The elevation in the Chihuahuan Desert varies from 1970 to 5500 feet, as there are several smaller mountain ranges contained in the area, namely the San Andres, Doña Anas, and Franklin Mountains. The Chihuahuan is a rain shadow desert, formed between two mountain ranges (the Sierra Madre Occidental on the west and the Sierra Madre Oriental on the east) which block oceanic precipitation from reaching the area. The Chihuahuan Desert is considered the most biologically diverse desert in the Western Hemisphere and one of the most diverse in the world, and includes more species of cacti than any other desert in the world. The most prolific plants in this region are agave, yucca and creosote bushes, in addition to the ubiquitous presence of various cacti species.