5 88 HISTORICAL GEOLOGY 



it was generally believed to have been the cause of the accumulation 

 of ice which resulted in the Glacial Period. 



High Plains and Bad Lands. — The great sheets o f clay, sand, 

 and gravel which during the Tertiary were burying the eroded sur- 

 face of the Upper Cretaceous and other rocks in the region east of 

 the Rocky Mountains, gradually built up a great plain, in some 

 places 500 feet thick, stretching from the foothills of the Rocky 

 Mountains for hundreds of miles. This is known as the High Plains 

 region. The deposition that formed the Great Plains was not con- 

 tinuous in any one place throughout the period, but shifted from 

 time to time, being local and contemporary with more or less erosion. 

 Eolian deposits (loess) were building up the level, grassed surfaces 

 (as, indeed, they are to-day) and constitute a not inconsiderable part 

 of the formation. In recent times, however, erosion has been in 

 excess of aggradation, and the plain is being cut away. Uneroded 

 remnants of this plain, remarkable for their level surfaces, remain in 

 western Kansas, Nebraska, and westward. 



Where the plain has been dissected by canyons and ravines, it is 

 seen to be composed of unconsolidated gravels, sands, and clays. 

 Since the region has a scanty rainfall, although with occasional heavy 

 downpours (cloud-bursts), vegetation, except on the level surfaces 

 of the plain, is sparse. The scantiness of the vegetation on the 

 sides of the ravines, combined with the looseness of the sediments 

 of which the country is built, affords conditions most favorable for 

 rapid erosion when the torrents of water from the occasional heavy 

 rains rush down the ravines. As a result, in certain places along 

 the edges of the High Plains we find a maze of hills and ravines 

 (Fig. 533) with almost no vegetation except on the tops of the mesas 

 (the remnants of the former surface). These are the " Bad Lands," 

 " Mauvaises Terres " of the early French explorers, and constitute a 

 scenery as weird as any on earth. 



Pliocene of Other Continents. — The emergent condition of 

 Europe during the Pliocene was in contrast to the widespread seas 

 of the previous epoch. In the north of Europe, with the exception 

 of Belgium and a little of northern France, the seas had withdrawn. 

 Great Britain, as throughout the early epochs of the Tertiary, had 

 a greater land area than now, since only a small portion of the south- 

 ern part was beneath the sea at this time, while England, Ireland, 

 and Scotland were probably connected; and the northern coast 

 extended farther out than now. 



