I I 8 SURFACE GEOLOGY. 



through Greenfield. The last blockade of the ice-sheet in its retreat to 

 the north may have been at Long fall, in the west part of Henniker, 

 where the high hills leave a narrower space than usual for the passage 

 of the river. The large proportion of sand in the kames of the north 

 part of Bennington is what we should expect, if their deposition was at 

 the mouth of glacial rivers where they entered the lake. The most im- 

 portant testimony, however, is given by high deposits of sand and fine 

 gravel, like that on Hedgehog hill in Deering. The widened lake now 

 filled the whole valley ; and these deltas, brought in by glacial rivers or 

 tributary streams, mark its height and shore line, and enable us to gauge 

 the floods which were supplied from the melting ice. 



The earliest of these lake-shore deposits are the plain of Greenfield 

 and that of Hancock village. Both of these have the same height 

 with the outlet, over which there as yet flowed only a shallow stream. 

 When the lake had advanced north to Clinton village in Antrim, the 

 depth of its outflow was probably 20 feet, as shown by a level-topped 

 ridge of sand and fine gravel exposed on the north side of Great brook 

 and the road, a quarter of a mile east from Hastings's mill. This deposit 

 extends a quarter of a mile to the north, and also occurs south of this 

 stream, by which it was formed about at the level of the lake. High 

 sand was also found three miles farther north, on the water-shed between 

 Cochran brook and North Branch, at the south-west side of Riley moun- 

 tain. This is two and a half miles due west from that on Hedgehog 

 hill. Both these deposits are level-topped deltas of glacial streams that 

 descended to the lake from the north, having the place of their inlet de- 

 termined by the gap of the adjacent hills. Their heights are the same, 

 and show that at the time of their formation 50 feet of water poured 

 over the outlet in Greenfield. Somewhat later, when the lake reached 

 its greatest extent and received its largest tribute from the more rapidly 

 melting ice-sheet, the depth of water discharged was 80 feet, as shown 

 by a delta-terrace half a mile south-west from Hillsborough Centre, and 

 by plains which occur at the same height north-east of Hillsborough Up- 

 per Village. All these deposits are level-topped, or nearly so; and their 

 position is generally on steep hillsides, with no barrier, if the drainage 

 had been the same as now, to prevent their being carried forward to the 

 bottom of the valley. Other deltas similar to these might probably be 

 found by a more thorough exploration of the ancient lake shore. 



