Basin and range topography results from crustal extension (extensional tectonics). As the crust stretches, faults develop to accommodate the extension. In the western United States, this topography is built by a number of normal faults that meet at a basal detachment fault. The basins are down-fallen blocks of crust and the ranges are relatively uplifted blocks, many of which tilt slightly in one direction at their tops due to the motion of their bottoms along the main detachment fault. The normal arrangement in the basin and range system is that each valley (i.e., basin) is bounded on at least one side by one or more normal faults that are oriented along or sub-parallel to the range front.