THE LAKES OF SNOWDONIA AND EASTERN CARNARVONSHIRE. 455 



VII. Origin of the Lakes. 



The frequency of lakes in regions which have been well glaciated has often been 

 noted, and it is generally agreed that this is something more than a mere coincidence. 

 That part of North Wales with which we are concerned in this memoir is remarkable 

 for the evidences which are there displayed of a past Glacial Epoch ; and a glance 

 at a general map of the district will show that its surface is sprinkled over with 

 lakes. The close association subsisting- between the glaciation of a district and the 

 development of lake-basins suggests a causal connection between the one and the 

 other ; the basins in some way or other owe their origin to glacial action. The late Sir 

 Andrew Ramsay, while admitting that some of the lakes of North Wales are at least 

 partially dammed up by moraines, held that many occupy basins scooped out of the 

 solid rock by glaciers. It is now forty years since Ramsay first advanced his theory of 

 ice-erosion to account for the formation of lake-basins lying in the path of the old 

 glaciers. Arguments in support of the theory were brought forward by Tyndall, 

 A. R. Wallace, and other writers ; and it was adopted by the late Mr Clifton Ward 

 to explain the origin of the lake-basins of Cumberland, and by Professor James Geikie 

 to explain many of those of Scotland. Other geologists have been unable to accept 

 Ramsay's hypothesis, and a keen controversy has been carried on for many years con- 

 cerning the power of glaciers to erode their beds. As this question is of vital import- 

 ance in discussing the possible modes of origin of the lake-basins in such a glaciated 

 region as North Wales, it becomes necessary for us to obtain clear ideas as to what 

 power a glacier has to excavate hollows in the solid rock. That glaciers often abrade, 

 smooth, and polish the rocks over which they flow is admitted by all. But there is no 

 such unanimity regarding the capacity of ice to excavate rock-basins. 



According to some writers, the erosive power of glaciers is insignificant ; others admit 

 that it may be sufficient under favourable circumstances for the production of small rock- 

 basins, but are opposed to the view that the larger lake-basins have been formed in this 

 way. Professor Bonney, for instance, maintains that the hypothesis of glacial erosion will 

 not suffice to explain the origin of the great Swiss and Italian lakes, but thinks it possible 

 that smaller lake-basins, such as some of those in the Lepontine Alps, have been produced 

 in this way ; and M. Delabecque holds similar views with regard to the lakes of France. 

 In connection with the Swiss lakes, Mortillet and Gastaldi, many years ago, 

 suggested that the work done by glaciers has been the ploughing out of the alluvium 

 which in pre-glacial times filled the valleys. Again, it is quite possible that before the 

 advent of the Glacial Epoch, the rocks of a region like North Wales had decayed and 

 rotted to a considerable depth owing to their exposure for a vast lapse of time to 

 sub-aerial weathering agents. Such sub-aerial disintegration has been observed to occur 

 especially among crystalline schists and eruptive rocks. Many instances are given by 

 Sir A. Geikie in his Text-Book of Geology (ed. 3, p. 350). He states that in Brazil, the 



