Geology
A hard outer layer of rock covered softer rocks. Erosion relatively quickly removes the softer minerals, and they are washed away from under the harder rock. The harder rocks eventually collapse under their own weight and a crater-like valley structure is formed. In Negev and Sinai makhteshim, the hard rocks are limestone and dolomites, while the inner softer rocks are chalk or sandstone.
The center of the Negev is dominated by northeast-southwest anticlinal ridges. The crests of four ridges host five deep valleys surrounded by steep walls. The upper half consists of hard limestone and dolomite and the bottom of friable sandstone. Each valley, known as a makhtesh, is drained by a narrow river bed.
Read more about this topic: Makhtesh