The shaping of Sydney by its urban geology

Abstract

Sydney's surface geology of Hawkesbury Sandstone capped with Wianamatta Shale has produced poor soils but good building material. The topography, derived from the subaerial erosion and extensive dissection of the Permian/Triassic sedimentary strata of the Sydney Basin, has been the strongest constraint on the city's growth. Steep-sided valleys carved out of sandstone plateaux forced the direction of settlement along the flatter interfluves, and in recent decades out onto the tectonic depression of the Cumberland Plain to the west. Holocene marine transgression flooded the eastern incisions of the eroded mass, producing some of the few safe anchorages on the southeast Australian coast, as well as bedrock-controlled zeta-curved coastal beaches. Some of the geological disadvantages have turned into advantages over time. The sterility of the skeletal sandstone soils made farming difficult, but has allowed much of the native vegetation to be retained for conservation and recreation. Coastal quartz sand deposits produced equally infertile soils but provided pure sand fill for Sydney's many attractive beaches, along with industrial uses. The depth of underlying coal deposits discouraged early mining within the city boundaries, to the ultimate benefit of residential development, while tectonic stability reduced complications for infrastructure engineering works.