Formed about 80 million years ago, the Cumberland Plain consists of not exactly flat plains; overall it is a low-lying area, largely over shale and labile sandstone, which derives its recognition largely by comparison with the surrounding uplands of harder quartzose Sydney sandstone. Relative to the surrounding higher sandstone lands, Hornsby and Blue Mountains Plateaux, it was an early matter of debate in Sydney physiographic circles as to whether the Cumberland Plain had gone down, or the surrounding plateaux had been raised up. Despite much study, especially along the western side at the Lapstone monocline, this complex matter is still not fully understood. There are volcanic rocks from low hills in the shale landscapes. Swamps and lagoons are existent on the floodplain of the Nepean River.