Sahara desert sands...

[Externet]

Radar imaging of the Sahara desert shows rivers carved rocky soil underneath the sand.

Where did all that huge amount of sand migrated from carried by winds, or was created and ended deposited there ?

Is the sand depth currently increasing or decreasing ?

[arc]

The Sahara was a wetter place 3 1/2 thousand years ago. And then the rain stopped and the top soil dried out and blew away. Much of this soil was blown out into the Atlantic and maybe even as far as N. America. The wind continued and made the remaining heavier soil components of sand into an abrasive erosional medium that reduced the exposed bedrock and ancient sand stone into even more airborne abrasives.

 

This cycle continued until large expansive areas had no exposed rock left, just a massive desert covered with wind driven sand dunes slowly moving across most of it's 9,400,000 square kilometres (3,600,000 sq mi). There are around the world deposits of sands that are covered over in forests and plains that are ready to get to work as soon as the overlaying soil and plant growth are removed beginning the process of desertification.

 

The American dust bowl was as close to a false start as you would want to get, the exposing by the plow of the soil to a climate dry period created an analog to the Sahara's beginning, that only for the sake of soil conservation and the eventual rains it ended.