ANDROSCOGGIN LAKES-PORTLAND SYSTEM. 223 



western border plain the broad valley is covered by sand, silt, and clay. 

 At Trap Corner the fine alluvium extends for a considerable distance up 

 two small tributary valleys to the same height as the clay plain of the 

 main valley at that place. This proves that most of the broad valley was 

 at one time covered by rather still water, approaching the condition of a lake, 

 and this must have happened after the melting of the ice at that place. If 

 the great glacial river that deposited the osar-plain to the north had flowed 

 into the broad triangular valley below West Paris after the ice had melted, 

 it must have filled up the valley with a delta-plain. Instead, the plain of 

 rounded gravel and cobbles is confined to a strip along the west side of the 

 broad valley hardly more than one-fourth of a mile wide. It is thus proved 

 that an osar-plain was formed in a broad g'lacial chaimel along the western 

 border of the triangular valley at a time when the rest of the valle}^ was 

 covered by ice. Later, when the ice over the valley melted, this broad valley 

 foi-med, for a time, a lake, owing partly to the great breadth of the valley at 

 this point as compared with its narrowness at Snows Falls, and partly per- 

 haps to the osar-plain's acting as a dam across the valley near Snows Falls. 

 In the northwestern part of this lake coarse sediment would be deposited by 

 the swollen river of that time, consisting in part of portions of the eroded 

 osar-plain, while east and south only the finer sediments would be laid 

 down. It thus becomes reasonably certain that the drift of the broad 

 ti'iangular valley that extends from West Paris to Snows Falls consists 

 of an osar-plain more or less covered by alluvium of fluviatile and lake- 

 delta origin. 



Not far south of Snows Falls the valley of the Little Androscoggin 

 widens so that the alluvial plain has an average breadth of about half a 

 mile. It is finer in composition than it is north of Snows Falls, sand and 

 gravel being most abundant, but it contains numerous pebbles and some 

 small cobbles. For 1 or 2 miles south of the falls the plain shows num- 

 bers of low ridges and shallow kettleholes. Then it becomes more level 

 on the top, and soon a two-sided ridge is formed near the river and extends 

 for about 3 miles to South Paris. It is locally known as the "Horseback." 

 It has the same height as the rest of the plain, and the material appeared to 

 be little if any coarser than that of the plain at the sides of the valley. 

 The ridge is the result of erosion of the alluvial plain on each side of the 

 horseback to a depth of 10 to 40 feet. There must be a reason why this 



