Sydney’s geological foundation is primarily composed of Sydney sandstone, which underpins much of the exposed rock throughout the city. This sandstone is mainly Triassic rock, with some igneous features such as dykes and volcanic necks, particularly found in the Prospect dolerite intrusion located to the west of Sydney. The formation of the Sydney Basin dates back to the early Triassic period, with the sandstone itself being deposited between 360 and 200 million years ago.

This sandstone formation includes shale lenses and ancient fossil riverbeds, offering insights into the geological history of the area. The Sydney Basin bioregion is characterized by its coastal features, including cliffs, beaches, and estuaries. During the Triassic period, deep river valleys known as rias were carved from the Hawkesbury sandstone. These rias were later flooded by rising sea levels between 18,000 and 6,000 years ago, forming the estuaries and deep harbours we see today, such as Port Jackson, which is commonly known as Sydney Harbour.

Sydney's soil composition is primarily two types: sandy soils, derived from the Hawkesbury sandstone, and clay soils, originating from shales and volcanic rocks. Some areas feature a mixture of both soil types, contributing to the diverse ecology of the region.

Directly overlying the ancient Hawkesbury sandstone is the Wianamatta shale, a geological formation located in western Sydney. This shale was deposited during the Middle Triassic as part of a large river delta. The Wianamatta shale comprises fine-grained sedimentary rocks, including shales, mudstones, ironstones, siltstones, and laminites, with occasional sandstone units. The Wianamatta Group, a subset of this formation, consists of Bringelly Shale, Minchinbury Sandstone, and Ashfield Shale, which together contribute to the complex geological landscape of Sydney.

Understanding Sydney's geology not only illuminates the city’s natural history but also provides essential context for urban planning and environmental management in this vibrant coastal metropolis.