112 GLACIERS OF THE CANADIAN ROCKIES AND SELKIRKS. 



rapid streams, where it was immediately assorted into boulders, cobbles, gravel, 

 sand, and clay. In these various forms it was built into the terraces, flood plains, 

 deltas, etc., which characterize the drainage streams. The finer materials made 

 their way to the Pacific and Hudson Bay. The argument against great glacial 

 erosion that the material removed cannot be found, seems to the writer to carry 

 little weight. If one looks far enough and is able to distinguish the Pleistocene 

 and post-pleistocene deposits from the earlier, it seems probable that enough 

 would be located to restore the mountains and valleys to the condition in which 

 the glaciers found them. In the Bow and Cascade valleys, near Banff, Wilcox 

 discovered two distinct till sheets, indicating that there were, at least, two main 

 advances of ice through this section of the mountains. Eastward from the moun- 

 tains McConnell and Dawson found three such sheets, derived either in whole 

 or in part from the Rockies. 



2. Precipitation. 



a. Geographic distribution. Owing to the north to south trend of the four 

 mountain systems that here constitute the great Cordillera, their limited 

 breadth, their nearness to the warm waters of the Pacific, and the relation of the 

 region to the great cyclonic areas that enter from the Pacific, conditions are 

 favorable for an abundant precipitation upon the western slopes of the moun- 

 tain systems. The arrangement of the four systems being such that they 

 increase in height successively from the Coast Range to the Rockies, enables 

 all of them to get a fair share of the available precipitation. The prevailing 

 winds are from the west and laden with moisture. In ascending the windward 

 slopes much of this moisture is precipitated as rain, or snow, owing to the expan- 

 sion and consequent cooling of the air. In the condensation of this moisture 

 its latent heat is liberated and raises the temperature of the air. In being 

 drawn down the leeward slope by the general cyclonic movement of the atmos- 

 phere, the air is still further warmed by the compression to which it is subjected, 

 its capacity for holding moisture is increased, and it reaches the same elevation 

 upon the leeward slope much dryer and warmer than it was at the con-esponding 

 level upon the windward slope. This gives rise to the well-known "chinook 

 wind," the equivalent of the Alpine foehn. The Selkirks, lying to the west of the 

 Rockies, receive the heaviest precipitation, are more completely forested, expe- 

 rience more frequent avalanches of snow, and send their nev^s and glaciers 

 to lower levels. The shifting of the centers of the cyclonic areas to the south 

 of this region would give rise to prevailing easterly winds, which in the winter 

 would be colder and drj-er and in the summer warmer than those which now 

 prevail, and, without doubt, bring about the disappearance of glaciers from this 

 part of the mountains. 



b. Climatic cycles. Precipitation records are too scanty and fragmentary 

 for safe generalization concerning the occurrence in this region of oscillations 

 known to occur in the other parts of the world. Still there are several lines of evi- 

 dence which indicate that a phase of reduced precipitation closed in the Selkirks 



