CANTON-AUBUEIS^ SYSTEM. 207 



and Mexico; and perhaps there should be added the Androscoggin Valley 

 from the mouth of Swift River to Canton. There has been a large amount 

 of erosion along the Androscoggin and Swift rivers, and this makes it 

 doubh^ difficult to discover what was the original condition. 



A well-developed osar- ridge begins not far from the Androscoggin 

 River at Grilbertville (Canton Point), and passes southward tlii'ough the 

 wide plain covered by sedimentary clay and sand which here borders the 

 Androscoggin on the south. It passes about half a mile east of Canton 

 Village, and then ascends the valley of Bog Brook to its source at a small 

 pond in Livermore. In this valley the gravel takes a somewhat unusual form. 

 A two-sided ridge is found along the axis of the valley, bordered on each side 

 by a ravine of erosion, while on each side of the A'alley is a level terrace of 

 fine gravel. The central ridge consists of gravel with cobbles and bowl- 

 derets, all very much rounded. It rises 10 to 20 feet above the terraces at 

 the sides of the valley. Evidently a glacial stream at one time flowed in a 

 rather narrow channel in the midst of the valley, and in this narrow chan- 

 nel was deposited the central ridge of coarse matter. Later the channel 

 Avidened until it extended nearly or quite across the valley, and in this 

 broad channel the finer gravel was deposited as a plain extending from the 

 central ridge to each side of the valley. The current in the broad channel 

 was not so rapid as in the narrow one, and the gravel was finer and did not 

 reach to so great a height as the original osar. Finally valleys of erosion 

 have been excavated in the osar-plain along each flank of the osar. 



In several places the terraces along the sides of the valley can be seen 

 to overlie till. Many bare ledges appear in the southern part of the pass, 

 as if the till had been washed away by the glacial river. The top of this 

 pass is so level that for a considerable distance Ave find a stream flowing 

 nortliAvard on one side of the osar ridge and on the other side a stream 

 flowing in the opposite direction. 



On the west and southwest sides of Brettuns Pond, at Livermore Post- 

 Office, the gravel takes the form of a narrow plain of reticulated ridges and 

 hummocks of gravel, cobbles, and boAvlderets. Extending from this plain 

 eastward around the south end of the pond is a rather level plain' composed 

 of gravel on the west but becoming sandy tOAvard the east. It is about 

 one-third of a mile in diameter, and is eAddently a delta-plain. It lies 

 between Brettuns Pond and the valley of Martins Stream. This valley is 



