The area was not always so dry. Episodically during the Pleistocene, large lakes occupied many of the basins, which accounts for the exceedingly flat basin floors. Some old lake terraces are 100 m or more above the adjacent lake beds. Two thousand meters of relief between adjacent ranges and valleys is not uncommon. The precise processes and mechanics of tectonic extension are not entirely clear.
Several factors seem to operate.
The subduction zone, lying between the coast of Washington and Oregon and the Gorda and Juan de Fuca rises offshore to the west, acts as a free face that accommodates the resultant northwestward extension of the Basin and Range section of the North American plate. This northwestward expansion forces the subduction zone to the northwest, closer to the spreading rises. Internally within the Basin and Range, the extension results as a combination of attenuation of the lower crust and normal faulting of the more brittle upper crust. In some instances, asymmetry of the mountains indicates scarp and dip slopes. Most ranges are straight, at least at the base where the steep mountain sides abruptly intersect the basin floor.