G. H. Stone — Glacial Sediments of Maine. 141 



12. Much of the so-called Valley Drift. — The deposits hereto- 

 fore described are so unmistakably of glacial origin that I antici- 

 pate no difference of opinion on this point among those who study 

 the facts in the field. We now approach a class of sediments 

 concerning which the interpretation is more doubtful, and 

 probably more facts may be needed before we solve the 

 problem in its details. It is but justice to add that my con- 

 clusions have been very slowly formed and even yet are in 

 part tentative. 



Where was the material derived from that forms the thick 

 sheets of alluvium which cover the bottoms of most of the 

 larger valleys of New England ? 



If this sediment was derived from post-glacial erosion of the 

 till and if the deposition of the alluvium took place after all the 

 ice had melted, we ought now to find the marks of this erosion. 



1. Since the alluvium was deposited in the larger valleys, 

 the erosion must have taken place in the steeper lateral valleys 

 and on the higher hill-slopes. Ravines of erosion in the till 

 ought to abound on all the highlands, and their aggregate 

 volume ought to be very great in order to account for so large 

 a body as the valley drift. Now there are multitudes of 

 ravines of erosion on the hill-sides and in the upland valleys, 

 but they are mostly small. Each year it has appeared more 

 and more improbable that the existing ravines represent the 

 vast erosion required by the hypothesis that the valley drift is 

 due to post-glacial erosion of the till. Nor do I see any proof 

 of the obliteration of ravines such as must have been formed. 

 Nor is it admissible to postulate, in so hilly and uneven a 

 country as Maine, any great diffused or general ablation from 

 the whole surface of the till, taking place after the ice had all 

 melted. The contour of the surface is such that surface 

 waters necessarily gather into rills and these into larger 

 streams, thus largely localizing the erosion in ravines 



2. The valley drift is chiefly composed of the finer parts of 

 the till. If this fine detritus was washed out of the till after 

 the final melting of the ice and deposition of the till in its 

 final position, we ought now to find in the upland valleys and 

 ravines great masses of residual gravels composed of the coarser 

 matter of the till left after the removal of the finer matter to 

 form the valley drift. No such extensive bodies of coarse 

 residual matter are found in such positions as ought to occupy 

 them on this hypothesis. 



The problem resolves itself into this : How can we account 

 for a very great erosion of the finer matter of the till, yet so 

 diffused over the surface as not to leave very large erosion 

 ravines and masses of residual gravels ? 



The subject is too complex to properly present within the 

 limits of a single article. The outlines are as follows : 



