I’m thinking particularly of the large erg deserts like the Gobi or the Sahara, but I suppose I’m really curious about any or all of them. How deep would one have to go to find something of a substrate? Would that be some sort of bedrock? Are there many places where such digging has been done? Are we talking about 50 feet of sand? 100’s of feet of sand? Thousands? What I find when I look at Wiki are discussions of surface area. What is known about this? Thanks, dopers. xo, C.
http://www.newscientist.com/backpage.ns?id=mg19225742.300
Apparently, the sand can be a couple hundred meters deep or even deeper and it’s all sand until you get to rock (or sometimes earth).
It really will depend on where you’re talking about as those deserts are so vast that the depths can be quite varied depending on the regional geology. For a general answer though, I believe you can safely say it could be thousands of feet.
One sense of the scale of the present is to look at that of the past. There are ancient deserts that have been indurated and uplifted over time and they can provide both a reasonable and tangible approximation of your Sahara and Gobi. Here’s a good read on such, the Navajo Sandstone found across portions of the Colorado Plateau in the American west.
I understand that it is all sand until you hit the turtles. Then it is turtles all the way down.
Consarnit Sapo I was getting ready to say that stupid line. 
One should draw a distinction between sand and sandstone, generally speaking sand is unconsolidated and sandstone is consolidated. The term consolidated is a pretty ambiguous and when ever one tries to get a geologist to nail something down specifically they get all evasive.
Anyway let’s consider sand to be the loose stuff that blows around and moves with the dunes. Out in the oil regions of the middle east, if you are in a dune region and assuming you are at the bottom of a dune then generally one only has to scrape way a few feet with a bulldozer to get onto something pretty solid, if you were not on something solid already. That something solid tends to be sandstone, limestone or shale of various ages, depending where you are in the sedimentary basin. The western dessert of Libya is pretty much the same deal. Sure there are deeper pockets of sand, which are normally some eroded chasm in the hard rock which has subsequently filled up with soft sand. They can be quite a shocker if you are bashing your way around the place in a 4x4 and come across one.
hope that helps some
NBC
Well, yes, it does help,** Chap**. At least to the extent that it helps narrow my question, which, indeed, meant to deal with the soft, unconsolidated stuff. I figured that in MiddleEastern deserts, where oil wells are pitched, that at some point, the soft stuff would be underlain with sandstone or shale or some porous material that would hold the oil. Same thinking in terms of oases. But I think my question’s been pretty much answered in scale. Sounds like there are places that could be well over 1000 feet and maybe even “thousands”, whatever that could mean. But I take it to mean that there may be places where the sand could be close to a mile thick, or more. Jeez. Thanks, dopers. xo, C.
Careful, you might get admonished for posting deliberately wrong answers in GQ.
We used to go sand boarding on the miles of dunes around Stovepipe Wells in Death Valley. When you started walking over the smaller dunes, they were all solid sand from top to bottom, but the largest dunes (which were about 200 feet tall) had very hard sandstone at their bases.
We had to bail off the boards about 3/4 of the way down or we would crash into the hard soil at the bottom.
oops! missed that one. Still, I am feeling confident (read hopeful) that nobody would mistake mine for a real answer.
Wow, this is so fascinating to me. I’ve never seen a dessert and I can’t even really imagine how vast it must be. I grew up going to the Outer Banks and I used to think those sand dunes were so impressive. How amazing it must be! I’ve always wanted to go sand boarding or else hang gliding off dunes.
You’ve been dieting too long. 
Officially, “Deserts are defined as areas that receive an average annual precipitation of less than 250 mm (10 in).” (Wikipedia) There’s no requirement for sand. Many places in the Gobi in particular are plain rock. So your answer is: 0 to thousands of feet.
Also from Wiki: Cold deserts (also known as polar deserts) have similar features but the main form of precipitation is snow rather than rain. The largest cold desert is Antarctica (composed of about 98 percent thick continental ice sheet and 2 percent barren rock).
On the desert where I live, not very deep in most places. The surface is coarse sand or fine gravel. All of the fine sand and dust has long since blown away to form sand dunes or areas of blow sand somewhere else. The soil is low in humous but it otherside like soil elsewhere.
I hate to think of the day when humor would be banned from this forum. Although the cited example in the ATMB thread is a good example of bad whooshing in bad form.
As far as this thread goes, I thought Sapo’s line was humorous, but I think the reference escapes me. Any help?
It’s a very old (ancient, I think) idea reinvigorated by Terry Pratchett’s Discworld novels. (I think Carl Sagan had a hand in it as well, with one or more of his essays.)
Deserts aren’t sand piles. At the foot of mountain canyons are features called alluvial fans. These are triangular piles of rocks and soil that has washed down from the canyon over thousands of years. They can be pretty deep. In the basin and range territory the basin floors can contain alluvium that has washed down from the mountains, again over thousands of years, and it can be pretty deep to the bedrock. In neither case, however, would they be called sand piles.
Areas of sand, such a dunes, are strictly localized phenomena. They occur where some surface feature slows the wind velocity or causes and eddy so that the blow sand falls to the surface.
I remember it from a story about Bertrand Russell.
Russell was giving a public lecture on astronomy, and an elderly lady in the audience presented the counter-proposal that, rather than being a round planet orbiting the Sun, the Earth was actually a flat disk resting on the backs of enormous turtles.
Russell asked her, “But dear lady, what are the turtles resting on?”
She replied, “You’re very clever, young man, very clever, but you won’t fool me! It’s turtles all the way down!”
When I lived in the Mojave in California, I don’t remember any sizeable dunes. We did have frequent dust-storms, though, which would take the paint off my mom’s Chrysler.
As a related question, I wonder how long something (or someone) could be buried under loose sand in the Sahara, before being uncovered by a storm or the gradual movement of the sand? Maybe there’s ancient stuff hidden from view that the archeologists would love to get their hands on.
Kelso Dunes is part of the larger Devils Playground system of Sand dunes inthe Mojave. But dunes do not make up a significant area of the Mojave.
That’s the one. (except that I couldn’t remember the names of the parts involved)