THE GLACIAL PERIOD. 



235 



great river-valleys of Europe, and that we are not called upon 

 to suppose that the vast sheets of Ehenish and Danubian loss 

 only began to be accumulated after the valleys had been ex- 

 cavated to their present depths. These deposits are the results 

 of the great floods that took place doubtless at stated times all 

 through the Glacial Period — those at the higher levels having 

 been laid down at a much earlier date than the loams which we 

 find overlying the low-level gravels towards the bottoms of the 

 present valleys. But even with this consideration in view we are 

 under the necessity of inferring the former frequent occurrence 

 of floods and inundations, which it would be hard to parallel at 

 the present day. I have referred to the suggestion made by my 

 brother, and to the view supported by Mr. Belt, that when the 

 great ice-sheet extended south as far as the latitude of the 

 Thames, the large rivers that flowed north from Central Europe 

 would be dammed back so as to inundate vast areas, which 

 might thus become overspread with glacial mud. It is hardly 

 possible to escape from this conclusion. But even without any 

 such dam to the passage of the water northward, the valleys of 

 the Ehine and similar rivers must have been filled to overflow- 

 ing. The waters pouring in at the upper ends of these valleys, 

 supplemented by the torrents and floods received from tributaries, 

 and the water derived from melting snow on the low grounds 

 and from excessive rains, could not escape at once in a great 

 tumultuous current. The rivers frozen over in winter would 

 themselves tend to choke a passage to the north, just as is the 

 case with the great rivers of Siberia at present. Thick ice 

 might continue to bind them in the north for some time after 

 their icy covering had melted in the south : and wide areas in 

 the upper reaches of the valleys would thus become inundated. 

 Again, if we consider the enormous quantities of water which 

 would be discharged from the melting snows in spring and 

 summer, we must see that the valleys themselves would be 

 insufficient to carry these waters immediately away. Even in 

 our own little country we may observe how after unusually 

 heavy rains all the rivers rise in flood, and wide areas become 



