It is also subject to periodic, intense droughts and frosts. The North American Deserts are comprised of a mix of physiographic features but, in general, the area consists of plains with hills, plains with mountains, and tablelands of high relief. In the north, the flat to rolling topography consists of loess and volcanic ash deposits on basaltic plains. The lowest basin point is 86 m below sea level. Within the basin are found many dry lake beds, or playas, with alluvial fans and bajadas at the margin slopes.
Sand dunes occur in some areas.
Spectacular landscapes occur on uplifted and deeply dissected sedimentary rocks. Wind and water erosion has left impressive canyons, cliffs, buttes and mesas. Soils of the region are dry generally lacking organic material and distinct soil profiles and are high in calcium carbonate. This aridity is the result of the rain shadow of mountain ranges as they intercept the wet winter air masses brought by the westerly and easterly winds. High mountains also block some moist Gulf Coast air masses that cross the Great Plains.