374 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



It is evident that before this glacier period the land had already 

 received its present grand contour, and this conclusion may be gene- 

 rally applied to European and American surfaces on a large scale 

 underlying the "drift." If the data previously stated be correct, it 

 appears that, after the great glacier period, much of the country was 

 depressed beneath its present level, at least 2300 feet, by which means 

 the glacier-markings were covered by accumulations of superficial de- 

 tritus. The higher parts of Wales, at the utmost from 800 to 1000 

 feet above water, must have formed a group of islands, perhaps too 

 insignificant and low to admit of the formation of glaciers on their 

 flanks. 



The scratches and polished surfaces in Anglesea (like those in the 

 vale of the Firth of Forth and in the lowlands of Ireland) seem to me 

 to be due to the action of floating ice, the direction of the grooves 

 being quite unconnected with those of the glaciers in the neighbouring 

 mountains of Caernarvonshire. In Anglesea the grooves (at Har- 

 lech, in Tywyn- trewan, near the Holyhead Railway, on the "Yellow 

 Sandstone," near Penrhos Llugwy, on the coast near Carmels Point, 

 and at various localities near Llanfairynghornwy) generally run about 

 E. 30° N. It is worthy of remark that near Penrhos-Llugwy, on the 

 polished and grooved surfaces, potholes occur (where no stream could 

 have run) of the kind frequently made on sea-coasts by the gyration 

 of stones in hollows, showing that, since the rocks were smoothed 

 and scratched, they have formed a sea-margin, from which the drift 

 has been removed by denudation. 



Whatever the conditions were under which boulders were dispersed 

 from the height of 2300 feet downwards, they were brought to a close 

 by the gradual re-elevation of the country. One of the characteristic 

 features of the scenery of North Wales is, as I have elsewhere ob- 

 served*, due to this elevation, the outlets of certain valleys being 

 dammed up by greater accumulations of sediment towards their open- 

 ings, the free egress of the drainage being prevented, and lakes having 

 been formed, after the manner indicated by Mr. Darwin in his " Geo- 

 logical Observations on South Americaf ." Examples of this may be 

 seen at low levels in Caernarvonshire in Llyn Cwellyn and Llyniau, 

 and on high ground in some of the lakes on the north side of Cader 

 Idris in Merionethshire, and in Marchlyn-bach and Marchlyn-mawr 

 in Caernarvonshire, where, in the latter case, the barrier of " drift " 

 reaches an elevation of about 2000 feet. 



But there are other lakes, such as Llyn Llydaw, on the Capel Curig 

 side of Snowdon, and the celebrated Llyn Idwal, which are clearly 

 dammed up, not by marine deposits, but by the moraines of glaciers 

 (see fig. 1). As an example of the moraine-dammed lakes I select 

 Llyn Idwal, in Cvrai Idwal, the moraines of which have been de- 

 scribed by Mr. Darwin J. A terminal moraine spreads across the 

 valley, and dams up the lake. Lateral moraines extend up the sides 

 of the valley on either side. 



* Athenaeum (No. 1171), 1850, p. 377. t P- 24. 



t Lond., Edinb. and Dubl. Phil. Mag. 1842, vol. xxi. p. 180 ; and Edinb. N. 

 Phil. Journ. 1842, vol. xxxiii. p. 352. 



