372 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



and Llyn Padarn there is a wild moorland tract, covered by the same 

 deposits, in which I found fragments of shells at an elevation of about 

 1000 feet. On the rough slopes on either side of the lakes of Lan- 

 beris, the " drift " has again in a great measure been removed, small 

 patches alone remaining nestled amid the smaller hollows of the hills. 

 Surrounding these more fertile spots, the ruined surfaces of masses of 

 rock rounded and polished by glacier-action often rise almost destitute 

 of vegetation. Between Llanberis and Nant Francon there is another 

 broad high moor formed by " drift " of great thickness, and which, 

 from an average height of about 1 100 to 1300 feet, stretches eastward 

 into the valley of Marchlyn-mawr, where it attains an elevation of 

 about 2000 feet. Standing on this moor, above the left bank of the 

 Ogwen, the eye easily detects on the opposite banks a corresponding 

 accumulation, stretching smoothly up the higher valleys towards Aber, 

 and bending on the east and south-east towards the sources of Afon 

 Berthan, the Llafar, and Afon Gaseg ; streams rising in the higher 

 recesses of Carnedd Dafyd and Carnedd Llewelyn, on their seaward 

 flanks. I am informed by Mr. Trimmer that he has good reason to 

 believe that this part of these deposits contains shells at a height of 

 about 1000 or 1200 feet. In the valleys, through which flow the 

 streams last named, the " drift " attains an elevation of about 2300 

 feet, stretching into their wide recesses with a smooth outline, broken 

 only by long lines of faintly-marked terraces, indicative of sea-margins 

 during pauses in the later upheavals of the country. From this 

 highest point it may be followed without a break to the present sea- 

 level. Having ascertained by this unbroken continuity that the drift 

 actually attains an elevation of 2300 feet on the flanks of Carnedd 

 Llewelyn and Carnedd Dafyd, there is no difiiculty in understanding 

 the reason of the existence of similar masses in neighbouring isolated 

 valleys high amid the mountains. This occurs in the elevated valley 

 between Y Glyder fawr and Y Gam, in which a deposit of " drift " (see 

 fig. 1, p. 375) stretches from the summit of the clifl^s that overhang 

 Cwm Idwal (2300 feet high), to the top of the hills overlooking the 

 Pass of Llanberis. 



In Anglesea, although by no means scarce, the larger boulders are 

 much less numerous than in the "drift," where it approaches the 

 mountains of Caernarvonshire. As far as I could discover, they are 

 principally composed of fragments of the rocks of the island. On the 

 opposite shores, approaching the mountains, they gradually increase in 

 quantity, being scattered in and on the " drift," until at its margin, 

 where it surrounds the higher points of land, the boulders often form a 

 large proportion of its material, packed closely together along the sides 

 of the hills, and in the nooks of the higher valleys. Even these coarse 

 accumulations, however, when viewed at a little distance, present a 

 generally smooth and regular outline. Sometimes on isolated hills, 

 or on the summits and sides of high ridges, where the smaller " drift " 

 has been removed by denudation, boulders and subangular masses lie 

 singly scattered on the surface. A good example occurs on the east 

 side of Nant Francon, on the long ridge of Pen-yr-olen-wen. One of 

 these masses lies on the summit of this ridge, about 2000 feet above 



