GLOLOGICAL SOCIETY OF LONDON. 



53 



face of the Dorothea slate quarry at Tal-y-Sarn, near the 

 West end of this valley, below the lakes of Nanty-y-Uef 

 and Llynian. Half a mile N. W. of this qnarry, groves and 

 furrows appear on a large horizontal surface of slaty por- 

 phyry. Polished and striated surfaces likewise occur on the 

 left side of the valley of Gwynant, and the rocks which occa- 

 sionally protrude from the bottom and sides of this valley 

 are ground to the condition of huge domes or bosses. 



Thus in each of the vallies here described we find two 

 common evidences of glacial action, viz.: the rounded sur- 

 faces or striee and the mounds of gravel, bearing the forms of 

 moraines, and probably modified by water. 



Diluvium or Drift. — In the mounds of gravel that have 

 been noticed near Pen Tre Voelas and Llyn Ogwyn, we have 

 as yet found no trace of those two strong evidences of di- 

 luvial currents from the north, which consist in the pre- 

 sence of erratic blocks from distant regions, and of frag- 

 ments of marine shells amidst vast masses of clay and gra- 

 vel ; — both these evidences abound along the N. W. base of 

 the mountain chain of Snowdonia. Mr. Trimmer has found 

 shells mixed with pebbles of Cumberland granite on Moel 

 Tryfan, near Caernarvon, 1,892 feet above the level of the sea, 

 and at more than 1000 feet on Moel Faban, near Bethesda, 

 in the valley of the Ogwhyn. The same shells occur in di- 

 luvium along the coasts of Caernarvonshire, Denbigh and 

 Flintshire. Similar shells have been found in the cave of 

 Cefn, near St. Asaph, above the bones of elephants and 

 hysenas ; but neither shells of this kind, nor blocks from 

 distant regions have been found on the S.E. flank of Snow- 

 donia. On the N.W. flank a large plain is covered with 

 gravel intermixed with granite and chalk flints from Cumber- 

 land and Ireland. 



Dr. Buckland attributes all this diluvium or drift to inunda- 

 tions by marine currents from the north, bearing with them 



