246 RE-ELEVATION OF SCOTLAND. CHAP. xin. 



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

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

 mountain torrent by a transverse barrier, 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 specially 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 be 

 come interstratified with the gravel 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 inferred from the height to which erratic blocks derived 

 from distant regions could be traced, besides the want of 

 conformity in the glacial furrows to the present contours of 

 many 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 

 3,000 to 4,000 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 1,500 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. 63, 1841. 



