130 PREHISTORIC EUROPE. 



10,800 years ; and if five feet be worn out of the valleys in 6000 

 years, one foot will be worn out in 1200 years. This is equal to 

 a loss of only -^ of an inch from the tableland in 75 years, 

 while the same amount is excavated from the valleys in 8J 

 years." 1 Hence at a rate which may with some reason be taken 

 as the present mean average rate of erosion in valleys, a valley 

 as deep as the Somme, say 150 feet, might be excavated in 

 180,000 years. 



But, as I have said, we have reason to believe that during 

 certain periods of the past the erosion of valleys has proceeded 

 more rapidly than at present, so that the Somme and other 

 ancient river-valleys may have been scooped out in less time 

 than the mean average rate of denudation now in progress 

 would allow. There is abundant evidence to show that the 

 rivers of the Pleistocene Period frequently flowed in much larger 

 volume than the streams of to-day, — that they very often as- 

 sumed a torrential character, and ever and anon rose in flood 

 and inundated wide tracts of country. Their torrential character 

 is shown by the coarseness of much of the gravel — the flints 

 being often very little rolled — by the absence of mud-sediment, 

 and by the confused and irregular disposition of the bedding 

 — all bespeaking the action of tumultuous waters that hurried 

 along promiscuous heaps of stones and scattered them in confu- 

 sion over the slopes and bottoms of the valleys, while the finer 

 sediments were swept away in suspension. "Where the water of 

 the flooded river was in less commotion the finer sediment 

 held in suspension would be deposited, and this, as Professor 

 Prestwich points out, has doubtless been the origin of many of 

 the so-called brick-earths and loss of such valleys as the Thames, 

 the Somme, the Seine, and their tributaries. They are simply 

 the flood-loams laid down by the same rivers that deposited the 

 valley-gravels. Thus the higher deposits of brick-earth, which 

 rise 60 or 80 feet above the upper gravel -terraces, were formed 

 during floods, when the valleys were beginning to be excavated, 



i Student's Manual of Geology, Jukes and Geikie, p. 430 ; see also Trans. 

 Geol. Soc. of Glasgow, vol. iii p. 153. 



