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.
Emerging around 80 million years ago, the Cumberland Plain is not a perfectly flat area; rather, it is a low-lying region, mainly consisting of shale and labile sandstone, which is recognized primarily by its comparison to the more rugged uplands of harder quartzose Sydney sandstone. The relative positions of the Cumberland Plain and the surrounding higher sandstone lands, including Hornsby and Blue Mountains Plateaux, were a topic of early debate in Sydney's geographical community.