246 RE-ELEVATION OF SCOTLAND. chap. xiii. 



have once filled the valley of the Tumrael to the height of the 

 stratified drift, it may have dammed up the mouth of a 

 mountain torrent hy a transverse harrier, giving rise to a 

 deep pond, in which beds of clay and sand brought down by 

 the waters of the torrent were deposited. Charpentier in his 

 work on the Swiss glaciers has described many such recep- 

 tacles of stratified matter now in progress and due to such 

 blockages, and he has pointed out the remnants of ancient 

 and similar formations left by extinct glaciers of an earlier 

 epoch. He si)ecially notices that angular stones of various 

 dimensions, often polished and striated, which rest on the 

 glacier, and are let fall when the torrent undermines the side 

 of the moving ice, descend into the small lake and become 

 interstratified with the graved and fine sediment brought down 

 by the torrent into the same.* 



The evidence of the former sojourn of the sea upon the 

 land after the commencement of the glacial period was for- 

 merly infeiTed from the height to which erratic blocks derived 

 from distant regions could be traced, besides the Avant of con- 

 formity in the glacial furrows to the present contours of man}' 

 of the valleys. Some of these phenomena may now, as we 

 have seen, be accounted for by assuming that there was once 

 a crust of ice resembling that now covering Greenland. 



The Grampians in Forfarshire and in Perthshire are from 

 3000 to 4000 feet high. To the southward lies the broad 

 and deep valley of Strathmore, and to the south of this 

 again rise the Sidlaw Hills to the height of 1500 feet and 

 upwards. On the highest summits of this chain, formed of 

 sandstone and shale, and at various elevations, I have 

 observed huge angular fragments of mica-schist, some three 

 and others fifteen feet in diameter, which have been conveyed 

 for a distance of at least fifteen miles from the nearest 

 Grampian rocks from which they could have been detached. 



* Charpentier, Essai sur les Glaciers, p. Go, 1S41. 



