﻿636 ME. W. G. EEA.KNSIDES ON THE GEOLOGY OF [Aug'. 1905, 



roughly parallel to the sides of the cwm, enclose the morass 

 between Ceunant-y-gareg-ddu and Amnodd-bwll. 



The retreat of the ice from the 1800- to the 1500-foot level must 

 have been comparatively rapid, and but little moraine can now be 

 found between those limits. Below 1500 feet, however, moraines 

 become abundant, and some of their terraces are exceedingly-well 

 marked. One along the north side of the higher Tryweryn Valley 

 is almost like a railway-embankment, aud the same might also be 

 said for that on the south-western flank of the Afon Tai-Herion 

 Yalley. Where these two meet, they have so overshot the outstand- 

 ing crag of Craig Henllan that they enclose a triangular hollow, 

 which (formerly a lake or tarn) is being rapidly drained by a small 

 outflow-stream, and is now only an impassable bog. 



The moraine which holds up Llyn Arenig Eawr is also mainly 

 at this level. Unlike the moraines at 1800 or 2000 feet, those at 

 1400 to 1500 feet are almost wholly local in their origin ; and I am 

 inclined to think that, at the time of their formation, the ice-shed lay 

 much nearer the present watershed than at the earlier stage. The 

 later glaciation, however, was no small phenomenon, for the Drift 

 includes boulders of enormous dimensions ; and though only the 

 andesitic dolerites and intrusive andesites retain their scratches, all 

 rocks within its boundaries are exceedingly- well moutonnes. 



A still later stage in the glaciation is marked by the accumula- 

 tion of various tumps and roughly-crescentic mounds of Boulder-Clay 

 and ill-sorted gravel in the valley-bottoms. One of these, occupying 

 the upper end of the old valley-channel south of Foel Bodrenig, 

 through which the railway now runs, was probably responsible 

 for the diversion of the Afon Tryweryn over the beautiful falls of 

 Boch-y-rhaidr, and the consequent formation of the great peaty flat 

 above the falls. The origin of the lake of Tryweryn is not at all 

 clear ; but as it lies almost upon the watershed, is shallow, and 

 has its exit over obvious Drift, we may reasonably conclude that 

 it too is due to similar causes. 



No evidence of any glacial overthrow^- valleys has been detected, 

 but as the ice never advanced into any important valleys, this is 

 hardly to be wondered at. The dip-slope valleys on the east of 

 Arenig do not seem to have ever been seriously glaciated, and are 

 much choked with frost-talus. 



In conclusion, I desire to express my best thanks to Dr. J. E. 

 Marr, F.R.S., P.G.S., for his constant good advice and encourage- 

 ment in the working-out of the succession ; to Miss Gertrude L. 

 Elles for her aid in identifying my graptolites ; to Mr. R. H. 

 Rastall, F.G.S., and all the various junior members of the Cambridge 

 Geological School, who have shared and helped me to enjoy the 

 glorious solitudes of Merionethshire ; and to all the various land- 

 owners, tenants, and hospitable farmers, who have allowed me to 

 wander free over the country and find out whatever new infor- 

 mation is contained in this paper. 



My thanks are also due to the Director-General, and especially 



