1. Change the terrain sand layer to a darker, slightly more orangey yellow, it'll look a bit more like a desert
2. Move the sun across a bit so we can see the rock-face in the background a bit better, but leave enough of an angle so that there are some lovely dramatic shadows on it
3. Calm down the displacement on the fake stones a bit, it's a tad unrealistic.
4. The rocks need a bit of a colour change, try a deep-ish brown, or an orange maybe.
Good luck with your renderings!