recent igneous dykes and volcanic necks (typically found in the Prospect dolerite intrusion, west of Sydney). The Sydney Basin was formed when the Earth's crust expanded, subsided, and filled with sediment in the early Triassic period. The sand that was to become the sandstone of today was washed there by rivers from the south and northwest and laid down between 360 and 200 million years ago. The sandstone has shale lenses and fossil riverbeds. The Sydney Basin bioregion includes coastal features of cliffs, beaches, and estuaries. Deep river valleys known as rias were carved during the Triassic period in the Hawkesbury sandstone of the coastal region where Sydney now lies. The rising sea level between 18,000 and 6,000 years ago flooded the rias to form estuaries and deep harbours. Port Jackson, better known as Sydney Harbour, is one such ria. Sydney features two major soil types; sandy soils (which originate from the Hawkesbury sandstone) and clay (which are from shales and volcanic rocks), though some soils may be a mixture of the two. Directly overlying the older Hawkesbury sandstone is the Wianamatta shale, a geological feature found in western Sydney that was deposited in connection with a large river delta during the Middle Triassic period which shifted over time from west to east. The Wianamatta shale generally comprises fine grained sedimentary rocks such as shales, mudstones, ironstones, siltstones and laminites, with less common sandstone units. The Wianamatta Group is made up of the following units (listed in stratigraphic order): Bringelly Shale, Minchinbury Sandstone and Ashfield Shale. Ecology The most prevalent plant communities in the Sydney region are grassy woodlands (i.e. savannas) and some pockets of dry sclerophyll forests, which consist of eucalyptus trees, casuarinas, melaleucas, corymbias and angophoras, with shrubs (typically wattles, callistemons, grevilleas and banksias), and a semi-continuous grass in the understory. The plants in this community tend to have rough and spiky leaves, as they're grown in areas with low soil fertility. Sydney also features a few areas of wet sclerophyll forests which are found in the wetter, elevated areas in the north and the northeast. These forests are defined by straight, tall tree canopies with a moist understory of soft-leaved shrubs, tree ferns and herbs. The predominant vegetation community in Sydney is the Cumberland Plain Woodland, which is found in Western Sydney (Cumberland Plain), followed by the Sydney Turpentine-Ironbark Forest which is scattered in the Inner West