Fig. 45.—Map showing the general relations of land and water in North America during part of the middle Tertiary period. (After Willis, courtesy of the Journal of Geology.)
The whole of the Cenozoic era, including both the Tertiary and Quaternary periods, has been a time of profound crustal disturbances throughout much of the continent, certain of these movements having continued right up to the present time, with positive evidence that some of them are still continuing. These great movements have included notable foldings of strata, uplifts without folding, faulting, and igneous activity, the whole effect having been to greatly increase the general altitude and ruggedness of the continent. In fact, North America is not known ever to have been at once higher, broader, and more rugged than it was very late in the Tertiary, or early in the Quaternary, period. Since that time the only notable change (barring the great Ice Age and its effects) has been a restriction of the area of the continent to its present size by spreading of sea waters over the borders of the continent, that is over the continental shelves.
We shall now rather systematically consider the more profound earth changes which have affected the continent, producing the existing major relief features, from west to east.
The “Coast Range Revolution” took place in the midst of the Tertiary period. Over the site of the Coast Ranges, strata had accumulated, especially during Cretaceous and earlier Tertiary times, to a thickness of thousands of feet. In middle Tertiary time these strata were subjected to a mountain-making force of compression and more or less folded, faulted (fractured), and uplifted into the Coast Range Mountains. Some portions of the range were intensely folded and faulted and upraised many thousands of feet, while other portions were only moderately folded and uplifted. It is an interesting fact that the great San Francisco earthquake rift or fault originated at this time. It was a renewed, sudden movement of a few feet along this fault which caused the disastrous earthquake of 1906. Still other considerable earth movements took place in the Coast Range region during late Tertiary and Quaternary times, as, for example, uplift without folding, as proved by distinct sea-cut terraces at altitudes of more than a thousand feet, like those north of San Francisco and south of Los Angeles. A moderate amount of still later subsidence has caused the development of San Francisco Bay. The large islands off the coast of southern California have in very recent geologic time (probably Quaternary) been cut off from the mainland by sinking of the land.
Plate 11.—(a) Part of the Mammoth Hot Springs Terrace in the Yellowstone Park. The view shows the deposit with boiling water flowing over it. The water enters the earth back on the mountain, travels underground in contact with hot lava, rises through limestone, from which the boiling water takes into solution much carbonate of lime which is deposited when the water reaches the surface. (Photo by Jackson, U. S. Geological Survey.)