HISTORY OF LANDSCAPE 179 



are much harder than those they come into contact with, and so 

 stand up as hills, which may be lumpy and massive like Dartmoor, 

 or extended into lines like many in Wales, or sometimes may form 

 terraced and stepped slopes like those of some of the Western Isles 

 of Scotland and the north-east of Ireland. 



While adjustment to rock structure is the chief determining 

 factor of the broad features of landscape, the details, and also to 

 a less extent some of the principal features themselves, depend 

 upon the nature and the date of the latest denudation to which 

 they have been subjected. 



While the summit crags of our mountains have plainly been 

 riven by frost, the valleys and lower shoulders of the peaks are 

 often rounded and smoothed in a remarkable manner. The 

 rocks may even show polishing and grooving of precisely the same 

 character as that exhibited by the rocks from which the Swiss 

 glaciers have recently retreated. Thus the Glacial Epoch has 

 left a conspicuous mark upon the scenery of the country, although 

 the ice-sheets and glaciers disappeared from it many thousands 

 of years ago. This is an instance of a special form of denuda- 

 tion, whose influence has not yet been destroyed by the agencies 

 at work at the present day. In contrast to this we may study 

 the effect of long-continued water denudation. This can be best 

 illustrated in the case of a typical river. 



In the upper part of its basin innumerable torrents come down 

 steep slopes with abundant erosive energy and plenty of transported 

 material. They join one another usually at an acute angle, and 

 group themselves into a number of larger streams, each with 

 considerable erosive power, and capable of excavating a valley 

 along which it flows downwards, carrying most of the material 

 brought to it, but usually dropping some, at least temporarily, 

 in the form of gravel banks on its bed or flanks. Gradually the 

 slope decreases, but the incoming of larger trunk tributaries keeps 

 up the flow, velocity, and most of the transporting power. The 

 valley generally widens out, and as it does so the river meanders 

 along its floor, now depositing and now removing gravel. 

 Sometimes it floods its lower ground and lays alluvium down over 

 the gravel. As the valley opens wider, floods become more 

 frequent, and the alluvium - covered flat wider and more con- 



