 There are volcanic rocks from low hills in the shale landscapes.Swamps and lagoons are existent on the floodplain of the Nepean River.The Wianamatta Plain, with many undulating hills of Wianamatta Group shales and sandstones, bounded by the Woronora and Illawarra Plateaus to the south, the Blue Mountains Plateau to the west and the Hornsby Plateau to the north/northeast.