To the student of land forms there is an everpresent contrast
between those due directly to the processes which build up the
earth's surface and those due to the erosive forces which destroy
what the others have built. In the great plains of North America
two of the divisions, that is, the Atlantic coastal plain of the
southeast and the peneplain of the northwest, owe their present
form to the forces of erosion. The other two, that is, the
prairies and the high plains, still bear the impress of the
original processes of deposition and have been modified to only a
slight extent by erosion.

A similar but greater contrast separates the mountains of eastern
North America and those of the western cordillera--the fourth and
last of the main physical divisions of the continent. In both the
Laurentian and the Appalachian highlands the eastern mountains
show no trace of the original forms produced by the faulting of
the crust or by volcanic movements. All the original distinctive
topography has been removed. What we see today is the product of
erosion working upon rocks that were thousands of feet beneath
the surface when they were brought to their present positions. In
the western cordillera, on the contrary, although much of the
present form of the land is due to erosion, a vast amount is 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