Yol. 65.] GLACIAL EROSION IN NORTH WALES. 341 



by the height of the main valley-floor steps ; but in both 

 allowance must be made for the glacial erosion of the lateral 

 valleys, or of the bench at the top of the rock-steps. As well as I 

 can estimate it on this basis, the total deepening of the main 

 valleys by glacial erosion in the Snowdon district may be from 

 200 to 400 feet, and in some cases 500 or 600 feet. It is important 

 to note that, with the downward erosion, a much greater lateral 

 -erosion must be associated : this may easily amount to 1000 or 

 1500 feet, both right and left, in some of the larger valleys where 

 the downward erosion measures 300 or 400 feet. These estimates 

 for downward erosion correspond roughly with those obtained from 

 the cwms. In both cwms and valleys glacial erosion has therefore 

 been of importance in shaping the present landscape. 



Glacial overflows.' — A striking case of glacial overflow occurs 



■ at the head of Nantlle Valley. The growing ice-streams from 

 the Snowdon centre must have filled the valley of Afon Gwarfai 

 (with outlet to the north-west) and Afon Colwyn (with outlet to the 



^south), and have become confluent across the divide between these 

 valleys ; then the ice must have overtopped the somewhat higher 

 divide at the head of Nantlle Valley, of which the outlet is to the 

 west. From this divide a fairly considerable ice-stream, overflowing 

 westwards, must have been joined by several small glaciers, and 

 must have had a comparatively rapid slope to the sea. It would be 

 reasonable, even under the theory of ice-protection, to expect some 

 -moderate amount of erosion as a result of the intensified action 

 that should follow from the accelerated motion of a glacier over- 

 flowing a pass into a valley-head : under the theory of ice-erosiou, 

 such points of overflow are naturally enough the very places where 

 strong erosion would be expected. Further, the more a pass is 

 worn down, the more ice will flow through it ; and the more ice 

 flows through the pass, the deeper will it be worn down; at the 

 .same time, the col will be shifted towards the higher valley. 

 Changes of this kind are clearly indicated at the steep head of 

 Nantlle Valley. Great cliffs rise on the north to Mynydd Mawr, 

 •and somewhat lower cliffs on the south to Y Garn. If reasonable 

 moel-slopes are reconstructed between the cliffs, the situation of 

 the pre-Glacial pass would be found at a greater altitude and 

 farther west than the present pass (at the Nantlle reservoir). By 

 drawing a valley- profile from the pre-Glacial pass westwards, having 

 due regard for the level at which several hanging lateral valleys 



■ are found on the way, the pre-Glacial Eantlle Valley may be 

 roughly reconstructed. The present Nantlle Valley is several 

 .hundred feet deeper than its reconstructed predecessor. 



It seems probable that a similar transection and shifting of the col 

 at the head of the Llanberis Valley has taken place, and the depth 

 of erosion in that case would be distinctly greater than at the head 

 *of Nantlle Valley; it may be roughly placed at 600 or 700 feet, 

 with a widening at mid-depth of more than twice those figures. 

 Q. J. G. S. No. 259. 2 a 



