Yol. 65.] GLACIAL EROSION IN NORTH WALES. 331 



profile (Cwm Llan) would have but one step ; and this one is of 

 exceptional form, being cut back by a rather wide notch, a feature 

 that does not appear in the other steps. A profile might be added 

 through the north-eastern cwm (Glas); this would, I believe, show 

 two steps, but the wide-spaced, high-level contours of the 1-inch 

 Ordnance map do not enable one to draw the profile satisfactorily. 

 Some of the steps in the profiles occur at the junction of smaller 

 with larger valleys : such are the second steps in the north-western 

 and western cwm-valleys, and the fourth step in the eastern cwm- 

 valley. The valley of Afon Gwarfai has two steps of 100 or 150 

 feet, above and beloAV Llyn Cwellyn. A third step or slope seems 

 to occur several miles farther north-west, beyond a stretch of broad 

 and flat floor (Bettws Garmon) ; and this, I venture to suggest, is 

 due to the confluence of the local Welsh glacier with the great ice- 

 sheet from the north. 



No consistent explanation of the steps can be found under the 

 theory of ice-protection, for they do not stand in the particular 

 relation to one another, nor in the proper position with respect to 

 the cwm-head cliffs, that is demanded by that theory. But, apart 

 from their disagreement in number and height, another difficulty 

 has already been mentioned in the explanation of rock-steps by 

 ice-protection : namely, that the normal erosion of a broadly open, 

 flat-floored valley below a rock-step — such a valley, for example, 

 as the Upper Gwynant Yalley below the steep 700-foot step from 

 Cwm Dyli — must have required a long period of time ; and yet 

 during all this period, the end of the protecting glacier must have 

 been nicely held at the top of the step. If it advanced forward for 

 a part of the period, then the excavation of the part of the valley 

 thus ice-covered would have been delayed, and the step would have 

 been of uneven slope ; if the glacier-end retreated for a part of the 

 period, then its water-stream would have cut a large notch in the face 

 of the step ; and such notches are characteristically wanting — the 

 notch in the rock-step at the lower end of Cwm Llan being the only 

 one of the kind among ten others in the immediate neighbourhood 

 of Snowdon. Even if a glacier should hold its end nicely at the top 

 of a step, it is not to be understood why the stream which issues 

 from the glacier did not trench itself in the face of the step, during 

 the long period in which the same stream deepened the unprotected 

 valley below the step, and during the much longer interval required 

 for the widening of the lower valley by the slower process of 

 lateral erosion, after its deepening was practically completed down 

 to grade. I believe that each one of these difficulties is fatal to the 

 ice-protection theory. It may be added that the prevailing absence 

 of deep gorges in the face of rock-steps is a strong reason for 

 ascribing very little glacial erosion to subglacial streams ; for 

 precisely at these points of rapid descent in the glacial bed, where 

 subglacial streams ought to be working most effectively, there is 

 practically no sign that they worked at all. The trifling depth of 

 the channel in which most of the streams run down the face of the 



