146 ORIGIN OF SCENERY OF BRITAIN 



by being protected by submergence. It was to a 

 large extent covered with Secondary rocks, though 

 the covering of these may have been but thin over 

 what are now the higher grounds. The present ter- 

 restrial areas emerged at some period later than the 

 Chalk. 1 In England there were three chief tracts of 

 land Wales, the Pennine Chain, and the Lake District. 

 The eastern half of the country, covered with Second- 

 ary rocks, was probably the last portion to be uplifted 

 above the sea; hence the watersheds and drainage 

 lines in that tract may be regarded as the youngest 

 of all. 



The history of some of the valleys of the country 

 tells the story of the denudation. The Thames is one 

 of the youngest rivers, dating from the time when 

 the Tertiary sea-bed was raised into land. Originally 

 its source probably lay to the west of the existing 

 Jurassic escarpment of the Cotswold Hills, and it 

 flowed eastward before the Chalk escarpment had 

 emerged. By degrees the Chalk downs have appeared, 

 and the escarpment has retreated many miles eastward. 



1 That the Secondary formations once extended far beyond the 

 limits within which they are now confined has been impressively 

 demonstrated by the discovery of large masses of Rhaetic, Liassic, 

 and Cretaceous strata in a great volcanic vent of Tertiary age in 

 the Isle of Arran (see Messrs. Peach, Gunn, and Newton, Quart. 

 Journ. Geol. Soc., Ivii., 1901). It may consequently be inferred 

 that the present drainage system and topographical features of much, 

 if not most, of the country have been established since the time 

 of the Chalk (see the * Geology of Eastern Fife ' in Memoirs of the 

 Geological Survey, 1902, p. 281). The stupendous erosion of the 

 Tertiary lava-plateaux of the west of Scotland and the north of 

 Ireland shows, in the most impressive way, how greatly the topo- 

 graphy has been carved out since older Tertiary time. 



