CERTAIN PHASES OF GLACIAL EROSION 197 



prevalent air movement of a region may be nearly constant in 

 general, the cyclonic movements that are the immediate agents 

 that bring on precipitation introduce variation in the particular 

 direction from which the wind blows at the critical time when the 

 storm is on and the distinctive work in question is done. In the 

 mid-latitudes of the northern hemispheres, the general air movement 

 is toward the east but at the times of storms the wind not uncom- 

 monly comes from the eastward. However, the general law that 

 snow lodgment is most abundant on the prevailingly leeward sides 

 of prominences seems to hold good. This is greatly aided by the 

 shifting that takes place in the intervals between storms. 



The fact that the eddies formed in the lee of crests, domes, 

 and knobs are the common spots of lodgment carries as a corollary 

 the observation that the forms of the snowfields are usually broad, 

 or ovoid. The windward edge is usually arched, and is often 

 thickened near its upper border. Not unfrequently the thickened 

 snow mass is wider transversely than in the line of slope. Often, 

 too, it must be noted, the lodgment is concentrated in ravines and 

 valleys that were shaped previously by drainage erosion, and in 

 such cases the localization is less distinctive. 



The case best suited to a discriminative study is a broad or 

 transversely elongate lodgment of snow in the lee of a well-rounded 

 eminence from which the normal run-off is divergent. So long as 

 such a snow mass lies passively where it lodged, there can be little 

 doubt that it is protective rather than erosive, when compared 

 with normal surface action. So long, too, as the later action is 

 confined to a slow annual melting of the snow and a quiet run-off 

 of the resulting water, the snow and water combined perhaps do 

 less erosive work, on the whole, than would be done by the more 

 forceful impact and the more prompt run-off of the equivalent 

 rain, though qualifying conditions must be recognized on both 

 sides. 



The case of snow vs. rain, under these conditions, is not more 

 than debatable at most and the modes of erosion in the two cases 

 are essentially identical. 



But when the snow accumulates perennially so as to move as 

 snow-ice in glacier fashion, the modes of erosion become diverse, 



