614 THE JOURNAL OF GEOLOGY. 



By any or all of these processes, stony or earthy material may 

 descend into a valley which has no lgacier. It may there accu- 

 mulate at the base of the slope, or it may be carried away by 

 waters coursing through the valley. It may descend into a 

 glacier valley below the end of the ice, when its fate is the same. 

 If it descend into a valley above, but near the end of a glacier, 

 it may fail to reach the surface of the ice, since near the lower 

 end of a glacier, where melting is rapid, the ice often fails to fit 

 snugly against the bounding slopes of rock. Under these cir- 

 cumstances, material descending from the slopes above is liable 

 to fall between the glacier and the valley wall. It is only where 

 the glacier fits snugly against the sides of its valley, that material 

 descending from the slopes can reach its surface so as to con- 

 tribute to a lateral moraine. In general, an alpine glacier fits 

 snugly against its valley walls in its upper stretches only, and it 

 is here, therefore, that lateral moraine material is most likely to 

 be acquired in quantity, by the processes indicated. 



It will be readily seen that steep slopes above the surface of 

 a glacier favor the accumulation of moraine debris upon it. 

 While expansion and contraction due to changes of temperature, 

 and while freezing of water in rock crevices, need not be more 

 effective on steep slopes than on gentle ones at the outset, 

 material once loosened will be much more likely to travel down 

 steep slopes than down gentle ones. Steep slopes, therefore, 

 will be more likely to lose such incoherent material as develops 

 upon them, and so be kept bare, while the gentler slopes will be 

 more likely to retain the disrupted and disintegrated material to 

 which they have given rise. On the gentler slopes, therefore, 

 the rock surface will ultimately come to be protected against 

 changes of temperature and other disrupting surface influences, 

 by its mantle of unremoved debris. Thus it comes about that in 

 the long course of time steep slopes above glaciers will contrib- 

 ute much more debris, than gentle ones for the making of lat- 

 eral moraines. 



Beyond a certain point, steepness of slope would not in all 

 ways favor the development of lateral moraines. Beyond a 



