CERTAIN PHASES OF GLACIAL EROSION 195 



sparsely as a modifying factor, it is usual to find the flowing con- 

 tours of the wear-and-weather type replaced in certain spots by 

 a type that may be said to be unconformable to the prevalent one, 

 a type in which concavity replaces convexity, a type in which the 

 surface has been broadly scooped out locally rather than rounded 

 off generally or narrowly incised. The broad scoop-like mode 

 of excavation, as distinguished from the gully-form mode of narrow 

 incision, is held to be distinctive in that it implies an agency that 

 deployed its effects laterally rather than one which concentrated 

 its action on axial lines. This, it is to be noted by way of precau- 

 tion, is a distinction that applies chiefly to the initial stage of the 

 two modes of erosion. They remain distinguished throughout 

 but are not so declaredly diverse in later stages. 



The lodgment of snow, which is the primary factor in glacial 

 work and determines its initial deployment, is controlled by the 

 wind to an exceptional degree, and wind action is chiefly horizon- 

 tal in its effects and is thus distinguished from rainfall and run-off, 

 whose dominant actions are vertical. While the very first phases 

 of this difference of action are not very important in themselves, 

 they are believed to be significant as the initial factors in the 

 localization as well as the deployment of the two classes of erosion. 



The relative locations of greatest rain-work and greatest snow- 

 work respectively. — Precipitation is intimately dependent on the 

 ascent of air so well laden with moisture that it reaches saturation 

 by reason of the expansion and cooling caused by the ascent. It 

 is for this reason that the ascent of moist air caused by rising over 

 the windward face of any marked relief of the topography deter- 

 mines precipitation on or near that face. As is well known the 

 windward sides of mountain chains thus receive more precipitation 

 than the leeward sides, as a rule. This holds true of snow-pre- 

 cipitation as well as rain, though the snowfall is less prompt and 

 less well localized. Where mountain ranges are broad and com- 

 plex the snow caught on the windward side is usually greater 

 than that which lodges on the leeward side, and the glaciers on 

 the windward sides of mountain ranges are usually larger than 

 those on the leeward sides. But such general community of 

 distribution does not hold in detail, for the wind comes in as a 



