In geology and physical geography, a plateau, also called a high plain or a tableland, is an area of a highland, usually consisting of relatively flat terrain, that is raised significantly above the surrounding area, often with one or more sides with deep hills. Plateaus can be formed by a number of processes, including upwelling of volcanic magma, extrusion of lava, and erosion by water and glaciers. Plateaus are classified according to their surrounding environment as intermontane, piedmont, or continental. A few plateaus may have a small flat top while others have wide ones.