The Hawkesbury Sandstone is overlain by a series of fine grained rocks – the Wianamatta Group – the uppermost unit of the thick Permo-Triassic sequence which infills the Sydney Basin. The Wianamatta Group rocks, their weathering products and derived soils are of engineering importance as they form the foundations for most of central and western Sydney and provide earth-fill and brick clay for construction projects throughout the Sydney Region.