i68 



THE BOOK OF NATURE STUDY 



denudation of the country behind would have slackened down 

 and ceased. It is needless to emphasise the fact that the head 

 waters will always be higher than the floors of the transverse 

 gorges, so that the streams maintain sufficient downward 

 gradient to ensure their flow and work. 



The general outcome of the system of denudation just outlined 

 will be, that the average landscape will be an echo of the resistance 

 of the rocks. The streams will be adjusted to the structure, the 

 majority of them situated in soft rocks, flowing off the harder 



FIG. 60. Map of two transverse streams with their longitudinal tributaries. 



ones by the shortest possible route, but still in many places 

 trenching these hard rocks. These trenches are the chief apparent 

 exceptions to the law of adjustment, but, if the foregoing line of 

 argument has been followed, it will be clear that they are the 

 prime cause of all the adjustment, and that they supply the 

 motive power by which the adjustment is maintained. 



A country which possesses the simple tectonic relations 

 postulated, an arch with alternating harder and softer beds, should 

 have impressed upon it sufficient transverse streams to carry 



