474 GLACIAL- GEAVELS OF MAINE. 



VALLEY DRIFT OF SEMIGLACIAL ORIGIN. 



The evidence that the valley ch-ift was derived from the drainage of 

 the ice-sheet is as follows: 



1. The valley drift can not be due to the erosion of till after it has 

 become bare of the ice, either by meteoric and fluviatile waters or by the 

 sea. We have no other assignable origin than glacial. 



2. The shapes of the stones of the valley drift are in general those of 

 the glacial gravels after they have been rolled several miles by the glacial 

 streams. The stones are in most cases much more worn and rounded than 

 those contained in the channels excavated in the till by existing streams, 

 except on the steeper slopes of the mountains. The stones of the gravel of 

 the valley drift are often as much worn as stream gravels knoAvn to date 

 from Tertiary time, but this they can not be, since the stream gravels of 

 preglacial age were removed by the ice or incorporated with the till. We 

 have no machinery for the production of such great masses of rounded 

 gravel, acting within valley-drift time, except glacial streams. 



3. We find in the valley drift here and there masses of coarse matter 

 bearing no relation to the local land slopes. Now coarse matter collects 

 near the ice where the subglacial streams emerge from the ice. The 

 lingering of the ice front at a given place would cause local accumulation of 

 coarse matter near that point. The occm-rence within the valley di-ift of such 

 a mode of assortment of sediments as does not depend on the slopes of 

 the land requires us to postulate glacial conditions. An instance like this 

 is found near North New Portland. (See p. 188.) 



4. The last-named argument would be strengthened if at the same 

 time with the local coarseness of material it was found that the body of 

 coarse matter formed a low bar or ridge across the valley and rose above 

 the level of the valley drift both to the north and to the south of it. This 

 is the condition at North and East New Portland. In various places lakes 

 within the valley drift have probably been formed in this manner. 



5. The glacial origin of the valley drift would be confirmed if near 

 the supposed overwash plain of sediment terminal moraines were found. 

 Such occur at East New Portland, in the valley of the Androscoggin River, 

 at the State line, and near the Katahdin Iron Works. 



6. In several cases an osar broadens southward and passes by degrees 



