158 GLACIAL GRAVELS OF MAINE. 



the other stream. If the two channels were formed simultaneously, I can 

 conceive no reason why the south chaiinel on a down slope should not 

 enlarge as fast as the soiithwestern channel on an up slope. If they enlarged 

 with equal rapidity, the larger amount of sediment ought to have been 

 deposited by the eastern instead of* the western stream. These facts all 

 combine to justify the conclusion that the diverging delta streams were for 

 most of the osar period not simultaneous, but that the Liberty branch was 

 the earlier. 



During the final melting there must have come a time when the thin- 

 ning ice could no longer flow southward over the hills and when the supply 

 of glacial water from the north would be diminished. The glacial river 

 flowed sluggishly, and presently in the osar channel northeast of the north 

 end of Hogback Moimtain Pass there was deposited an osar-plain of fine 

 gravel and sand, and finally cla}^ As the ice continued to melt, the ice 

 front began to retreat northward from the hills, and there came a time when 

 a lake occupied the valley of Half Moon Stream. This valley is widely 

 covered by a sheet of sedimentary clay to a height of at least 260 feet 

 above the sea. The small Half Moon Stream could not have deposited 

 this clay as ordinary valley alluvium, for it reaches at least 30 feet above 

 the stream. The most probable interpretation is that in Thorndike, Knox, 

 and Unit)^ there was a glacial lake several miles long confined between the 

 ice on the north and the hills on the south, east, and west. It may not 

 have always stood at the same height. For a time this lake may have 

 overflowed south through Hogback Mountain Pass. Into the lake still 

 poured a supply of glacial Avater from the north, and in it was deposited 

 the delta-plain situated between Unity and Thorndike which overlies the 

 ridges previously deposited in narrow ice channels in the midst of the 

 valley of Sandy Stream. This delta reaches from near Unity Village 

 almost to Thorndike station. But in the meantime the sea had been 

 advancing up the valleys of the Kennebec and thence eastward over the 

 broad valley of the Sebasticook River. If it first advanced along the south 

 side of the high hills which border the Sebasticook Plain on the south, 

 then it might be that the extreme northern part of the delta-plain south of 

 Unity Village was deposited in the sea. With possibly this exception, the 

 advance of the sea was so simultaneous over the Sebasticook Plain that no 

 marine deltas were formed by this glacial river in tliat part of the State, 



