IX.] THE WORK OF RAIN AND RIVERS. 139 



often be witnessed. And, from these, it is possible to 

 pass, by insensible steps, to brooks and streams of larger 

 size, until, at length, you come to a true river. If it be 

 admitted that the little stream has worn out the gutter 

 in which it runs, it is hard to deny that the larger stream 

 has not done similar work on a larger scale. The whole 

 affair is indeed a mere question of time. The smallest 

 cause can produce a vast effect if it is only allowed to 

 work long enough. 



It needs but little boldness to apply such reasoning 

 to the valley of the Thames. On looking at the two 

 opposite sides of the valley it may often be seen that 

 the rocks exactly correspond ; a bed of gravel on one 



Wititbledon Valley of the Wandsivorth Clnflmm 



Connnon. Wandle, Cottnnoti. Coininon. 



Fig. 34. — River-valley worn through gravel and London clay. 



bank, perhaps, has its counterpart on the other. Fig. 

 34 is a section from Wimbledon Common to Clapham 

 Common.' Here it is seen that the surface of Wim- 

 bledon Common is covered by a bed of gravel spread 

 over the London clay. On descending from the summit of 

 the common to the valley of the river Wandle, the gravel 

 is seen to be abruptly cut off and succeeded by the 

 underlying clay ; but, on going up the opposite slope of the 

 valley, the gravel again appears, at about the same level, and 

 covers the surface of Wandsworth Common ; and similarly 

 the gravel will be found on the surface of Clapham Common. 

 There can be little doubt, then, that the gravel once spread 

 in a continuous sheet over these three commons, as indi- 



1 From Piof. I'restwich's Groimd Beneath Us. 1857, 



