132 LAND MAMMALS IN THE WESTERN HEMISPHERE 



These beds extend as a mantle of wind-drifted and compacted 

 dust from South Dakota to Texas and in places contain multi- 

 tudes of fossil bones; they correspond to one of the early 

 interglacial stages and in South Dakota pass underneath a 

 glacial moraine. 



The upheaval which came at or near the end of the Pliocene 

 had raised the continent, or at least its northeastern portion, 

 to a height considerably greater than it has at present, and this 

 must have facilitated the gathering of great masses of snow ; 

 but before the end of the Pleistocene a subsidence of the same 

 region brought about important geographical changes. The 

 depression, which lowered the coast at the mouth of the Hudson 

 about 70 feet below its present level, increased northward to 

 600 feet or more in the St. Lawrence VaUey and allowed the 

 sea to invade that valley and enter Lake Ontario. From this 

 gulf ran two long, narrow bays, one far up the valley of the 

 Ottawa and the other into the basin of Lake Champlain. The 

 raised beaches, containing marine shells and the bones of whales, 

 seals and walruses, give eloquent testimony of those vanished 

 seas. The recovery from this depression and the rise of the 

 continent to its present level inaugurated the Recent epoch. 



When the ice had finally disappeared, it left behind it 

 great sheets of drift, which completely changed the surface 

 of the country and revolutionized the systems of drainage by 

 filling up the old valleys, only the largest streams being able 

 to regain their former courses. Hundreds of buried valleys 

 have been disclosed by the borings for oil and gas in the Middle 

 West, and these, when mapped, show a system of drainage 

 very different from that of modern times. Innumerable 

 lakes, large and small, were formed in depressions and rock- 

 basins and behind morainic dams, the contrast between the 

 glaciated and non-glaciated regions in regard to the niunber 

 of lakes in each being very striking. 



On the west coast events were quite different; marine 

 Pleistocene beds in two stages are found in southern Call- 



