In places bentonite has been mined altered volcanic ash beds in Cenozoic intermontane sedimentary basins. The region is composed of intermontane plateaus and mountain ranges. These ranges cover an extensive area and are often separated by intermontane basins or lowlands. The Laramide orogeny produced intermontane structural basins and adjacent mountain blocks by means of deformation. Plateaus are classified according to their surrounding environment as intermontane, piedmont, or continental. Most of the province and its intermontane valleys have an arid climate. The Tinajani Basin, in which Lake Titicaca lies, is an intermontane basin. It is a relatively low range, cut across by swampy intermontane basins. Qaidam is an intermontane basin, surrounded on all sides by mountain ranges. The road ran along the narrow Tangimarmoul passage in the intermontane area. The average height of the intermontane areas is between and . Tanana Uplands province, which in turn is part of the larger Intermontane Plateaus physiographic division. The Intermontane and Insular Coalfields of British Columbia also contain deposits of Cretaceous bituminous coal. The Intermontane Plateaus come to an end at the Cascade Range and the Sierra Nevada.