The Tanezrouft region in south-central Algeria is known as the Land of Thirst and the Land of Terror. The Tanezrouft region, a western extension of the more massive Ahaggar Mountains to the east, is comprised of sandstone formations, whereas the Ahaggar Mountains are metamorphic in origin. The physiography of the region is dark sandstone hills with many steep canyon walls rising 800 to 1600 feet (250 to 500 meters) above the surrounding arid terrain; sandy areas with dune fields; and numerous sandstone outcrop patterns of concentric loops and ellipses. The reddish color of some of the sand results from metal oxides being dissolved from the parent rocks. Rainfall amounts in the Tanezrouft area rarely exceed 3 inches (8 centimeters) annually. In an earlier geologic period the region experienced substantial water erosion; now the landscape is being modified by wind erosion. Since there is very little vegetation in the area, wind erosion is a serious environmental degradation process.