How deep is the sand under those sweeping dunes of the Sahara or Arabian Deserts? In other words, how far below the troughs of the dunes would you find the underlying bedrock?

Zbeckabee

Sahara: Surface sand seldom piles higher than a couple of hundred metres above underlying earth or bedrock, except by filling ancient valleys or lakes. Such deep sands and spongy sandstone form important groundwater reservoirs. And yes, sand does form and re-form constantly as water erosion, frost and wind-driven particles flake grains off rocks. Conversely, deep moist layers of sand become cemented into sandstone, which in turn may go through the same cycle after millions of years. Far deeper sand occurs in submarine detritus fans which build up at the mouths of rivers. Even more intriguingly, the Mediterranean has dried up repeatedly in the last tens of millions of years. Each time that happened, rivers flowing into the basin eroded their fans into massive canyons, which silted up again whenever the sea returned. The bed of the lower reaches of the Nile consists of silt, more or less compacted, kilometres deep, turning it into an underworld canyon that has dwarfed the Grand Canyon in the past. http://www.newscientist.com/article/mg19225742.300-shifting-sands.html Mar 17 10, 10:54 PM