444 



THE AGE OF MAMMALS 



Alaska. — The coast of Alaska was elevated, a broad land bridge with 

 Asia existed, and the continental shore line extended far north into the 

 present Arctic Ocean, but the northern portions of the peninsula itself were 

 never covered with ice. 



2. Subsidence. — Continental depression was the grand feature of 

 post-Glacial times on both the Atlantic and Pacific coasts. The San 

 Pedro Stage of California corresponds with the Cham-plain Stage of the East. 

 In Champlain times a long arm of the sea probably extended up the Hudson 

 River to Lake Champlain, converting it into a small mediterranean sea 

 which attracted seals and even white whales {Delphinapterus leucas). The 

 subsidence carried both the North Atlantic and North Pacific shores several 

 hundred feet below their present level. This was a warm and moist climatic 

 period. 



3. Reelevation. — This depression- was followed by a renewed eleva- 

 tion, both of the Atlantic and Pacific coasts, which apparently coincided 

 in time with the Terrace Epoch of the New England river valleys. 



These elevations and subsidences naturally exerted a profound influence 

 (a) on temperature, (6) on moisture and precipitation, (c) on the migrations 

 and extinctions of animal and plant life. 



Glacial and Interglacial Stages 



Glacial Stages 



Wisconsin 



Drift from the Keewatin centers. 

 Excessively calcareous. Drift of light 

 yellow color. A moraine-forming ice- 

 sheet covering Iowa. Divided into 

 earlier and later phases. 



lowan 



Drift from the northwest Keewatin 

 center barely reaching Iowa. Light yel- 

 low in color. Large granitic boulders. 



niinoian 



Yellow drift covering small part of 

 Iowa. Ice chiefly from the northeast 

 Labradorian center. 



Xansan 



Interglacial Stages 



Fifth interglacial 



Fourth interglacial, PEORIAN 



Interval very short as compared 

 with the Yarmouth or Sangamon. 

 Yellow layers containing terrestrial 

 molluscs (Coccinia pwpa) in the Missis- 

 sippi valley, in Illinois, in Iowa. 



Third interglacial, SANGAMON 



Clearly represented in Illinois. 

 Buried forests, peat beds, and pond 

 silts. Forests of tamarack {Larix). 

 Long interval, though shorter than the 

 Yarmouth. 



Second interglacial, YARMOUTH 



Forests and terrestrial faunas typi- 

 fied at Yarmouth. Apparently the 

 longest of the interglacial intervals, 

 " perhaps longer than all post-IUinoian 



