In the Central East Sahara, crack polygons of different sizes occur in almost all soils. Cracks controlled by wetting and drying in clay and loamy soils are called desiccation cracks or dehydration cracks (Hydro-Crack) and those in sandy soils are classified as thermal cracks (Thermo-Cracks). In sandy soils, cracks are formed through expansion and contraction of the extreme dry pedon as a result of daily and annual temperature fluctuations as well as the absence of any biological activity. Extremely dry sand particles can expand and shrink due to temperature fluctuations and arrange themselves into a higher bulk density. When a high bulk density is reached, the sand particles are pressed together by expanding and bound together by cohesive force. At last, the coherent soil mass must crack into fine fissures by shrinkage. The cracks created in this way can be filled with drifting sand and thus preserved. This means that the crack cannot close when the soil expands when heated. But it can get wider when it cools down. This causes the sand to slide deeper and more sand can trickle into the crack. As the cracks get wider, the soil surface has to bulge up to match the increase in volume. This can create a Gilgai-like micro-relief.