The main Rocky Mountains and their foothills have been strongly compressed and have developed overthrusts directed toward the east. The beds of the foreland or Plains area are comparatively flat and undisturbed.

The main Mackenzie Mountains are open folds with some normal faulting. Their foreland has been broken by block faulting of the basement, through which most of the mountain-building stresses were transmitted.

By applying information obtained from the Mackenzie Mountains foreland to problems of the Rockies it becomes apparent that in the geosynclinal belt the basement was so deep and so broken that it was ineffective to control structure. The Paleozoic limestones were the agents of structural control in this tectonic belt.

In the Plains the basement was rigid and unyielding except in a few localities.

The degree of yielding to compression is a measure of relative strength of the basement and of the other rocks in the sequence in different localities, and bears little relation to the intensity of compressive stress present in a particular locality. With this understanding of basement control more logical interpretations of characteristic structures of the Rocky Mountains and foothills can be given.