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. The steep slopes are heavily ravined with sharp V-shaped gorges.
Triangular facets are common on interfluves between valleys.
Figure T-4.1 is an oblique view looking south toward the Arrow Canyon Range, Nevada. The tilted nature of the fault block is evident. Note the difference in stream length, depending on steepness of the dip of strata. Landforms on the east and west sides of the range are different. On the west, the steep range front is being covered with debris that is now being trenched, with the material removed into the center of the basin.