182 THE BOOK OF NATURE STUDY 



would indicate that they had been formed in an earlier cycle of 

 denudation, when the land stood at a lower level. At the same 

 time the river courses would be the site of the earliest and most 

 rapid denudation, and, until the lateral agencies caught them up, 

 some trace of the old base-level in the heights between the valleys 

 would be left to tell the story of an earlier cycle of denudation. 



The landscape of any large country invariably gives evidence 

 of one or more reductions to base-level, followed by rejuvenation 

 of the denuding agencies. The base-levelling may have sometimes 

 been effected under the sea, at other times by rivers and atmo- 

 spheric denuding agents ; but with each new uplift the rivers have 

 incised new or old courses and developed drainage systems which 

 have gradually passed through the stages of youth and maturity 

 to old age. Traces of such a phase will for a time be found in the 

 existence of plateaus (the former base-levels) into which the new 

 streams are incising their valleys. But by degrees the valleys 

 widen and deepen, reducing the amount of intervening plateau 

 until nothing is left but a ridge separating the new valleys, and 

 even this is gradually consumed from both sides by further valley 

 widening. 



As an example we may again turn to the Weald, which bears 

 evidence of an earlier cycle of erosion preceding that to which its 

 present hills and valleys are mainly due. One of the most striking 

 features of the Weald is the level lines given by the summit of 

 the chief escarpments. A plane 900 feet high, if placed on the top 

 of the Weald, would nearly touch all the summits of the Downs, 

 the Ragstone Range, and the Wealden Heights. It is below 

 some such plane that the valleys are cut. Their sides; where they 

 cut one or other of the ranges, rise abruptly on either hand up to 

 nearly this height, and then the ridge sweeps away right and left 

 at that level (Fig. 70). This would not occur if the country had 

 been carved out from an unbroken arch, say of Chalk, spreading 

 right over the whole country. If that had been the case the Chalk 

 outcrop would run steadily inward and upward from each valley, 

 and then downward and backward to the next. Instead of that, 

 it rises onward and upward to a height of 600 to 700 feet, and then 

 passes away in a nearly straight line to the next valley, when it 

 plunges down, forward, and back again to about the same 600 or 



