254 W. Keeping — Glacial Geology of Wales. 



Terminal Moraines. — Various humraocky mouncls and semicircular 

 ridges seen in the upper parts of some of tlie valleys are clearly the 

 remains of morainic matter heaped up at the end of the melting 

 glaciers, i.e. Terminal moraines. In some cases they form banks at 

 the foot of lakes, as in the Llyn Glaslyn, Llyn Gwyddior, and 

 Llyn Llygad Eheidol, where they hold up the waters beyond the 

 levels of their rock basins. Other examples are seen just below 

 Diffwys ; and in the side valleys at Blaen y Cwm, and under Craigddu 

 in Cwm Ystwyth. 



Erratics. — The erratic and perched blocks are distributed in an 

 irregular and promiscuous manner, some areas being crowded with, 

 them, as over the country east of Llanfihangel and Cwm Ystwyth, 

 where they are dotted like sheep over the bills, while in other 

 places, e.g. around Llynodd leuan, we may walk for miles without 

 seeing one. They are found at great heights, as in the high ground 

 above Cwm Symlog towards Goginan, where fine groups occur at 

 1000 and 1200 feet levels. All are local rocks, namely, the Grey- 

 wacke and Shaly Slates (with certain exceptions to be mentioned 

 below), and but few are of gi'eat size. Fine examples occur around 

 the lakes Llyn Eiddwen and Llyn Berwyn, and at Nant y maen, 

 Tregaron. Some boulders of quartz east of Llanfihangel may have 

 come from mineral veins, and a fine boulder of conglomerate on 

 Gogerddan Hill, which contained a few fossils, has probably come 

 from the Plynlimmon hills.' 



These erratics are angular, and bear no marks of water- or ice- 

 wearing upon them, and I regard them as having been for the most 

 part launched off from the sides of the glaciers and left stranded 

 upon the hill-sides as the ice gradually fell to lower levels. 



Boulders from beyond the present drainage areas. — I have already 

 stated that it is only at the northern and southern borders of Car- 

 diganshire that any erratic blocks occur whose origin cannot be 

 referred to the present valley systems. The best example is in the 

 charming little Llyfnant Valley, south of Machynlleth, where large 

 blocks of felsite occur, which can only have come from the moun- 

 tains further north, in the Cader Idris or Aran Eanges. One of the 

 blocks lying in the Llyfnant stream measures about 12x9x11 ft. 

 These boulders show that the ice from the mountains of North Wales 

 came down in such volume that even the broad Dyfi could not con- 

 tain it, and it overflowed at the little pass south of Machynlleth and 

 streamed down into the Llyfnant Valley. This is the only case I 

 have met with of the invasion of Central Wales by the northern ice. 



Erratics of felsite and highly altered rocks occur in the main 

 valley soutb of Machynlleth, but these of course belong properly to 

 the North Wales Valley system. 



At the southern end of the county, around Cardigan, is a glacial 

 clay of a different character, which is worked for brickmaking at the 

 north end of the town. It is a stiff, unctuous clay, slightly gritty 

 in the mouth, of a purplish and brownish blue colour, with a 

 sprinkling of subangular, ice-scratched pebbles. Patches of more 

 1 See a paper on " Geology of Central Wales," Q. J. G.S. 1881, p. 158. 



