220 GLACIAL GEAVELS OF MAINE. 



deposited by an overflow of the Androscoggin River after the melting of 

 the ice. Only an ice dam at Rumford could cause an overflow up the 

 valley of the west branch of the Concord and over the col at North 

 Woodstock. 



The following is the probable history of this interesting valley: First, 

 a glacial river flowed southwestward through the North Woodstock Pass in 

 a narrow channel along the axis of the pass. This was bordered on each 

 side by ice walls, and in the channel was deposited an osar-ridge. Subse- 

 quently this channel gradually broadened, and in the broad channel was 

 deposited an osar-plain. At length a time came when the channel extended 

 from side to side of the valley, and the osai'-plain thus came to resemble a 

 plain of valley drift in its external form. The broader the channel became 

 the less rapid, on the average, was the glacial river and the finer were the 

 sediments deposited by it. The erosion of the plain has proceeded more 

 rapidly in the medium gravel than in the very coarse gravel, of the central 

 part of the valley or in the finer sand and gravel at the margins. Now a 

 dam of 125 feet at North Woodstock would flood back the water in the 

 broad osar channel for many miles xip the valley of the Ellis River. If 

 the channel was open on the top to the air, or for any reason the broad 

 osar river was not confined within the ice under high hydraulic pressure, 

 the dam would cause the glacial river to form practically a lake one-eighth 

 to one-half mile wide, extending from North Woodstock to Andover, where 

 it would be at least 50 feet deep. The glacial river pouring from the north 

 down Black Brook would deposit in this dammed osar-plain channel or 

 back-water lake the plains near Andover Village which so much resemble 

 lake deltas. In this long reach of quiet water would be deposited the 

 fine clays of the Ellis Valley that border the narrower osar-plain. The 

 osar-plain of the Ellis Valley had been deposited in still earlier times when 

 the channel of the glacial river was not so broad as that of the later osar 

 border clay. It is also possible that the sedimentary drift near Andover is 

 in part frontal matter. 



The highest part of this pass is a short distance north of North Wood- 

 stock. Here a small brook takes its origin and flows southward along a 

 gentle slope to Bryants Pond. The osar-plain continues in this valley and 

 the material becomes coarser, and near Bryants Pond contains very round 

 bowlders 2 and even 3 feet in diameter. Here the plain becomes a plexus 



