A makhtesh, Hebrew plural: is a geological landform considered typical for the Negev desert of Israel and the Sinai peninsula of Egypt. A makhtesh has steepwalls of resistant rock surrounding a deep closed valley, which is usually drained by a single wadi. The valleys have limited vegetation and soil, containing a variety of different colored rocks and diverse fauna and flora. The best known and largest makhtesh is Makhtesh Ramon.
Etymology
Although commonly referred to as "craters", these formations are "erosion cirques". Craters are formed by the impact of a meteor or volcanic eruption, whereas makhteshes are created by erosion. The word makhtesh is the Hebrew word for a mortar grinder. The geological landform was given this name because of its similarity to a grinding bowl.
Geology
Where a hard outer layer of rock covers softer rocks, erosion removes the softer minerals relatively quickly, 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 makhteshes, 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.
Makhtesh Ramon is exceptional as it is drained by two rivers. It is the largest makhtesh at over 40 km long, 2–10 km wide and over 500 m deep. The rocks in this makhtesh contain thousands of ammonite fossils, as well as volcanic and metamorphic rocks.
Makhtesh Gadol. At the time of naming, Makhtesh Ramon was uncharted, and so this was thought to be the largest makhtesh, at 10 km by 5 km.
Makhtesh Katan is the smallest major makhtesh at 7 km by 5 km and was charted in 1942 by Jewish explorers.
Sinai
The two makhteshes in Sinai, Egypt, have no names for the basin, but their walls have several names including Jabal al-Manzur or Gebel Maghara.
