What is under a (sand) desert?

Question

I mean, a desert is basically just a huge amount of sand but where does it stop, what is beneath? I googled it and it says there’s groundwater but what does that mean, and (how) is that even possible?

Edit: So, actually the main question I wanted to know is if there’s a exact border between sand and rock, or if the sand just gets denser and denser until it’s rock (but I believe the weight must not be that big), like Saturn or something.

(Yes, I know what a desert is but I specifically need to know about this certain type of desert)

In: 1165

Answer 1

I’m guessing you are thinking of the classic sand dunes you see in movies set in the desert. Like you get hills of sand with little or no vegetation and can often see the sand blowing around in little wisps up to full-blown sandstorms, depending of the amount of wind. *Please correct me if I’m wrong.*

You see these dunes in parts of New Mexico USA, the Arabian Peninsula, North Africa, western South America, etc. You also get dunes and similar landforms in other places where the sand blow around enough that plants can’t get established to hold it down.

And that’s the key. This type of sand dune desert is formed by sand that is blown around by the wind. The dunes actually move slowly across the landscape with sand being eroded away on the upwind side of the dunes and deposited on the downwind side. So this means the sand forms younger deposits on top of whatever older rock was there before. The older rock is commonly sedimentary rock like shale, sandstone, limestone or whatever. But it could be any type. The sand blows over top so there is typically a sudden change from the loose sand to the older harder rock.

One thing that can happen is that the wind-deposited sand stops moving, maybe due to a change in rainfall. Then with time the sand beneath the surface can be slowly cemented into sandstone. In that case the sand might grade down from loose near the surface to rock below.

There are other types of desert sediments and other things that can occur but I think that is the ELI5 version.