302 ALTERATION OF THE RIVER LEVEL 



and I must once more repeat that this is no mere hypothesis, 

 since the mass of sand and gravel cannot have been pro- 

 duced without an immense removal of the chalk. On the 

 whole, then, we may safely conclude that the upper-level 

 gravels were deposited by the existing river, before it had ex- 

 cavated the valley to its present depth and when consequently 

 it ran at a level considerably higher than the present. 



Far, therefore, from requiring an immense flood of water, 

 two hundred feet in depth, the accumulation of the gravel 

 may have been effected by an annual volume of water, differ- 

 ing little from that of the present river. 



A given quantity of water will, however, produce very 

 different effects, according to the manner in which it passes. 

 " We learn from observation, that a velocity of three inches 

 per second at the bottom will just begin to work upon fine 

 clay fit for pottery, and however firm and compact it may be, 

 it will tear it up. Yet no beds are more stable than clay 

 when the velocities do not exceed this : for the water even 

 takes away the impalpable particles of the superficial clay, 

 leaving the particles of sand sticking by their lower half in 

 the rest of the clay, which they now protect, making a very 

 permanent bottom, if the stream does not bring down gravel 

 or coarse sand, which will rub off 1 this very thin crust, and 

 allow another layer to be worn off 1 . A velocity of six inches 

 will lift fine sand, eight inches will lift sand as coarse as 

 linseed, twelve inches will sweep along fine gravel, twenty- 

 four inches will roll along rounded pebbles an inch diameter, 

 and it requires three feet per second at the bottom to sweep 

 along shivery angular stones of the size of an egg." * 



If, therefore, we are justified in assuming a colder climate 



than that now existing, we should much increase the erosive 



action of the river, not only because the rains would fall on 



a frozen surface, but because the rainfall of the winter months 



* Cyc. Brit. Article " Rivers," p. 274. 



