270 The Realm of Nature CHAP. 



even consolidated, consisting chiefly of clays and sands. 

 The Pleistocene formation in Northern Eurasia and America 

 consists almost entirely of boulder clay, the result of ice- 

 action, and the period has been termed the Great Ice Age. 

 Many exposed rock surfaces on the mountain-tops as well 

 as in valleys, in places where glaciers have never been seen, 

 closely resemble the roches moutonnees of Switzerland ( 338). 

 Perched blocks are scattered thickly over all parts of Northern 

 Europe and America, and from their nature many of them 

 are known to be far travelled. The conclusion is irresist- 

 ible that after the formation of the last tertiary rocks these 

 lands were subject to ice-action. Great and wide -spread 

 subsidence, and subsequent elevation of the land took 

 place during this period. Some writers, among whom 

 is the Duke of Argyll, maintain that the boulder clay, 

 perched blocks, and ice-scratchings were brought about by 

 this subsidence permitting fleets of icebergs sailing south- 

 ward to strand or rub against surfaces which were afterwards 

 elevated. To most geologists, however, the evidence of 

 true glacier action having occurred over the whole area is 

 overpowering, although the period is so remote that atmo- 

 spheric erosion has in many cases obliterated the work of ice. 

 352. The Great Ice Age. Glaciation probably oc- 

 curred on the grandest scale, the ice marching over mountain 

 and valley with little regard to the form of the surface. 

 In the Glacial period it appears that all Northern Europe 

 and Northern America (see light blue tint on Plate VII.), 

 were covered by vast ice-caps, thicker than that now over- 

 spreading Greenland, which polished and smoothed off the 

 mountains, and covered the valleys and plains with layers of 

 boulder clay. The ice seems to have spread beyond the 

 margin of the land, to have hollowed out deep furrows 

 across the Continental shelf, and sometimes even to have 

 ploughed up the shallow sea-bed and scattered the sand and 

 shells on the coast-lands. Professor James Geikie points out 

 that the Great Ice Age was divided into periods during which 

 the climate was very severe, while between them a genial 

 climate prevailed, and interglacial beds of peat were formed 

 containing a varied vegetation and the remains of insects 



