"Southwest Basin and Ranges" is the vague term this site uses for the countless small mountain ranges of southern Arizona, southern New Mexico, far western Texas, and the Mexican state of Sonora. Even though it is classified as part of the Intermountain West, this area is so far south that there are no longer any Sierra Nevada or Rockies to be between any more.
The main unity this area posesses is geologic. It is basin-and-range territory, characterized by endless waves of parallel ranges rising from dry plains, exactly like the Great Basin. However, this area is largely separated from the Great Basin by the massive, high uplift of the Colorado Plateau, and is much hotter and lower in elevation than the basins and ranges to the north.
Calling this area "Southwest" is problematical, too, since that is the most vague directional term used to describe a section of the United States. Arizona, New Mexico, and Texas are the core of "The Southwest", but southern California is geographically far more southwest than, say, Texas, and the term is applied in varying degrees to parts of Nevada, Utah, Oklahoma, Arkansas, Kansas, and sometimes even Missouri, former headquarters of the Southwestern Bell Telephone Company.
So we are left with this area as a logical unit, obviously not part of the Rockies, Colorado Plateau, Great Basin, Pacific Ranges, or Mexican Sierra Madre, instead filling up the irregular space between these more well-defined areas. The far eastern edge of the area is in many ways an topographic southward extension of the Rocky Mountains, and the Guadalupe Mountains or the Big Bend is sometimes called the southernmost extention of the Rockies. But those areas are so much lower and more isolated than the high Sangre de Cristo Mountains of the Rockies that to me they are clearly part of the grab-bag of the Southwest Basins and Ranges.
