542 PREHISTORIC EUROPE. 



so that the gravels at the higher levels are the oldest, and those 

 at the lower levels are the youngest of the series. Just as there 

 are high- and low-level gravels, so are there high- and low-level 

 loams. Such loams are the flood -deposits accumulated by 

 muddy rivers from time to time, when these rose to considerable 

 heights above their normal levels. But the great development 

 attained by the loss-beds in many regions cannot be accounted 

 for by ordinary river -floods. They imply the more or less 

 sudden melting in spring and summer of enormous reservoirs of 

 snow and ice, when water descended from the mountains and 

 elevated regions, and filled the valleys to overflowing. And the 

 mud deposited during such inundations we recognise as the 

 finely -levigated sediment which is only met with in glacial 

 rivers, or in streams which are busily engaged in washing and 

 denuding clays and loams of glacial origin. Now loss or lehm 

 occurs both upon valley-slopes and upon table-lands that extend 

 between the valleys. In the latter localities it is called upland- 

 or hill-loss, and it is evident that this high-level loss must have 

 been laid down before the valleys were excavated to their 

 present depths. We cannot believe in the former existence of 

 flood-waters that were sufficiently great not only to fill valleys 

 of such a depth, but to brim over and drown wide regions 

 beyond — covering high table-lands and rising to some height 

 upon many hill-slopes. It is evident, then, that the upland-loss 

 must date back to the earlier stages of the Pleistocene Period, 

 and we thus learn that even in those earlier times the rivers 

 were occasionally subject to floods of enormous magnitude. 

 But long after the valleys had been greatly deepened by river- 

 erosion floods continued to take place, and we know that after 

 the streams had succeeded in excavating their channels to nearly 

 their present depth, and when the valleys had assumed pretty 

 much the appearance which they now show, a final epoch of 

 great inundations succeeded. To these concluding floods of the 

 Pleistocene Period we attribute the formation of the greater 

 portion of the valley-loss. The Ehine, the Danube, the Seine, 

 the Garonne, and many other rivers then poured immense 





