W. Keeping — Glacial Geology of Wales. 255 



arenaceous and more stony Boulder-clay also occur in an irregular 

 manner. The included pebbles are mostly slates, including some 

 pale and some gritty slate, also greywacke, much Mountain Lime- 

 stone, Old Eed Sandstone, Coal-measure Sandstone, Felsites and 

 Greenstones. This deposit appears to have been formed by a large 

 glacier which had its origin in the Caermarthenshire hills of South 

 Wales, where the Upper Palgeozoic pebbles were obtained. Thence it 

 streamed westwards, overriding the intervening watershed, and forcing 

 its way down through the Teifi valley to the Irish Sea. Boulders of 

 the same rocks, foreign to the district, occur also in the country 

 further north towards Aberporth, up to a height of 700 feet. 



Lakes. — Small lakes and pools are of frequent occurrence in the 

 high ground (on the higher marine plane) in Cardiganshire, usually 

 in groups of two or more. 



The accumulation of morainic matter at the bases of some of these 

 lakes has already been mentioned, but all are in the main true rock 

 basins, the morainic accumulations (at Llyn Llygad Eheidol, etc.,) 

 adding nothing or very little to the depth of the basin. There is, I 

 believe, no case of a simple moraine dammed lake in the county. 

 Those examined by me proved to be nearly all shallow depressions 

 from 7 to 32 feet deep,^ irregular in form, several of them having 

 islets in their midst. 



An exception to this must now be noticed, namely, Llyn Llygad 

 Eheidol, a sombre pool under the precipitous head of Plynlimmon, 

 which is much deeper (60 feet) than the others. The position of 

 this lake is highly favourable for ice erosion, and the section of its 

 bed agrees better in shape with ice-worn hollows. It is indeed, 

 in my opinion, a very good example of an ice-formed rock basin. 

 The clayey terminal moraine at the foot of the lake acts as an 

 additional dam for deepening the waters.^ 



In all the other cases there is no high ground near where a local 

 glacier of any adequate size could have collected. Indeed, the general 

 arrangement and directions of the lakes, the sections of their beds, 

 their contours, and the features of the neighbouring land alike show 

 that a glacial erosion theory is utterly inapplicable to the forma- 

 tion of these rock hollows, and we must refer to other causes for 

 their origin. The neighbourhood of most of the lakes is remarkable 

 for the frequent foldings of the rocks, and in one case (Llyn 

 Fyrddyn Fawr) there is a fault, with slickensides well exposed near 

 the water's edge. I look to earth movements to account for their 

 formation. 



The Aberystwyth Beach Pebbles. — Aberystwyth beach is well 



1 The common belief of the people is that the lakes are very deep, but all that I 

 examined were shallow, the Teiii Pools and Llyn Gynon being from 7 to 33 feet, and 

 Llyn Gwj'ddior, Llyn Bugeilyn, and Llyn Fyrddyn Fawr about 33 feet. Some of 

 these measurements of depths were obtained by swimming across the lakes, and the 

 rest by waiting till they were frozen over, when the ice was broken thi'ough with the 

 hammer, and a line let down. 



" Aberystwyth now obtains its water from this lake, and I am indebted to the 

 engineer, Mr. Stooke, for information kindly sent me about the slopes of the lake 

 bed, and the section of the moraine at its base. 



