Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 50

FIGURE 2.20 Cross sections of rock structures resulting from low-to high-angle thrust faulting in the
Sawtooth Range of Montana. This commonly results in the juxtaposition of rocks of varying res-
istance and produces an extremely rugged landscape. Length of cross section is 16 km (10 mi).
(Adapted from Deiss 1943, in Thornbury 1965: 391.)
Most of the Basin and Range in the western United States resulted from tensional
forces. This region, centered in Nevada, has been broken into a series of fault zones ori-
ented roughly northwest-southeast, and the resulting blocks have been tilted, lifted, and
dropped relative to one another to create more or less parallel ridges and valleys. Many
of the blocks occur as asymmetric massifs with one side, a fault scarp, rising as much
as 1,500 m (∼5,000 ft) above the adjacent surface, with the other side dipping gently
into the surrounding lowlands. From the Wasatch Range in Utah to the Sierra Nevada
in California, the complexly eroded fault scarps of these ranges, and most of the oth-
ers in the Basin and Range, are the result, depending primarily upon rock type, either
of erosional downwearing of the original fault scarps to reduced angles, or of parallel
erosional retreat at approximately the same angle. All the fault scarps are the result of
abrupt incremental fault offsets of a few meters at a time, generally at intervals of cen-
turies between earthquake events. Over millions of years the recurrent offsets generate
the mountain ranges of the present day. Bull (2007) assessed the typical landforms of
these faulted mountain fronts in the southwestern United States to establish more than
five classes of relative tectonic activity, from active maximal to minimal, as well as vari-
ous types of inactive mountain fronts. He used various criteria to establish his classi-