36 SURFACE GEOLOGY. 



slope, descending with the river, and preserving a height about 150 feet 

 above it. 



This high and continuous flood-plain, extending from Thetford to Mas- 

 sachusetts line, seems to have been formed during a gradual and slow 

 melting of the ice along this distance. It would appear that the greater 

 part of the depth of ice, as far northward as to the Passumpsic river, had 

 been melted in the last part of this time, sending down its floods laden 

 with gravel to form the kame. A comparatively shallow mantle of ice 

 remained, and when the melting advanced to the north from Thetford 

 and Lyme this disappeared too rapidly to give time for the formation of 

 a kame, or the deposition of a high flood-plain. 



At the north line of Thetford, near Ely station, the highest terrace is 

 435 feet above the sea, or 55 feet above the river. This is the south end 

 of the continuous descending slope from the mouth of Passumpsic river. 

 The first intimation of change is a high terrace, which rises from 475 to 

 545 feet in going from one mile north to one mile and a half south of 

 North Thetford. Opposite to this place in Lyme the alluvium does not 

 appear as usual in distinct terraces, but lies in a slope rising from 400 to 

 450, and at one mile north to 490 feet. South from North Thetford the 

 high plain averages one half mile wide for eight miles, extending half 

 way through the town of Norwich. Along this distance in Lyme and 

 Hanover only narrow terraces of corresponding height remain. 



Child's pond, situated on the high plain one third of a mile north of 

 East Thetford, is worthy of notice. No terrace occurs here below the 

 plain, which has been so undermined as to slope from its top to the river 

 at an angle of 45°, excepting only the width of the railroad bed built on 

 its side near the bottom. Eighty-five feet back from the edge of this 

 plain, with a road between, is the pond, occupying some two acres, 142 

 feet above the river, and by our soundings 40 feet deep. Its range from 

 high to low water is said to be one foot and a half, with outlet to the west, 

 but no inlet ; and its surface is only from two to five feet below the plain 

 on its cast and south-east sides against the river. The clayey character 

 of this alluvium is shown by the impervious bank which holds in the 

 pond. The circumstance that only so narrow a width intervenes between 

 the pond and its edge is not specially remarkable, as this plain was origi- 

 nally continuous across the valley, all its east portion having been exca- 



