i64 PHYSIOGRAPHY. [chap 



the floating ice. If the bottom of the lake, or of the sea, 

 should at any time be upheaved, the glacial mud and gravel, 

 with angular blocks and ice-scratched boulders, may be 

 exposed to view ; and may thus furnish evidence of glacial 

 denudation in countries which are now free from anything 

 like glaciers or icebergs. 



Other evidence of ice-action is afforded by the peculiar 

 position of large angular blocks of stone, poised perhaps 

 upon the very edge of a precipice, or balanced upon a mere 

 point* Such stones, known 2S perched blocks or blocs perches, 

 could hardly have been brought into their strange position 

 by mere rolling, or by the action of running water ; but it is 

 easy to see that they might have been dropped by an ice- 

 berg, or left stranded by the gradual melting of a glacier on 

 which they were originally seated. 



It is now more than a quarter of a century since the late 

 Prof. Agassiz, who had been a diligent observer of glacial 

 action in Switzerland, visited this country, and in company 

 with Dr. Buckland pointed out the evidence of former ice- 

 action in many parts of Britain. The traveller in Scotland, 

 Ireland, Cumberland, or North Wales, will have no diffi- 

 culty in detecting roches moutonn'ees, perched blocks, and 

 occasionally the remains of old moraines ; while here and 

 there, where the rocks have been protected, the glacial 

 polish and striations are still preserved. Such evidence 

 conclusively proves that ice must have flowed over the sur- 

 face of the country. It is believed, indeed, that at one 

 period of geological history, known generally as the Glacial 

 Period, Britain must have been buried beneath a vast sheet 

 of ice, similar to that which now covers Greenland. This ice 

 played its part in rasping and grinding and polishing the 

 surface of the land ; and it has even been suggested by Prof. 

 Ramsay that many of the rock-basins which contain lakes 



