are found in the lofty St. Elias Mountain of Alaska, where much
of the erosion has been done by some of the world's greatest
glaciers. The western slope of the Wasatch Mountains facing the
desert of Utah is the wall of a huge fracture, 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