The initial uplifts of the Sierran mountain belt were made in Mesozoic time, and the strata involved were formed from the waste of the older lands in the present Great Basin region to the eastward. But it was not until late Tertiary time that the entire block or mass of the Sierras was lifted to a great height and tilted to the west. In connection with this uplift a lofty fault scarp developed, which now forms the steep eastern front of the mountains. This crest, therefore, is toward the east, and the principal drainage is down the gentler western slope into the valley of California. The Sierras continue northward as the Cascade Mountains of Oregon and Washington. The Willamette Valley in Oregon and the Puget Sound Valley in Washing ton are the analogues of the valley of California, and they are separated from the ocean by a young range of mountains, known in its various parts as the Coast Range, the Klamath Moun tains, and the Olympic Mountains.
Thus it appears that the Western United States have had a composite history. It began with island nuclei which grew by sedimentation and uplift. The several great ranges of moun tains mark several periods of folding, faulting, and uplift, while both mountains and plains rose by massive and wide-ranging or continental move ments, thus adding to the height of the moun tains and making the plains into plateaus of from 3000 to S000 feet in altitude. With these disturbances, especially in Tertiary times, were the most extensive outflows of lava of which this continent shows any record. These are found either as remnant sheets and volcanic necks, or as vast sheets scarcely changed since their out flow. They occur in nearly every Cordilleran State, as on both sides of the Rocky Mountains in Colorado, in New Mexico, in Utah, in Mount Sha-gta, and the great cones of the Cascades, and especially in the lava plateaus of the Snake and Columbia rivers in Idaho, Washington. and Oregon. As a lingering episode of their interior volcanic energy we may, perhaps, recognize the reyser phenomena of the Yellowstone Park.
Through all the which have been passed in review, the process of land sculpture was active and all varieties of relief have been shown, even in the same area, in successive cycles of denudation and uplift. Thus New England is everywhere mountainous in structure, although much of its surface is now reduced to hills, and to valleys whose bottoms are not far above base level. The Connecticut Valley, as it now exists, is rather a product of downwear along its course than of uplift on its sides. The same is true of the Hudson, and, indeed, of all the valleys of the Appalachian region. Much of the Arclilean core of this Eastern mountain system is well worn, like the coastal parts of New England.
Such a region is the Piedmont belt of hills and low mountains lying between the Blue Ridge and the coastal plain in the South Atlantic States. So, too, the Appalachian .lountains of post Paleozoic age were perhaps raised to Alpine heights, but have been greatly reduced and given their present relief of ridge and valley by proc esses of wasting and the survival of more resist ant formations.
The strata of the Mississippi basin have for the most part never been raised to considerable heights, and the streams have not had sufficient vigor to be the instruments of largest denudation. But among the mountains and plateaus of the Cordillera]] region the land forms owe their pe bets to long continued denudation, conditioned by the composition and structure of the rocks. For example, in the Grand Cation region, or more broadly. everywhere among the Colorado plateaus, vast. thicknesses of the upper strata have been bodily removed. Those that remain present south-facing escarpments of ragged out line, and their masses are in turn profoundly dissected by the swift river and its branches. The true measure of denudation is not the canons, however, hut the amount of stripping that has been accomplished over the whole region. Simi larly, the lava sheets have been deeply channeled, as by the Snake River, o• have often been nearly removed, and volcanic cones are in all stages of decay, from the little marred Mount Shasta to the 'necks' that mark the complete disappearance of overlying cones.
The principal reliefs were given to the country prior to the glacial invasion. But within the field of glacial movement important changes were effected, and in some cases the combined effect of the wearing of hill and mountain tops, and of the filling of valleys, was to diminish the total relief by several hundred feet. The territory affected includes all of New England, the 'Middle States into northern New Jersey and Pennsyl vania, and the Central States to lines not far from the Ohio and Missouri rivers. Eastern Ne braska was included, with much of the Dakotas, Montana, and the more northern Cordilleras, Small remnant glaciers are still found in the high Sierras, on the volcanic cones of the Cascades, and in Montana and Colorado. The general ef fects of the ice sheet were the grinding and trans port of rocky waste, coarse and fine, the com mingling of this material with the prOxistent soils, the formation of moraines and other bodies of drift during retreat, the blockading of ancient valleys causing innumerable changes of drainage, and the formation of thousands of lakes.