<P> Most of the area was part of the Viceroyalty of New Spain in the Spanish Empire before becoming part of Mexico . European settlement was almost non-existent outside New Mexico in 1848, when it became part of the United States through the Treaty of Guadalupe Hidalgo; far southern Arizona was added in the later Gadsden Purchase . </P> <P> The geography of the region is mainly made up by four features: the Mojave, Sonoran, and Chihuahuan Deserts, and the Colorado Plateau; although there are other geographical features as well, such as a portion of the Great Basin Desert . The deserts dominate the southern and western reaches of the area, while the plateau (which is largely made up of high desert) is the main feature north of the Mogollon Rim . The two major rivers of the region are the Colorado River, running in the northern and western areas, and the Rio Grande, running in the east, north to south . </P> <P> Formed approximately 8000 years ago, the Chihuahuan Desert is a relatively dry desert, although it is slightly wetter than the Sonoran Desert to the west . The Chihuahuan Desert spreads across the southeastern portion of the region, covering from southeastern Arizona, across southern New Mexico, and the portion of Texas included in the Southwest . While it is the second largest desert in the United States, only a third of the desert is within the United States, with the rest in Mexico . El Paso is the major city in this desert, with other smaller cities being Las Cruces and Roswell in New Mexico . </P> <P> The elevation in the Chihuahuan varies from 1970 to 5500 feet, as there are several smaller mountain ranges contained in the area, namely the San Andres and Doña Anas in New Mexico, and the Franklin Mountains, which span Texas and New Mexico in the Southwest region . 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 cacti . </P>

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