The interior of the western united states.  The closing stages of the Mesozoic were marked, as we have seen, by the upheaval of the Rocky Mountains and other western ranges.  The bases of the mountains are now skirted by widespread Tertiary deposits, which form the highest strata of the lofty plateaus from the level of whose summits the mountains rise.  Like the recent alluvium of the Great Valley of California, these deposits imply low-lying lands when they were laid, and therefore at that time the mountains rose from near sea level.  But the height at which the Tertiary strata now stand—­five thousand feet above the sea at Denver, and twice that height in the plateaus of southern Utah—­proves that the plateaus of which the Tertiary strata form a part have been uplifted during the Cenozoic.  During their uplift, warping formed extensive basins both east and west of the Rockies, and in these basins stream-swept and lake-laid waste gathered to depths of hundreds and thousands of feet, as it is accumulating at present in the Great Valley of California and on the river plains of Turkestan.  The Tertiary river deposits of the High Plains have already been described.  How widespread are these ancient river plains and beds of fresh-water lakes may be seen in the map of Figure 260.

The Bad lands.  In several of the western states large areas of Tertiary fresh-water deposits have been dissected to a maze of hills whose steep sides are cut with innumerable ravines.  The deposits of these ancient river plains and lake beds are little cemented and because of the dryness of the climate are unprotected by vegetation; hence they are easily carved by the wet-weather rills of scanty and infrequent rains.  These waterless, rugged surfaces were named by the early French explorers the Bad lands because they were found so difficult to traverse.  The strata of the Bad Lands contain vast numbers of the remains of the animals of Tertiary times, and the large amount of barren surface exposed to view makes search for fossils easy and fruitful.  These desolate tracts are therefore frequently visited by scientific collecting expeditions.

Mountain making in the tertiary.  The Tertiary period included epochs when the earth’s crust was singularly unquiet.  From time to time on all the continents subterranean forces gathered head, and the crust was bent and broken and upridged in lofty mountains.

The Sierra Nevada range was formed, as we have seen, by strata crumpling at the end of the Jurassic.  But since that remote time the upfolded mountains had been worn to plains and hilly uplands, the remnants of whose uplifted erosion surfaces may now be traced along the western mountain slopes.  Beginning late in the Tertiary, the region was again affected by mountain-making movements.  A series of displacements along a profound fault on the eastern side tilted the enormous earth block of the Sierras to the west, lifting its eastern edge to form the lofty crest and giving to the range a steep eastern front and a gentle descent toward the Pacific.