due directly to so-called "tectonic" activities such as the breaking of
the crust, the pouring out of molten lavas, and the bursting forth of
explosive eruptions.

The character of these tectonic activities has differed widely in
different parts of the cordillera. A broad upheaval of great blocks
of the earth's crust without tilting or disturbance has produced the
plateaus of Arizona and Utah. The gorges that have been rapidly cut
into such great upheaved blocks form part of the world's most striking
scenery. The Grand Canyon of the Colorado with its tremendous platforms,
mesas, and awe-inspiring cliffs could have been formed in no other way.
Equally wonderful are some of the narrow canyons in the broadly upheaved
plateaus of southern Utah where the tributaries of the Virgin and other
rivers have cut red or white chasms thousands of feet deep and so narrow
that at their bottoms perpetual twilight reigns. It is a curious proof
of the fallibility of human judgment that these great gorges are often
cited as the most striking examples of the power of erosion. Wonderful
as these gorges certainly are, the Piedmont plain or the northwestern
peneplain is far more wonderful. Those regions had their grand canyons
once upon a time, but now erosion has gone so far that it has reduced
the whole area to the level of the bottoms of the gorges. Though such
a fate is in store for all the marvelous scenery of the western
cordillera, we have it, for the present at least, as one of the most
stimulating panoramas of our American environment. No man worthy of the
name can sit on the brink of a great canyon or gaze up from the dark
depths of a gorge without a sense of awe and wonder. There, as in few
other places, Nature shows with unmistakable grandeur the marvelous
power and certainty with which her laws work out the destiny of the
universe.

In other parts of the great American cordillera some of the simplest and
youngest mountain ridges in the world are found. In southern Oregon, for
example, lava blocks have been broken and uplifted and now stand with
steep fresh faces on one side and with the old surface inclining more
gently on the other. Tilted blocks on a larger scale and much more
deeply carved by erosion 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 frac