Vol. 65.] GLACIAL EROSION IN NORTH WALES. 309 



same time, let it be recognized that normal erosion continues on 

 the higher slopes above the ice, reducing them to gentler declivity ; 

 and also, farther down the valleys, deepening and widening them, 

 as in fig. 9. In order to examine certain details, let ABMCD, 

 fig. 10, represent a pre-Glacial stream- profile, and BC represent a 

 short glacier ; then, while normal summit-erosion changes AB to 

 EB, and normal valley-erosion changes CD to CFG, BMC will 



Fig. 10. — Successive profiles of a protective glacier. 



remain unchanged. Several special consequences of these sup- 

 positions should be noted : First, the points B and C must stand in 

 such relation as would represent the upper and lower ends of a 

 glacier formed in the space between them. Second, if the lower 

 stretch of the valley FG is to be widened as well as deepened, then 

 the slope FGr must be first worn down to grade with respect to the 

 controlling base-level of the stream-system. Third, all the important 

 branch streams down valley from F will develop mature branch- 

 valleys, joining the deepened main valley at accordant level. 

 Fourth, the distinctness of the valley-floor step (CF) will depend in 

 great measure upon the steadiness with which the little glacier 

 holds its end in one position. Fifth, the rock-step will have no 

 necessary relation to rock-structure. Sixth, the more pronounced 

 the rock-step, the more distinct should be the angle at B in the 

 slope EBM, and the more distinct should be the corresponding angle 

 or ' edge ' along the sides of the glacier. All these consequences 

 are illustrated in fig. 9 also, but on a smaller scale than in fig. 10. 



Now let it be supposed that the glacial climate becomes more 

 severe, so that the glacier increases in size, as shown in fig. 11 

 (p. 310). Its profile is shown in LKH, fig. 10. The mountain- 

 summit above the glacier will now be worn to still gentler slopes 

 (if the summit is not completely enveloped in a protecting neve'- 

 cover), and the valley down-stream from the glacier-end will be 

 further widened by normal stream-action. But it cannot be sig- 

 nificantly deepened, unless there is an elevation of the district 

 contemporaneous with the increase in the size of the glacier ; for 

 the widening of the valley-floor in the previous stage can have 

 taken place only after the stream had worn down its channel to 



Q. J. G. S. No. 259. y 



