I40 PHYSIOGRAPHY. [chap. 



cated by the dotted line, and has been cut through by 

 running water. The Wandle is a tributary of the Thames, 

 and what is true of this small river is also true of the larger. 

 In some places the Thames has cut through similar high- 

 level gravels ^ and through the London clay. In other 

 parts of the river, again, the chalk forms the two sides of 

 the valley through which the water flows. In the case of 

 the chalk, it is evident, from what was said in the last 

 chapter, that the mechanical erosion would be greatly 

 assisted by chemical solution, the carbonate of lime being 

 easily soluble; while the flints, so commonly found em- 

 bedded in the chalk, would resist such chemical action 

 and, to a great extent, would also withstand mechanical wear. 

 Hence their broken fragments will be found still rolling 

 about as flint-gravel ; and every bit of such gravel is, in fact, 

 a memorial of a quantity of white chalk that has long ago 

 been dissolved and washed away by running water. In 

 other parts of its course, the Thames flows through rocks of 

 a different character, which will be subsequently described ; 

 but they are all affected by the mechanical or by the chemical 

 action of the river, in the way just explained. 



Passing from the study of the Thames Valley to that 

 of the general surface of the country, abundant evidence 

 IS to be found that rain and running water have been 

 actively at work. Indeed there is good reason to believe 

 that these almost silent workers have been the chief 

 instruments in producing the present physical features 

 of the ground. They have eaten out river-courses and 

 worn away valleys, leaving masses of rock which stand out 

 as hills and crags. But, while giving them credit for effecting 



1 The meaning of the term "high-level gravels" will be explained 

 in Chapter XVII. 



