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 east 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- 
