458 MR T. J. JEHU ON 



must have been nicely gradated in relation to each other. The discordance of the 

 hanging valleys and the main valley, seen to-day, is due to the fact that the bed of the 

 main glacier has been worn deeper than the beds of the side glaciers. 



It ought not to be forgotten that as long ago as 1893 Dr A. E. Wallace had, in 

 an article which appeared in the Fortnightly Review, suggested that erosion had been 

 carried on more rapidly in the main valleys. Referring to large lake-basins, he says : 

 " On looking at the maps of any of these lakes, one cannot but see that the lake surface, 

 not the lake bottom, represents approximately the level of the pre-glacial valley, and 

 that the lateral streams and torrents enter the lake in the way they do because they 

 could only erode their channels down to the level of the old valley before the ice over- 

 whelmed it." 



Somewhat similar phenomena to those described by Davis had already been observed 

 in Norway, and are referred to in Professor Geikie's Earth-Sculpture. In that country 

 we have a markedly ice-worn plateau land, intersected by deep, chasm-like, fiord valleys, 

 which are U-shaped in cross section. The erosion of the main or fiord valleys is greatly 

 in advance of that of the lateral valleys. The tributary streams, after winding through 

 the plateau land in broad and shallow valleys, suddenly cascade dow T n the precipitous 

 walls into the fiord. The explanation given by Dr Richter is that while at certain 

 stages of the Glacial Epoch glaciers and glacier streams were deepening the main valleys 

 and making their walls steeper, erosion was practically at a standstill in the side valleys, 

 which lay buried under the firn and ice of the plateau. 



In 1901 a paper was communicated to the Royal Society of Edinburgh by Mr 

 Alfred Harker, on "Ice-erosion in the Cuillin Hills, Skye " [Transactions, vol. xl. 

 part ii. No. 12). In that region, also, straight steep-sided valleys, giving U-shaped cross 

 sections, were in evidence, and are attributed to the work of local glaciers as distinguished 

 from merely aqueous erosion. 



The fact that glaciers are thus proved to be powerful agents of erosion, capable not 

 only of quarrying their beds at certain spots where the conditions are favourable, but 

 also of deepening and widening for miles together the main valleys along which they 

 flow, simplifies greatly the discussion of the origin of lake-basins in such a strongly 

 glaciated region as that of North Wales. Indeed, the lack of similar evidences of erosion 

 in such an area would be a fact so strange that it would need some explanation. It 

 becomes natural, therefore, for us to expect to find lakes resting in basins hollowed out 

 of the valley floor, and also, possibly, to find indications that some of the main valleys 

 have been over-deepened as compared with the side valleys. 



The district with which we are concerned presents a highly complicated geological 

 structure. It is made up of a heterogeneous assemblage of sedimentary and aqueous 

 rocks of Cambrian and Ordovician Age. Slates, grits, and sandy and calcareous be<ls 

 are found interstratified with felspathic ashes and lavas, and intruded into these are sills 

 and bosses of greenstone, felspathic porphyry, and other massive igneous rocks. 



It is owing to this intermingling of rocks of varying character, and to the unequal 



