The Valley of Desolation contains a network of caves and underground tunnels in which the Clay People live. Like the walls of these tunnels, the Clay People largely consist of clay and have the ability to melt or blend into these walls, and possibly actually pass right through them. However, the Clay People seem dependent upon this environment for survival and cannot leave.
They have a simple social structure with a single ruler, the Clay King.
The Clay People all seem to have once been normal Martians. Most, if not all of them have been reduced to their current state as punishment by Queen Azura, whose magic has both transformed them and transported them to the Clay caves. However, it is also said that a normal person would permanently become one of the Clay People by spending too much time in their habitat. Dr. Hans Zarkov speculates that this is an effect of the vapours emanating from the cavern walls; it is possibly these same vapours upon which the Clay People are dependant.
Clay People are impervious to bullets. Despite their limitations, the Clay People have developed technology. They have extended their network of tunnels all the way to Queen Azura's palace, using a special subway car to travel the long distance. They have built traps to ensnare trespassers, and also have a chamber which emits healing gases.
The Clay People know of the Forest People of Mars but apparently have had no interaction with them. They believe the Forest People to be honest people.
Being victims themselves, the Clay People are distrustful of all others and are prepared to use any means necessary to restore themselves to their original normal state. Eventually they come to trust and befriend Flash Gordon, who gets Queen Azura to reveal the method to release the Clay People from their curse. Flash destroys the White Sapphire and the Black Sapphire and the Clay People become human once again.