THE PLEISTOCENE OR GLACIAL PERIOD. 333 



the complex body of ice crossed the boundary of Canada, and encroached 

 somewhat on the United States in the Flathead, Kootenay, Columbia 

 Okanagon, and Colville valleys. The northern lobes descended the 

 valleys tributary to the Yukon, but, so far as now known, did not 

 cross the Canadian boundary into Alaska. It is not known that the 

 Cordilleran plateau glacier escaped the Rockies to the east, or even 

 sent tongues through their gaps in the more southerly latitudes of 

 Canada, though glaciers formed on the mountains crept out on the 

 western borders of the plains. In the more northerly uninvestigated 

 latitudes, where the mountains are lower and the gaps deeper and broader, 

 the descent of ice from the plateau on the west to the plains on the 

 east is not improbable. On the west, the plateau ice-cap seems to 

 have sent tongues of ice through the gaps in the coast ranges at many, 

 points, and to have discharged thence into the Pacific. Though ham- 

 pered by its environment, the Cordilleran ice-sheet seems to have 

 conformed to the habit of the Larbradorean and Keewatin sheets in 

 expanding chiefly to windward. If the whole glaciation, plateau and 

 alpine, be regarded together, the westward movement of the Cor- 

 dilleran complex was perhaps even more .pronounced than that of 

 the Keewatin and Labradorean. 



Mountain Glaciation. — In Alaska, mountain glaciation was strongly 

 developed on the ranges adjacent to the Pacific, particularly on the 

 side next to the ocean. On the north side, the ice pushed well out 

 from the higher mountains, but did not reach the Yukon. Some 

 ancient glaciation has recently been discovered on the divide between 

 the Yukon and the Arctic Ocean, but with this, and perhaps some 

 undiscovered exceptions, the plains of Alaska seem to have been free 

 from glaciation even at the stages when the waters of the Ohio and 

 the Missouri were being tinned from their courses by encroaching 

 ice-sheets, 2000 miles farther south. In view of these and other sin- 

 gular features of distribution, the localization of the ancient glacia- 

 tion becomes one of its most significant problems. 



South of the continuous Cordilleran glaciation of Canada, local 

 glaciers were widely distributed from the Rockies on the east to the 

 Sierras and Olympics on the west, while on the south, within the United 

 States, they appeared in New Mexico, Arizona, and southern Cali- 

 fornia. Within this broad area, the deployment of ice was greatest 

 at the north. Of glaciation in the mountains of Mexico little is known. 



