can be categorised as sedimentary rock from the Triassic period. The oldest rocks from this period, that are found in the area, are classified as the Narrabeen Group (e.g. at Long Reef and the Upper Hacking River), the Hawkesbury Group (i.e. most of the sandstone occurring across the eastern section of the area), and the Wianamatta Group (e.g. shales across the west and south-west of the area). The earlier (and underlaying) Permian sedimentary rock is only evidenced as isolated pockets of surface rock. Vulcanism has affected the area at three main geological times, during the early Jurassic period, late Mesozoic era and Cenozoic era. Little evidence of that volcanic activity remains except for several diatremes (extinct volcanic vents containing magma based rock) found across the area (such as the Dundas Valley diatreme). Present-day alluvial material was deposited during the Cenozoic era and Quaternary period. The current inter-glacial period began around 18,000 years before present, with sea levels stabilising approximately 6,000 years ago. Therefore, the alluvial deposits have been reworked over the last 6,000 years as sea and river base levels have stabilised. This has resulted in depositional features such as infilling adjacent to offshore islands (e.g. the Towra Point formation) and sedimentation of estuaries (e.g. Port Hacking). Longshore marine transport of coarse grained sands, along Sydney's coastal fringe, has led to the formation of a number of oceanic beaches. These include some of Australia's best known beaches such as Bondi, Manly and Cronulla Beaches. The interaction of bedrock type, climate and sea level rise and fall has resulted in the area exhibiting a diverse range of landforms, including;
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