324 



PROF. W. 310EEIS DAVIS ON 



[Aug. 1909, 



contradict all experience. Subdued monadnocks of normal erosion, 

 even when their valleys are deepened by revived streams, do not 

 show sharp ridges; and to suppose that sharp and narrow ridges 

 could persist when the valley-floors had gained as great a breadth 

 as that of the Snowdon cwms is altogether unwarrantable. If, 

 however, instead of supposing that these serrated spurs have 



iFig. 22.— Sketch of Crib Goch, 

 looking north- eastwards from 

 Snowdon summit. 



Fig. 23.— Sketch of Lliwedd, 

 looking south-eastwards from 

 Snowdon summit. 



preserved their pre-Glacial form by burial, it is supposed that they 

 rose above the neve in the adjoining cwms, then all sharpness of 

 form ought to have been lost in the long epochs required for the 

 deepening and widening of the lower valleys, below the rock-steps 

 at the cwm fronts. Surely the explanation of the valley-head cwms 

 and the sharp dividing cribs by the theory of protective glaciers is 

 attended with difficulty. 



Figs. 24 and 25. — Diagrams illustrating the origin of the summit of 

 Snowdon, on the theory of protective and of eroding glaciers. 



The summit of Snowdon receives very unlike explanations by 

 the rival theories. If glaciers are protective, pre-Glacial Snowdon 

 must have been a sharp and high peak, which may be roughly re- 

 constructed by continuing the present cwm-walls upward till they 

 meet, as in fig. 24. Glaciers then headed up against the walls 

 to the height of the present wall-tops and preserved them while the 



