GEOLOGICAL MAPS 167 



slacken, its erosive power would be checked, its course would 

 become choked with its own debris, and the deepening and cutting 

 back of its valley would be in abeyance, until the transverse 

 stream got rid of its obstruction and was again in full working 

 order. Thus the rate of denudation of the tributary streams, 

 including the longitudinal ones, that means to say, the denudation 

 of the country as a whole, is regulated by the erosion and deepening 

 of the main and primitive transverse gorges, and that again is 

 dependent on the resistance of the hard rocks to erosion. In 

 other words, the rate of denudation of the country as a whole 

 is regulated by its resistant rocks. 



Provided the transverse gorges are kept open, the denudation 

 of the whole country will be in activity, and as a consequence 

 the head waters of all the tributaries, and even the rocks on the 

 crown of the arch from which the original drainage started, 

 will be steadily lowered, especially if made of soft rock. On the 

 other hand, the ridges along the hard rock bands between the 

 transverse gorges will undergo much less erosion, and will only 

 become slowly lower by the general retreat of the scarp con- 

 sequent on undercutting. Consequently these ridges will be 

 the part of the country undergoing least rapid denudation, and 

 eventually they may come to stand out, not only high above the 

 floors of the transverse gorges by which they are cut, but above 

 the very drainage ground from which come the streams cutting 

 these gorges. Thus we have the paradox that at the present 

 day the drainage of an important section of the country may 

 rise at an inconspicuous height, may proceed outwards and plunge 

 straight at ridge after ridge, whose crests may be higher than the 

 source of the streams, trench them by steep-sided, deep gorges, 

 and escape by this difficult path out to the sea or to an outside 

 drainage system. The paradox, however, ceases to be one when 

 we follow out the history of the system, and realise the three 

 main factors. First, that the sources were originally the highest 

 ground ; secondly, that the relief of the ridges and wolds is due 

 to the lowering of the intervening bands of softer rock ; and thirdly, 

 that all the denuded matter has been carried out through the 

 transverse trenches, and that the keeping open and downward 

 cutting of these has been a condition without which the general 



