GEOGRAPHICAL DEVELOPMENT OP THE AMERICAS 131 



was followed by the reestablishment of the ice, as recorded 

 in the upper sheet of drift. Even far to the north, on the 

 Hudson's Bay slope, an interglacial forest is embedded between 

 two glacial drift-sheets. In Iowa and South Dakota numerous 

 mammals of temperate character occur in interglacial beds. 



At the time of their greatest extension, the glaciers covered 

 North America down to latitude 40° N., though the great 

 terminal moraine, which marks the ice-front and has been 

 traced across the continent from Nantucket to British 

 Columbia, describes a very sinuous Une. The ice was not a 

 homogeneous sheet, moving southward as a whole, but flowed 

 in all directions away from several, probably four, centres 

 of accumulation and dispersal. At the same time, the western 

 mountain ranges had a far greater snow-supply than at present, 

 and great glaciers flowed down all the valleys of the Rocky 

 Mountains as far south as New Mexico and in the Sierras to 

 southern California, while the Wasatch, Uinta and Cascade 

 ranges and those of British Columbia and Alaska were heavily 

 glaciated, but, strange to say, the lowlands of Alaska were 

 free from ice. During the periods of greatest cold the rain- 

 belt was displaced far to the south of its normal position, 

 bringing a heavy precipitation to regions which are now ex- 

 tremely arid. In the Great Basin were formed two very large 

 lakes; on the east side, rising high upon the flanks of the 

 Wasatch Mountains, was Lake BonneviUe, the shrunken and 

 pygmy remnant of which is the Salt Lake of Utah, and on the 

 west side, in Nevada, was Lake Lahontan. Lake Bonneville, 

 which was nearly two-thirds the size of Lake Superior, dis- 

 charged northward into the Snake River, a tributary of the 

 Columbia, but Lahontan had no outlet. Each of these lakes had 

 two periods of expansion, with a time of complete desiccation 

 between them. 



Over the Great Plains the principal Pleistocene formation 

 is that known as the Sheridan, or, from the abimdanee of 

 horse-remains which are entombed in it, the Equus Beds. 



