ANDROSCOGGIN LAKES PORTLAND SYSTEM. '217 



cross the lake obliquely — at least the ridge soon appears on the western 

 shore and continues thus to the south end of the lake, where it forms a 

 prominent two-sided ridge. The region lying south and southeast of the 

 lake is so low that onl}- a few feet of digging would be required to drain 

 the lake southeastward down Black Brook. I am informed that in time 

 past it has repeatedl}' been proposed to cut a canal at this place in order to 

 use the water for lumbering purposes on Black Brook and the Ellis River. 

 One branch of Black Brook takes its rise within a half mile of the foot of 

 the lake. The osar continues southeastward along the broad and level val- 

 ley of Black Brook for about 3 miles, sometimes broadening into a plain 

 resembling an osar-plain in appearance. It then enters a narrow V-shaped 

 pass where the hills rise steeply, almost precipitously, on each side up to 

 near 1,000 feet. The glacial river flowed through this pass, but in its nar- 

 row part I saw no glacial gravel for a short distance. It can hardly be 

 expected that au}^ but the larger stones and bowlders would be left by the 

 stream in the narrow gorge, and if there were an}^ such they have been 

 covered out of sight by dcibris that has fallen from the high cliffs. South 

 of the narrow pass Black Brook has for several miles a fall of 50 feet or 

 more per mile, and here most of the gravel was swept away by the force of 

 the glacial river. Approaching Andover the slopes become gentle, and then 

 for 3 or 4 miles the valley is covered with a hummocky plain which soon 

 becomes nearly horizontally stratified. This plain is composed of coarse 

 gravel, cobbles, etc., at the north, and passes by degrees into sand at the 

 south. It fills the valley from one side to the other and is of varying breadth 

 up to nearly a mile. The valley of the Ellis River in Andover forms a 

 broad valley or mountain cirque several miles in diameter, surrounded on 

 all sides by high hills, except on the south. Into this rather level plain 

 pour the Black and Sawyer brooks, also tlie east and west branches of the 

 Ellis River, all uniting not far south of Andover to form the main Ellis 

 River. Sedimentary plains of gravel, sand, and silt extend up all these val- 

 leys for a mile or more. Part of these plains must have been brought 

 down by these streams as fluviatile alluvium, yet the alluvium is so abun- 

 dant near the mouth of Black Brook as to suggest the theory that the gla- 

 cial river here flowed into a lake which extended up the tributary valleys. 

 The cause of such a lake will be discussed presently. 



The valley of the Ellis River narrows near South Andover, and from 



