The Southwestern United States (also known as the American Southwest) is the United States portion of the Southwest (which is situated in both the United States and Mexico). It is a region of the United States defined in different ways by different sources. The definition of what constitutes the American Southwest has evolved over the last one hundred and fifty years, moving steadily farther south and west, as well as becoming more confined in scope. Broad definitions have included nearly a quarter of the United States, including Arizona, California, Colorado, Nevada, New Mexico, Oklahoma, Texas, and Utah. Narrowly defined, the "core" Southwest is centered on Arizona and western New Mexico, the extreme western part of Texas, and the very southern parts of Colorado and Utah. This is one of the most common definitions of the term, as exemplified by archeologists such as Erik Reed and geologists such as D. W. Meinig. The other variations are discussed in a separate section, the main article will deal with this core definition, which includes all of Arizona, the western portion of New Mexico, bordered on the east by the Llano Estacado, the southern portions of Colorado and Utah below the 39th parallel, the "horn" of Texas (the Trans-Pecos section, below New Mexico, which includes El Paso), the southernmost triangle of Nevada, and the very most southeastern portion of California, which encompasses the Mojave and Colorado Deserts.