VALLEY DRIFT OF FLUVIATILE ORIGIN. 471 



on the hillsides have eroded channels in the till, sometimes 10 to 20 

 or even 30 feet deep, but in general they are small and their united 

 volumes insignificant compared to the great sheets of valley drift. The 

 surface of the land is such that there never could be a great diffused or 

 general ablation, but the erosion must have been chiefly confined to the 

 hillside ravines. 



While, then, there must have been considerable erosion of the upland 

 till since the disappearance of the ice, especially immediately after the 

 melting, while the till was still unprotected by vegetation and the upper 

 till somewhat unconsolidated, yet this furnished only a small part of the 

 valley drift. 



The impossibility of thus accounting for the valley drift is still further 

 emphasized by the relatively short time in which this supposed erosion of 

 the uplands must have been accomplished. The upper stratum of the 

 valley drift often extends beneath the sea level of that time as deltas 

 deposited by the rivers in the sea. No matter what origin we assign to 

 the valley drift, the great mass of the deposit must have been laid down 

 between the time of the melting of the ice at the place of deposit and the 

 retreat of the sea to its present level. The limited erosion by the sea dur- 

 ing this time proves it to be geologically a very brief period. 



Furthermore, we must remember that the till resists erosion far better 

 than the sedimentary drift. For a large part of postglacial time the 

 streams have been able to erode and transport the sediments of the valleys 

 more rapidly than the upland erosion. This is proved by the great size of 

 the valleys of erosion which the streams have excavated alike in the marine 

 clays, in the valley drift, and in the glacial sediments proper. Only here 

 and there locally has deposition exceeded transportation in the lowland 

 valleys. Any such relation of the comparative difficulty of erosion of the 

 till and the sediments, or of land slopes to rainfall, as now exists, could 

 plainly never have caused the great accumulation of alluvium in the 

 valleys. When once the tenacious till was eroded, the streams would have 

 been able to transport most of the loosened matter direct to the sea. Only 

 the coarser matter would be left in the valleys, and a fine clay, like the 

 lowest layer of the valley drift, would be impossible under the conditions 

 assumed. • 



Moreover, we must account for the coarse residual matter that would 



