Simple sheet cake
Double layer on one corner to simulate a hill.
Chocolate candy rocks and bolder all over the cake that were very hard to find from where I live.
Crushed Grahm crackers for the desert sand.
I found some hard sugar succulent decorations at Hobby Lobby, so I used one pack of those.
Cactus decorations are all pipped from soft frosting. Short ones are pipped directly onto the cake and the tall barrel cactus are pipped onto a rose nail that has a piece of parchment paper on it then removed onto a pan, then placed into a freezer and frozen. These were put on the cake later when I finished off the decorating. 
One problem with this cake, at least in the environment in which I live, is that the month I made it was during what we call the monsoons. All that moisture in the air interacted with the coating on the candy rocks and they began to weep. They kind of looked like the sailing rocks of Death Valley. It was all eaten in a matter of a couple of hours though, so it didn't get too bad.