SEDIMENTARY DRIFT OF UPPER KENNEBEC VALLEY. 177 



distance in the southern part of Forks Plantation it also contains many 

 bowlders 2 to 4 feet in diameter. 



Is there an ordinary osar along the axis of the valley, which was sub- 

 sequently covered and flanked by river drift? I regret that the numerous 

 sections examined by me along the windings of the river were not free 

 from surface sliding. I could not find an ordinary osar of arched strati- 

 fication along the valley, but the sediments appeared to become finer 

 by degrees as we go back toward the sides of the valley, and the coarse 

 central belt showed no distinct border. This kind of assortment is charac- 

 teristic of the osar-plain, and probably also of fluviatile drift. The stratifi- 

 cation of the central ridge could not be distinctly made out, but appeared to 

 be rather horizontal. The pebbles and cobbles are well rounded, far more 

 so than in ordinary valley drift off from lines of glacial gravel. The 

 average slope of the Kennebec River from Moosehead Lake to tide water 

 at Augusta is about 9 feet per mile, and from The Forks to Norridgewock 

 it is probably not more than 4 to 6 feet per mile. Below Bingham the 

 central ridges contain bowlderets more than a foot in diameter at a point 

 where the alluvial plain is near three-fourths of a mile wide. Above 

 Bingham the valley narrows to about one-fourth of a mile, here and there 

 expanding to a half mile or more. If the whole alluvial plain is ordinary 

 river drift, a very broad and swift river is necessary to transport such large 

 stones and to roll and round them so thoroughly. The question as to 

 whether so moderate a slope could have given the necessary velocity of 

 current will be considered later. My exploration of this part of the Kenne- 

 bec Valley was made in 1879, before T recognized the osar-plain. I 

 therefore somewhat doubtfully outline the history of the upper Kennebec 

 Valley as follows: 



First, glacial gravels were deposited in the A^alley between The Forks 

 and Solon by glacial streams flowing in narrow channels between ice walls. 

 These gravels are now deeply buried and have been found only here and 

 there. Subsequently an osar-plain was formed along the axis of the valley 

 in a much broader ice channel. This channel gradually broadened, until 

 at last it extended across the whole valley, at which time the river ceased 

 to be a glacial stream. The gradual retreat of the ice from the center of 

 the valley is probably the explanation of the fact that the alluvial plain of 

 the Kennebec does not extend back into several of the lateral valleys which 

 MON xxxiv V-l 



