186 MAN AND THE GLACIAL PERIOD. 



the northward, at a period when, during the long and 

 severe winters, the annual accumulation of ice near their 

 mouths was excessive, ice-gorges of immense extent, such 

 as now form about the mouths of the Siberian rivers, 

 would regularly occur. We are not surprised, therefore, 

 to find, even in these streams, abundant indications of the 

 indirect influence of the great northern ice-sheet. 



The indications referred to consist of high-level gravel 

 terraces occasionally containing boulders, of from four to 

 five tons weight, which have been transported for a con- 

 siderable distance. The elevation of the terraces above 

 the present flood-plains of the Seine and Somme reaches 

 from 100 to 150 feet. We are not to suppose, however, 

 that even in glacial times the floods of the river Seine 

 could have filled its present valley to that height. The 

 highest flood in this river known in historic times rose 

 only to a height of twenty-nine feet. Mr. Prestwich esti- 

 mates that, without taking into consideration the more 

 rapid discharge, a flood of sixty times this magnitude 

 would be required to fill the present valley to the level of 

 the ancient gravels, while at Amiens the shape of the val- 

 ley of the Somme is such that five hundred times the 

 mean average of the stream would be required to reach 

 the high-level gravels. The conclusion, therefore, is that 

 the troughs of these streams have been largely formed by 

 erosion since the deposition of the high-level gravels. 



Connected with these terrace gravels in northern 

 France is a loamy deposit, corresponding to the loess in 

 other parts of Europe, and to a similar deposit to which 

 we have referred in describing the southwestern part of 

 the glaciated area in North America. In northern France 

 this fine silt overlies the high-level gravel deposits, and, 

 as Mr. Prestwich has pretty clearly shown, was deposited 

 contemporaneously with them during the early inunda- 

 tions and before the stream had eroded its channel to its 

 present level. 



