find on Boulders transported by Floating Ice. 187 



of Llanberis (310 feet above the sea), it is evident that gla- 

 ciers filled the valleys after the land had risen to nearly its 

 present height ; and these glaciers must have swept the valleys 

 clean of all the rubbish left by the sea. As far as my very 

 limited observations serve, I suspect that boss or dome-formed 

 rocks will serve as one of the best criterions between the ef- 

 fects produced by the passage of glaciers and of icebergs*. 



Dr. Buckland has described in detail the marks of the pass- 

 age of glaciers along nearly the whole course of the great 

 central Welch valleys ; I observed that these marks were evi- 

 dent at the height of some hundred feet on the mountain-sides, 

 above the water-sheds, where the streams flowing into the sea 

 at Conway, Bangor, Caernarvon, and Tremadoc, divide : 

 hence it appears that a person starting from any one of these 

 four places (or from some way up the valley where the gla- 

 cier ended), might formerly, without getting off the ice, have 

 come out at either of the other three places, or low down in 

 the valleys in which they stand. The mountains at this pe- 

 riod must have formed islands, separated from each other by 

 rivers of ice, and surrounded by the sea. The thickness of the 

 ice in several of the valleys has been great. In the vale of 

 Llanberis I ascended a very steep mountain, E.N.E. of the 

 upper end of the upper lake, which slightly projects where the 

 valley bends a little. For the lower 1000 feet (estimated, I 

 think, correctly) the marks left by the glacier are very distinct, 

 especially near the upper limit, where there are boulders 

 perched on bosses of rock, and where the scores on the nearly 

 vertical faces of rock are, I think, more distinct than any 

 others which I saw. These scores are generally slightly in- 

 clined, but at various angles, seaward, as the surface of the 

 glacier must formerly have been. But on one particular face 

 of rock, inclined at an angle of somewhere about fifty degrees, 

 continuous, well-marked and nearly parallel lines sloped up- 

 wards (in a contrary sense to the surface of the glacier) at an 

 angle of 18° with the horizon. This face of rock did not lie 

 parallel to the sides of the main valley, but formed one side 

 of the sloping end of the mountain, over and round which the 

 ice appears to have swept with prodigious force, expanding 

 laterally after being closely confined by the shoulder above 



* In the Appendix to my Journal of Researches (1839), I endeavoured 

 to show that many of the appearances attributed to debacles, and to the 

 movements of glaciers on solid land, would in all probability be produced 

 by the action of stranded icebergs. I have stated (p. 619), on the author- 

 ity of Dr. Richardson, that the rocky beds of the rivers in North America 

 which convey ice, are smoothed and polished; and that (p. 620) the ice- 

 bergs on the Arctic shore drive before them every pebble, and leave the sub- 

 marine ledges of rock absolutely bare. 



