Turning now to the Colorado Plateau, studies have shown that region to have been more or less periodically raised fully 20,000 feet since the beginning of Tertiary time, but because of profound erosion in the meantime its present altitude is only 6,000 to 9,000 feet. During late Tertiary time the land stood at a much lower level than to-day, so that, practically during the last period (Quaternary) of geologic time, the region has been elevated to its present position. As a direct result of this profound rejuvenation the Colorado River has had its erosive activity tremendously increased, and it has carved out the mightiest of all existing canyons--the Grand Canyon. The work of deepening and widening the canyon is still proceeding at a rapid geologic rate. As we have learned, the Rocky Mountains and many of its subsidiary ranges were formed by folding and uplift of strata toward the close of the Mesozoic era (Cretaceous period). During much of Tertiary time the newly formed mountains had been considerably reduced by erosion. Then, late in the Tertiary period, much of the Rocky Mountain region, as well as much of the Great Plains area just east of the mountains, became rejuvenated by differential uplift without any notable folding of strata. We can tell that this general uplift amounted to at least several thousand feet because definite formations of relatively late Tertiary strata, originally horizontally deposited under inland bodies of water, gradually rise so that at the base of the Front Range of the Rockies they are fully 3,000 feet higher than they are 200 miles or more farther east. Thus, the original folding and faulting of the Rockies, Tertiary volcanic activity, late Tertiary rejuvenation, and subsequent erosion account for the present altitude and relief features of the great Rocky Mountain system. Portions of the rejuvenated Great Plains region have been notably dissected by erosion since the late Tertiary, this being particularly true of the so-called "Bad Lands," especially in parts of Wyoming and South Dakota, where mostly relatively soft Tertiary strata have been cut to pieces. Turning our attention now to the eastern half of the continent we find that all, or nearly all, of it was more or less raised toward the close of the Tertiary period. Practically the whole Mississippi Valley east of the Great Plains, as well as much of the country to the north in Canada, was elevated some hundreds of feet and the streams have since the late Tertiary uplift (except where the land was ice-covered during the Ice Age) been at work sinking their channels below the newly upraised surface. As already pointed out, the lowlands of the Atlantic and Gulf Coastal Plains were mostly submerged under the sea during early middle Tertiary time. By the close of the period they had emerged practically to their present positions, and they have been only moderately affected by erosion.