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

It depends on what’s below the sand.

You can think of sand like soil, it forms and moves differently but in essence it accumulates in roughly the same sort of way. Just like soil, sand isn’t the bedrock itself, it just covers the bedrock filling in depressions or being pushed around by wind and water erosion.

So what’s below it? If the water table is high enough then if you dig down into the sand you will find water. This doesn’t really have much to do with what’s actually below the sand and more about the local hydrology of the region.

What else? Well if the sand is very very old and deep then it’s possible for the sand to compress and crystallize together into sand stone. This happens a lot in oceans but can happen on land as well if the conditions are right.

And finally, what you will find it most cases, is just bedrock. What kind of bedrock? That depends on the location. The Sahara, Gobi, Namib and other deserts all have various bedrock types of various ages below them. The sand simply forms and then is blown over top of the bedrock due to wind so there doesn’t need to be a special type of bedrock for the sand dunes to form.

In most deserts the sand dunes and drifts are scattered things that move around. The Sahara is less of an endless stretch of sand and more of several very large sand piles that slowly shift around as regional winds slowly blow them one way or the other. And often what was covered by sand dunes for hundreds of years can be uncovered or what was uncovered can be buried. That’s why we find buried ruins in places like Egypt, the sand simply shifts and covers everything like snow drifts that keep growing and never melt.