136 ORIGIN OF SCENERY OF BRITAIN 



series and the Palaeozoic rocks of the Pennine Chain 

 were worn away, and two rivers carrying off the drain- 

 age of the southern end of that chain flowed in oppo- 

 site directions, the Avon turning south-west and the 

 Trent northwards. By degrees these streams moved 

 away across the broadening plain of softer strata, as 

 the escarpments emerged and retreated. At the same 

 time, streams collected the drainage from the uprising 

 slope of Secondary rocks and flowed south-eastward. 

 Successive lines of escarpment have since been de- 

 veloped, and many minor watersheds have arisen, while 

 the early watershed has undergone much modification, 

 these various changes pointing to the continuous oper- 

 ation of running water. 



THE MOUNTAINS AND TABLELANDS. 



A true mountain-chain is the result of plication of 

 the earth's crust, and its external form, in spite of 

 sometimes enormous denudation, bears a relation to 

 the contours produced by the original uplift. Tried 

 by this standard, hardly any of the heights of Britain 

 deserve the name of mountains. With some notable 

 exceptions in the south of Ireland, they are due not 

 to local but to general upheavals, and their outlines 

 have little or no connection with those due to under- 

 ground movement, but have been carved out of 

 upheaved areas of unknown form by the various 

 forces of erosion. In the course of their denudation, 

 the nature of their component rocks has materially 

 influenced the elaboration of their contours, each 

 well-marked type of rock having its own character- 

 istic variety of mountain forms. 



