CERTAIN PHASES OF GLACIAL EROSION 



213 



may not develop, but this does not affect the behavior of the gla- 

 cier below. If a pre-existent step or down-set crosses the bottom 

 of the valley at any point beneath a glacier, or if a step is developed 

 by structural inequalities, and if the down-set is sufficiently great 

 in proportion to the thickness of the ice to cause effective crevassing 



4 



*#>•'* 



Fig. 10. — View from the Baregg in the Bernese Oberland, Switzerland. At 

 the bottom of the picture below the ice fall is the comparatively level Lower Eismeer 

 of the Unter Grindelwald Glacier. Above the ice cascade is another higher ice plateau 

 known as the Fiescherfirn, limited by the slope seen in the perspective. Peeping 

 through the mists high above is the point of the Grosse Fiescherhorn. Photo, by 

 R. T. C. 



through the whole depth of the glacier, the conditions at the base 

 of the step are not radically different from those at the base of the 

 cirque wall, for there is in effect a break in the continuity of the 

 motion of the glacier and the beginning of a new motion in the ice 

 mass below. The rock face of the step may be regarded as a 

 cirque wall in a modified sense. From it masses may be detached 

 and, falling against the ice wall, become attached and dragged 



