tn the Champlain and Terrace Periods. 467 
wholly concealed by the ordinary alluvium. In the twenty- 
four miles from Wells River to Lyme no similar ridge is found. 
In Thetford and Lyme we come to an abrupt change in the 
height of the upper terrace-plain. This slopes in thirty-three 
miles between the mouth of Passumpsic River and the south 
line of Orford, from 650 to 440 feet above the sea, gradual] 
declining from 190 to only 60 feet above the river. At Nort 
Thetford this line of the highest terrace suddenly rises to 525, 
and in a mile and a half farther south to 545 feet. This forma- 
tion is well shown through Thetford, with remnants in Lyme, 
and continues well developed and nearly level for twenty-five 
miles to Windsor, varying from 560 to 500 feet above the sea, 
and from 150 to 220 feet above the river. It forms extensive 
terraces or plains on one or both sides along the whole distance 
and is clearly the original flood-plain of the river. Frequent 
delta-terraces rise above it, sometimes 100 feet higher, being 
more than 300 feet above the present river channel. It isa 
notable coincidence, that along this same distance we have a 
continuous kame, occupying the center of the valley, commonly 
rising somewhat above the highest plain, but not seldom en- 
' tirely covered by it. Super-position and conformable stratifica- 
tion show the fine material of the terrace-plain to have been 
deposited upon this kame or gravel ridge, which beforehand 
extended like a windrow along the empty valley. To the south 
from Windsor the highest terrace shows a somewhat regular 
slope, descending with the river, and preserving a height about 
150 feet above it. 
This high and continuous flood-plain, extending from Thet- 
ford to Massachusetts 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- 
in. 
In Norwich we find an interesting example of a well marked 
ancient river-bed high above its present level. This extends 
two miles from Pompanoosuc River, one-third of a mile above 
its mouth, to the bend of Connecticut River a half mile south 
of Tilden pond, which occupies a depression of this old channel. 
Its highest point, from which there is a gradual descent both 
ways, is 520 feet above the sea or 145 feet above the river. On 
the east side of this ancient channel is the steep gravel kame, 
which for a while turned the Pompanoosue River in this course, 
‘till a direct passage was cut through its ridge. 
