376 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



portant here to recall the circumstance that the " drift " of this valley 

 attains an elevation about equal to that of similar deposits, which may 

 be traced without a break from the seaward flanks of Carnedd 

 Llewelyn to the shores of the Menai. 



If, therefore, the loose moraine-heaps of Cwm Idwal had been 

 formed during the great glacier period already mentioned, it would 

 either in all probability have been destroyed during the depression 

 and re-elevation of the land that followed that period, or it would 

 have been covered over and smothered in the succeeding drift. 



I am therefore forced to the conclusion, that there were two glacier 

 periods in this land ; first, one preceding our Pleistocene deposits, 

 and a second on a much smaller scale, either when the land was 

 rising from the Pleistocene sea, or possibly when during a portion of 

 this process of rising it attained a higher elevation than at present. 

 The moraines of these latter glaciers are to be seen in the mouths 

 and recesses of the higher valleys, such as Cwm Idwal and Cwm 

 Graianog, and in the valley of Llyn Llydaw and Cwm-y-Clogwyn, 

 on the flanks of Snowdon, and also in the upper part of Cwm-Uafar, 

 where a small lake has been drained by the stream cutting its way 

 to the base of the moraine*. These moraines are often of a massive 

 character, being composed of piles of heavy angular stones heaped 

 rudely together. In Cwm Graianog, on the summit of the pile of 

 smaller blocks, half way between the bounding hills, there is a stone, 

 now split by the weather, measuring 33 X 27 X 4i feet, and weighing 

 about 250 tons. The shortness of the courses of these glaciers at 

 first seemed to me a great difiiculty, varying as some of them do 

 from f of a mile to 1^ mile in length. An examination of the maps 

 of MM. Schlagintweitf removed this objection, for on ground of 

 similar form glaciers equally short are not uncommon. In Wales 

 true moraines never occur in the main valleys opening into low tracts 

 approaching the sea. In many of these valleys even most of the 

 drift has been removed, by means which at a future period I hope 

 to explain. 



* The glacier has originally extended beyond this point, having in its course 

 scooped out a long straight hollow in the drift. That this was not hollowed out 

 by the stream is evident from the circumstance that the surface of alluvial dieixitVi^ 

 is not thickly strewed with boulders accumulated by the gradual removal of inter- 

 mingled smaller sediment by means of river-action. The broad terrace of drift on 

 the right bank of the stream is thoroughly charged with such boulders, often of 

 large size. The removal of the lighter material by river-action would have con- 

 centrated these on the surface of the straight alluvial hollow through which the 

 stream flows. This concentration of boulders may be seen in many of the Caer- 

 narvonshire valleys. A well-marked instance occurs on the banks of the river 

 Gorfai, two miles S. and S.E. of Caernarvon. 



t Untersuchungen iiber die phys- Geogr. der Alpen, 8va. 1850. 



