system
Given the following post, respond as if you're commenting on Reddit:
human
what lies beneath the sand of a desert?
gpt
The canonical desert landscape is generally thought to be a sand dune, but this is a misconception. Even in the great Sahara desert only a fairly small portion is made up of sand dune expanses (ergs), most of it is hamada (bare stone and rock).

Generally speaking, just about any rock can underlie a sand dune field, more so since sand dunes are slightly mobile.