252 W. Keeping — Glacial Geology of Wales. 



mass of Plynliramon. From many points of view these two planes 

 and their cliif-like line of separation are strikingly shown, for 

 example, from the hills above Cwm Gwarchau, Machynlleth, and 

 between Goginan and Cwm Symlog. 



The Drift formations range over both these planes reaching up to 

 within 100 feet of the highest ground. They consist principally of 

 ice-formed stony clay or Till, which may be overlain by stratified 

 beds of loam, sand, and gravel, of later glacial age, these being 

 covered in the river valleys by the more modern river gravels. 



We may observe at once that over nearly the whole of this country 

 the drifts consist entirely of local materials, and when any difference 

 of character occurs in the general uniformity of the solid rock, we 

 can usually see that the drifts have been formed from the degrada- 

 tion of the rocks within the present drainage areas. Exceptions to 

 this rule are found only on the extreme N. and S. borders of the 

 county (Cardiganshire). 



Glaciated BocTc Surfaces. — The various surface features usual in 

 glaciated districts are well marked in this area, many of the hills 

 showing well the flowing contours on their sides, and the valleys, 

 such as Nant Myheran, exhibiting the broad U-shaped sections 

 characteristic of old glacier courses. Roches moutonnees, with polished 

 and striated faces, are still perfectly preserved, as fresh as if the old 

 glacier had but just left them. Large erratics and perched blocks 

 are abundant, and in places the crushing force of the ice has torn up 

 the ground rock, and carried the fragments onwards in a broken 

 stream at the base of the Till. Examples of the latter phenomenon 

 are seen in the cliffs under the old castle at Aberystwyth, and the 

 coast section four or five miles south of that town. In the latter 

 place the solid rock is fractured and disturbed to a depth of five feet 

 from the base of the stony clay. 



The ice-polished rocky mounds known as roches moutonnees are 

 particularly well preserved in the district, the polish and striation on 

 the rocks being actually as perfect as those seen around modern 

 glaciers. Good examples are exposed at the head of the Llyfnant 

 valley, particularly one fine broad back at the base of the waterfall 

 called Pistill y Llyn ; other good examples will be found in the 

 Claerwen valley, at Pont Faen, Machynlleth, on the side of Afon 

 Tarenig, Plynlimmon, also on the banks of Llyn Fyrddyn Fawr, L. 

 Glaslyn, and L. Bugeilyn, and half a mile west of Cwm Berwyn Farm. 



The Stony Clay or Till. — A good section of the Till is exposed in 

 the cliffs at Ffos las and Cwm Ceirw, south of Aberystwyth. The 

 deposit consists of a matrix of amorphous stiff blue clay crowded 

 with small angular grains and shaly fragments. This paste is thickly 

 studded with rocks of all sizes, from mere fragments to large masses 

 several tons in weight. Most of the stones are subangular and well 

 striated, a few are rounded, and others are unworn and angular. 

 Patches of angular blocks occur in several places, showing scarcely 

 any traces of rounding or striation. All the rocks are of local origin. 

 This deposit is so tough that it forms high overhanging cliffs, seventy 

 or eighty feet high. Above the Till occur roughly stratified clay 



