RIVERS. 60 1 



on the surface by rain and streams. The denudation is therefore 

 both superficial and subterranean. 



Escarpments, as before mentioned, are ahnost invariably formed 

 of porous beds resting on clays or marls, and although the inclina- 

 tion of the strata is usually away from the outcrop, the waste of the 

 ridges or their recession is due to springs carrying material in 

 solution and suspension, gradually undermining the beds, and pro- 

 ducing landslips of greater or less extent. They are wasted also 

 by means of rainwash, angular detritus, and other forms of talus, 

 such as the ' screes,' the material being carried away by streams 

 that flow in the vales on the impervious strata.^ Thus Chalk flints 

 derived from the waste of the Chalk escarpment are carried down 

 by some of the lateral streams into the Wealden area, and thence 

 into the main streams that run through the ' trumpet-shaped ' 

 valleys in that escarpment, — valleys that have been formed by the 

 Darent, IMedway, and Stour. 



In many portions of the Chalk tracts we find dry valleys or 

 Combes : similar hollows occur in the Inferior Oolite of the 

 Cotteswold Hills. In the latter case the beds were formerly 

 overlaid by Fuller's Earth clay, and streams flowed over the retentive 

 capping, and aided in forming the Combes; while in the case 

 of the Chalk the denudation may have commenced when that for- 

 mation was overlaid by Tertiary clays.* The removal of these clays 

 has allowed the rainfall to sink in at once instead of being collected 

 to form streams, and thus dry-valleys remain where but little or no 

 surface denudation is now carried on. 



The gorge of the Avon at Bristol was no doubt formed before 

 the surrounding low-lands of Lias and New Red rocks had been 

 excavated, and before the river had cut its deep and picturesque 

 channel through the Oolites between Bradford-on-Avon and Bath.^ 

 The Wye (see Fig. 98), the Dart, and other rivers must also have 

 commenced their courses through the harder rocks, before the 

 softer strata along their courses had been reduced to their present 

 relative levels. 



The courses of streams have in some instances been modified by 

 changes of level, by barriers raised by the sea, etc. The Trent in 

 ancient times passed through the Oolitic escarpment at Lincoln,* 

 while the Waveney formerly flowed into the sea at Lowestoft. 

 (See also p. 594.) 



The sources of streams may be above ground or subterranean. 

 Where the water-parting is formed of impervious strata, separate 

 streams may rise near together, as in mountain regions and also in 



1 The term Talus is used to designate the heaps of loose material that have 

 accumulated (by natural or artificial means) at the base of a cliff or quarry ; the 

 term Screes (or didders) is applied to the natural accumulations of rocky debris 

 that are found on the slopes or at the foot of mountains and cliffs. 



* See E. Witchell, Proc. Cottesw. Club, 1S67, p. 227 ; and Dixon, Geol. Sussex, 

 ed. 2, p. 103. 



* See Jukes, G. Mag. 1S67, p. 444 : Morgan, Proc. BristolNat. Soc. (2), iv. 171. 



* Jukes-Browne, Geol. S.W. Lincolnshire, pp. 90, 100 ; see also Q. J. xxxix. 

 596, xl. 160. 



