ture, as is the eastern face
of the Sierra Nevadas facing the deserts of Nevada. Each of these great
faces has been deeply eroded. At the base, however, recent breaking and
upheaval of the crust have given rise to fresh uneroded slopes. Some
take the form of triangular facets, where a series of ridges has been
sliced across and lifted up by a great fault. Others assume the shape
of terraces which sometimes continue along the base of the mountains for
scores of miles. In places they seem like bluffs cut by an ancient lake,
but suddenly they change their altitude or pass from one drainage area
to another as no lake-formed strand could possibly do.

In other parts of the cordillera, mountains have been formed by a single
arching of the crust without any breaking. Such is the case in the Uinta
Mountains of northwestern Utah and in some of the ranges of the Rocky
Mountains in Colorado. The Black Hills of South Dakota, although lying
out in the plains, are an example of the same kind of structure and
really belong to the cordillera. In them the layers of the earth's crust
have been bent up in the form of a great dome. The dome structure, to be
sure, has now been largely destroyed, for erosion has long been active.
The result is that the harder strata form a series of concentric ridges,
while between them are ring-shaped valleys, one of which is so level and
unbroken that it is known to the Indians as the "race-course." In
other parts of the cordillera great masses of rock have been pushed
horizontally upon the tops of others. In Montana, for example, the
strata of the plains have been bent down and overridden by those of
the mountains. These are only a few of the countless forms of breaking,
faulting, and crumpling which have given to the cordillera an almost
infinite variety of scenery.

The work of mountain building is still active in the western cordillera,
as is evident from such an event as the San Francisco earthquake. In the
Owens Valley region in southern California the gravelly beaches of old
lakes are rent by fissures made within a few years by earthquakes. In
other places fresh terraces on the sides of the valley mark the lines of
recent earth movements, while newly formed lakes lie in troughs at
their base. These Owens Valley movements of the crust are parts of the
stupendous uplift which has raised the Sierra Nevada to heights of over
14,000 feet a few miles to the west. Along the fault line at the base
of the m