588 G. K. GILBERT 



degradation. The glaciers of these low ridges, being able to develop 

 only on the slopes most favorable for snow accumulation, marked 

 the lower limit of neve conditions and were the feeblest of all the 

 Sierra glaciers. Their lives must have been short, for they could 

 exist only when glacial conditions were at or near a maximum; they 

 began long after, and ceased long before, the glaciers of the higher 

 districts. The topographic features they produced were subject to 



S. or S.W. N.or N.E. 



Fig. 8. — Diagrammatic cross-section of a ridge glaciated on one side only, with 

 hypothetic profile (broken line) of preglacial surface. 



the dulHng influence of atmospheric and aqueous attack during both 

 interglaeial and postglacial times. And yet the degradation they 

 accomplished was far greater than that of nonglacial agents working 

 on the opposite sides of the same ridges — agents working not only 

 during the same time, but during all interglaeial epochs and during 

 postglacial time. It is true that we cannot measure the nonglacial 

 work, which consisted of a general reduction of surface without 

 notable change of form; but whatever the amount, we may assume 

 that it would have been the same on both slopes of the same ridge, 

 had there been -no glaciers. The visible ice-made hollows therefore 

 represent the local excess of glacial over nonglacial degradation. 



G. K. Gilbert. 



Washington, D. C. 



