CENOZOIC ERA: AGE OF MAMMALS 587 



Pacific Coast. — Deposits of this age are less widespread on the 

 Pacific coast than those of the Miocene. A change from marine to 

 fresh-water conditions in a portion of the area may have been due to 

 a raising of the land near the coast, or to an elevation along faults 

 which excluded the sea. Volcanic activity took place during the 

 period in certain portions of northern and central California, and in 

 the Sierra Nevadas and Cascades. 



Pliocene Elevation. — The deformation of the peneplain of the 

 Appalachian region raised trn Coastal Plain and shifted the coast line 

 to the east, except in Florida, where there was a slight depression. It 

 is possible that during this period of elevation the now submerged 

 valleys of the St. Lawrence, Hudson, Delaware, and Mississippi were 

 eroded. 



The plateau region of the west was uplifted at various times prior 

 to the Pliocene, during that epoch, and later, and has since been in- 

 trenched to form the great canyons for which it is famous. 



Near the close of the Pliocene the Rocky Mountains and the Sierra 

 Nevadas began a period of growth which has given them their present 

 altitude. Instead of folding, as at the close of the Mesozoic, the ele- 

 vation was chiefly due to warping and faulting. A study of a cross 

 section of the Sierra Nevadas brings out the fact that the slope on 

 the west is long and gradual and deeply intrenched by such great 

 valleys as the Yosemite and Hetch Hetchy, while on the east it 

 is very abrupt and short. This marked difference in the eastern and 

 western slopes is due to a profound fault on the east, which was first 

 formed in the Miocene, along which an enormous block was raised 

 and tilted to the west, leaving its eastern edge to form the crest of the 

 range. The movement along the fault plane has apparently not yet 

 ceased, as is shown by a slip of 25 feet which occurred in 1872. The 

 raising of the Sierra Nevadas inclosed the Great Basin region, shutting 

 off the moist winds of the Pacific and making it a desert. 



During the Pliocene the Cascade Mountains seem to have been 

 peneplained, the mountain mass being raised shortly before or 

 after its close. The rugged scenery of these mountains is the result 

 of comparatively recent erosion. Volcanic activity continued in 

 the epoch and became marked at the end, many of the great vol- 

 canoes of the west dating from the close of this epoch, or later. 



The close of the Pliocene was a time of widespread elevation, the 

 outline of the continent being extended, with few exceptions, farther 

 out than now. So marked was this elevation that for many years 



