High Quality Content by WIKIPEDIA articles! The Basin and Range Province is a vast physiographic region defined by a unique topographic expression. Basin and Range topography is characterized by abrupt changes in elevation, alternating between narrow faulted mountain chains and flat arid valleys or basins. The region covers most of the western United States, extends into northwestern Mexico and is mostly desert, as well as numerous ecoregions. The physiography of the province is the result of tectonic extension that began around 17 Ma (million years ago) in Early Miocene time.