PLEISTOCENE RIVER-DEPOSITS. 127 



gravel-terraces can be due to the action of the present rivers 

 under existing conditions. " The greatest flood of the Seine on 

 record," he remarks, " is that of the year 1658, when it rose to 

 a height of 29 feet. Even in this case a flood of nearly sixty 

 times that magnitude would be required merely to fill the valley 

 to the level of the high-level gravels, without taking into con- 

 sideration the more rapid discharge. But neither in this nor in 

 the other cases of modern times are we aware of an increase in 

 the volume of water, during floods in these regions, to many 

 times the ordinary mean average, whereas we see that in a case 

 such as is presented at Amiens a flood having a volume five 

 hundred times that mean would be required to reach the beds 

 of St. Acheul." 1 The conclusion to which this sagacious 

 observer came, therefore, was that the gravels had been laid 

 down by the rivers during the gradual excavation of their 

 valleys ; that is to say that the gravels indicate the various 

 levels at which the rivers formerly flowed. Thus the high-level 

 terraces are those which the streams formed when they were 

 flowing 100 or 150 feet, as the case may be, higher than at 

 present, while the lower terraces on the slopes of the valleys 

 mark out the various stages in the slow process of excavation. 



When we bear in mind the fact that, between the time when 

 the higher terraces began to be formed, and the period when 

 the deposition of the lowest -lying Pleistocene beds had been 

 completed, the valleys were excavated in rock to depths ranging 

 from 50 to 150 feet, and to widths that sometimes reach and 

 even exceed a mile, we must be forcibly impressed with the 

 protracted duration of the Pleistocene Period, and the extreme 

 antiquity of its commencement. In the long time that has 

 elapsed since the deposition of the lowest-level Pleistocene beds 

 the valleys have suffered comparatively little denudation, and 

 did we measure the rate at which they were deepened in Pleisto- 

 cene times with that at which they are now being excavated, we 

 should be compelled to infer for them an almost inconceivable 

 age. There are abundant reasons, however, for believing that 



1 Philosophical Transactions, 1864, p. 266. 



