48 Ice Marks in North Wales. [Jan., 



together more than three miles long, are good examples of this 

 class, and illustrate very clearly their characteristic peculiarities. 

 There is a drainage into these lakes of about twenty square miles of 

 country, bounded on both sides by mountain ranges over 3,000 feet 

 high. The whole of the glaciers from these had to pass out between 

 the ridge of the Clegr and that which descends from Cefn du, form- 

 ing a pass about half-a-mile wide, while the shores of the lakes are 

 all along bounded by steep and lofty slopes which would throw the 

 whole weight of the accumulated ice into the nearly level trough 

 between them. That the grinding power here was very great is 

 evidenced by the fact of the shores of these lakes presenting finer 

 cases of striation and grooving, of mamellation, and of complete 

 planing off of the softer rocks, than are perhaps to be found any 

 where else in Wales. Now most of the other lakes show exactly 

 the same arrangement, — wide upland valleys with many tributaries 

 above them, and below them a sudden narrowing of the valley by 

 projecting spurs. This can in most cases be sufficiently seen on the 

 Ordnance maps, but it is still more striking to look down at the 

 lakes themselves from a moderate elevation. Look at the two 

 ridges that meet together at an angle and shut in the valley at the 

 lower end of Llyn Ogwen, or the precipitous slopes that confine 

 Llyn C welly n, west of Snowdon, and Talyllyn, south of Cader Idris. 

 In these and most other cases the valleys containing lakes are of 

 very moderate inclination or nearly flat, so that the motion of the 

 glacier would be slow and would chiefly arise from pressure. When 

 therefore a sudden narrowing of the channel occurred, the ice would 

 necessarily accumulate just above the obstruction, and thus give that 

 increased weight and grinding power which are exactly the con- 

 ditions said to have produced lake basins. Without going any 

 further into particulars, I may state generally that the situation and 

 surroundings of many of the lakes of North Wales are just such as 

 ought to exist if Professor Kamsay's theory be the true one. 



As a glacier can only be now grinding out a lake basin in the 

 very thickest part of its course, it is very difficult to see the opera- 

 tion going on. At the same time so much is known about glaciers, 

 and so many of the facts bearing upon this question are admitted 

 by all, that some conclusions seem quite clear. For example, all 

 admit that glaciers do (or once did) grind down the rocks over 

 which they pass, to some extent. The grinding is caused chiefly 

 by the weight of the glacier, and therefore where the glacier is 

 thickest the grinding will be the greatest. Glaciers behave like a 

 very thick semi-fluid mass, flowing and filling up channels of varying 

 widths, and therefore accumulating where there are obstructions to 

 their free passage. Now where such an accumulation takes place 

 in a valley of tolerably uniform slope, there will be more weight 



