134 ORIGIN OF SCENERY OF BRITAIN 



been mainly determined by erosion, independent of 

 geological structure ; while the details have been chiefly 

 guided by structure, but partially also by the rate and 

 kind of erosion. Ruggedness, for example, has re- 

 sulted primarily from structure, but has been aggravated 

 by greater activity of erosion. The mountainous west, 

 with a greater rainfall and steeper slopes, is more 

 rugged than the mountainous east. 



The Tablelands of Britain are of two orders — i, 

 those of Deposit, which may be either (a) of sedimen- 

 tary rocks, horizontal or nearly so, as in the Millstone 

 Grit and Jurassic plateaux of Yorkshire, or (b) of 

 volcanic rocks, as in the wide plateaux of Antrim, 

 Mull, and Skye ; 2, those of Erosion, where, as the 

 result of long-continued degradation, a series of plicated 

 rocks has been cut down into a more or less uniformly 

 level surface, as in South Wales. By the elevation of 

 such a surface into a high plateau, erosion becomes 

 more active, and the plateau is eventually trenched 

 into a system of ridges and isolated hills, as has 

 happened in the Highlands. 



The Valleys of Britain are the result of erosion either 

 (a) guided by geological structure, as in what are called 

 longitudinal valleys, that is, valleys which run along 

 the strike or outcrop of formations, as the Great Glen 

 and Glen Spey in Scotland and the valleys of the Trent 

 and Avon in England ; or (b) independent of geological 

 structure, as in the transverse valleys which embrace 

 the great majority of British examples. 



Our Plains are due to prolonged erosion, as in the 

 Weald ; to the deposit of detritus, as in river-terraces 

 and alluvial plains ; to the action of the sea, as 



