216 THE BOOK OF NATURE STUDY 



and peaks, which will, however, for a time remain at a height 

 approximating to that of the original plateau. 



Mountain chains are the result of localised earth-movement 

 on a very intense scale. Along lines of weakness in the earth- 

 crust the strata have been intensely folded and crumpled (Figs. 29 

 and 30), tilted to all angles, often overthrust and inverted, broken 

 by faulting, cleaved and jointed, intruded upon by igneous rocks, 

 and often, in part at least, subjected to important chemical 

 changes. Denudation acting on such a chain finds a great complex 

 of rocks of very varying hardness and composition brought 

 into j uxtaposition. The whole ground having been much elevated, 

 selective denudation has been in full activity and adjustment 

 is speedily obtained. So a mountain chain soon becomes the 

 site of a vast number of scarps and dip-slopes brought close 

 together, separated by longitudinal valleys and cut by trans- 

 verse gorges. Thus a series of broken, but on the whole parallel, 

 ranges are produced, in which the rock character and structure is 

 closely related to the relief (Fig. 48). The active denuding agencies, 

 having produced this type of relief, cut back into the ridges, pro- 

 ducing cwms or corries, isolating spurs, aretes, and peaks, and 

 obscuring to some extent the relation between structure and relief. 

 Examples of this are to be seen in all our chief mountain ranges, 

 and particularly in those of North Wales, Lakeland, and the 

 Highlands of Scotland and Ireland. They may even be seen in 

 numerous separated and isolated ranges which come out through 

 newer rocks elsewhere, such as the Malverns, in the Scottish 

 Uplands, and in isolated areas in Ireland. 



A feature in valley scenery not hitherto alluded to is that of 

 lakes, which are either situated high up in the mountain hollows, 

 when they are called tarns, or along the course of the major 

 valleys, such as Bala Lake and Windermere. Attention has 

 often been drawn to the association of these lakes with glaciated 

 regions, and numerous explanations have been given of them. 

 The irregular deposit of glacial moraines on a plain has often 

 given rise to hollows capable of holding water. Again, moraines 

 dropped across the course of a valley may dam up the rivers 

 which follow the disappearance of ice from a country, and form 

 lakes (Fig. 83). In some cases the lakes will drain over the 



