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.
The Cumberland Plain originated approximately 80 million years ago; it is not a perfectly flat region but rather a low-lying area, primarily made up of shale and labile sandstone, which is distinguished by its comparison to the surrounding more rugged uplands of harder quartzose Sydney sandstone. The positioning of the Cumberland Plain relative to the higher sandstone lands, including Hornsby and Blue Mountains Plateaux, was an early point of discussion in Sydney's geographical community.