186 Mr. Darwin on the Ancient Glaciers of Caernarvonshire, 



sea never freezes ; and 3rd, by these two agencies united. I 

 have further remarked that the condition and kind of the stones 

 transported, would generally be influenced by the manner 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 surface of glaciers, 

 even in the coldest times. I infer therefore that such pebbles 

 were probably inclosed by the freezing of the water on the 

 ancient sea-coasts. We have, however, the clearest proofs of 

 the existence of glaciers in this country ; 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 angular blocks of Welch rocks, 

 scattered over the central counties of England, were trans- 

 ported*. I looked carefully in the valleys near Capel-Curig 

 and in Nant-Francon for beds of pebbles, or other marks of 

 marine erosion, but could not discover any : when, however, 

 Moel Tryfan and Faban stood beneath the level of the sea, 

 inland creeks of salt-water must have stretched far up or quite 

 through these valleys, and where they were deep, the glaciers 

 (as at present in Spitzbergenf) would have extended, floating 

 on the surface of the water, ready to become detached in large 

 portions. From the presence of boss-formed rocks low down 

 in the valley of Nant-Francon, and on the shores of the Lakes 



* 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 has since been 

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

 mospheric disintegration for a longer period than any other in this part of 

 England. I was therefore interested in comparing the state of its lower 

 surface, which was buried two feet deep in compact ferruginous sand (con- 

 taining only quartz pebbles from the subjacent new red sandstone), with 

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

 the preservation of the sharp outlines of its sides. I had a hole dug under 

 another 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 boulder. 



t Dr. Martens on the Glaciers of Spitzbergen, New Edinb. Phil. Journ. 

 1841, (vol. xxx.) p. 288. 



