STUDIES FOR STUDENTS. 6 1 5 



certain point, steepness of slope does not favor avalanches, since 

 snow and ice cannot accumulate in sufficient quantity to produce 

 avalanches on slopes of too high gradient. The degree of slope 

 would affect landslides in a similar way. If a slope be too steep 

 there can be no slide of loose rock and earthy material, since 

 such materials cannot accumulate in sufficient quantity to give 

 rise to a slide. Since landslides and avalanches are at best no 

 more than subordinate sources of lateral moraine material, lateral 

 moraines are most likely to be well developed on those glaciers 

 which are bounded by high mountains with steep slopes. 

 All the drift which gains a superglacial position by any of the 

 processes thus far mentioned, is superglacial from the beginning 

 of its association with the ice. 



It is possible that lateral moraine material may reach its 

 position by another process. Where the bed of the ice is rough, 

 it may chance that the lateral portion of the glacier passes over 

 roughnesses of bed of considerable extent. If the lateral margin 

 of the ice passes over elevations which project up into it nearly 

 to its surface, those parts of the ice which pass around any given 

 elevation will presently come together below the same, carrying 

 with them some material from its slopes. Likewise the ice which 

 passes over the summit of the rock prominence may have worn 

 or torn away more or less rock material from its surface. Such 

 material is at first subglacial, with reference to the ice which 

 removes it, but it quickly becomes englacial as the ice moves on. 

 Some of it may be near the upper surface of the ice, after the ice has 

 united below the obstruction. Further down the valley, as a result 

 of melting, the ice surface may be brought down to the level of this 

 englacial material. When this happens, the englacial material 

 becomes superglacial. This superglacial material which has 

 passed through an englacial history may be an additional source 

 of lateral moraine material. Not all superglacial material 

 derived in this manner could enter into a lateral moraine. Only 

 those portions which reach the surface of a glacier near its mar- 

 gins would be so available. It is possible that, above the differ- 

 entiated alpine glacier, the ice of the snow-field may have passed 



