360 Mr Darwin on the Ancient Glaciers of Caernarvonshire, 



I have further remarked, that the condition and kind of the 

 stones transported would generally be influenced by the man- 

 ner of production of the floating ice. In accordance with 

 these views, I may remark, that it does not seem probable, 

 from the low level of the chalk- formation in Great Britain, 

 that rounded chalk-flints could often have fallen on the sur- 

 face of the glaciers, even in the coldest times. 1 infer, there- 

 fore, that such pebbles were probably inclosed by the freez- 

 ing of the water, on the ancient sea-coasts. We have, how- 

 ever, the clearest proofs of the existence of glaciers in this coun- 

 try ; and it appears that when the land stood at a lower level, 

 some of the glaciers, as in Nant-Francon, reached the sea, 

 where icebergs charged with fragments would occasionally be 

 formed. By this means we may suppose that the great an- 

 gular blocks of Welch rocks, scattered over the central coun- 

 ties of England, were transported.* I looked carefully in 

 the vallies near Capel-Curig and in Nant-Francon for beds of 

 pebbles, or other marks of marine erosion, but could not dis- 

 cover any; when, however, Moel Tryfan and Faban stood 



* On the summit of Ashley Heath in Staffordshire, there is an angular 

 block of syenitic greenstone, four feet and a half by four feet square, and 

 two feet in thickness. This point is 803 feet above the level of the sea. 

 From this fact, together with those relating to Moel Tryfan and Faban, we 

 must, I think, conclude that the whole of this part of England was^ at the 

 period of the floating ice, deeply submerged. From the reasons given in my 

 paper (Phil. Trans., 1839, Phil. Mag. S. 3, vol. xiv. p. 363), I do not doubt, 

 that, at this same period, the central parts of Scotland stood at least 1300 

 feet beneath the present level, and that its emergence since has been very 

 slow. The boulder on Ashley Heath probably has been exposed to atmo- 

 spheric disintegration for a longer period than any other in this part of Eng- 

 land. I was, therefore, interested in comparing the state of its lower sur- 

 face, which was buried two feet deep in compact ferruginous sand (contain- 

 ing only quartz-pebbles from the subjacent new red sandstone) with the 

 upper part. I coukl not, however, perceive the smallest difference in the 

 preservation of the shai-p outlines of its sides. I had a hole dug under an- 

 other large boulder of dark green felspathic slaty rock, lying at a lower 

 level ; it was separated by 18 inches of sand (containing two pebbles of 

 granite, and some angular and rounded masses of new red sandstone), from 

 the surface of the new red sandstone. One of the rounded balls of this 

 latter stone had been split into two. and deeply scored, evidently by the 

 stranding of the bouUlor, 



