The Sahara Desert is constantly shifting as a result of erosion and deposition from wind and water. This image of northwestern Algeria just south of Morocco shows evidence of both these effects. The darker areas at the top edge of this image are exposed bedrock. The light blue regions are ancient deposits left after water evaporated. As can be seen clearly in this image, the desert sands of the Grand Erg Occidental are slowly advancing over this earlier sediment.

The area covered by this image is approximately 40x55 miles. A full-resolution crop of the image file is available here that shows the detail present in the full image print.