80 SURFACE GEOLOGY. 
extent, lying on the south side of Contoocook river. Their northern and 
western portions are 125 feet above the Merrimack river at the head of 
Sewall’s falls, but they become slightly lower at the south. The mouth 
of Contoocook river is 249 feet above the sea. Its descent through Fish- 
erville, in the last mile and a half of its course, exceeds 100 feet. By the 
Borough dam, at the head of these falls, this river is held level to Con- 
toocookville in Hopkinton, six miles in a direct line. Along this distance 
and beyond we find extensive alluvial areas at small elevation above the 
river, continuous with these plains in the Merrimack valley. A descrip- 
tion of the modified drift of Contoocook river will be hereafter presented. 
The most extensive plains in Concord, and indeed in this entire valley, 
lie on the east side of the Merrimack between it and the Soucook river. 
They extend along Merrimack river six miles, from above East Concord 
to the mouth of the Soucook. Their area of greatest width, which ex- 
ceeds two miles, is opposite the city, being known as the “Dark plains.” 
The channel which has been excavated by Soucook river is very crooked, 
lying at first along their east edge, but at three miles from its mouth 
deviating towards the middle of the plains, and again returning eastward 
and southward. This excavation is 50 to 125 feet in depth, with areas of 
low terrace at its bottom bordering the river. The greater part of this 
large expanse of plain is very level, with occasional gullies, but with 
scarcely any undulations rising above the general surface. Its slope, in 
nearly four miles from its north-west limit to opposite the south part of 
the city, is only 10 feet, with a height 130 to 120 feet above the river, or 
the same above the river as the plains north and south of Fisherville, 
their difference in absolute height being equal to the descent of the river 
at Sewall’s falls. Farther south the slope of the plain becomes more 
rapid, descending 50 to 75 feet in about two miles, the highest portions 
at the south end being about 100 feet above the mouth of Soucook river, 
which is 199 feet above the sea. The total descent of the Merrimack in 
Concord is thus 50 feet, of which 20 feet are at Sewall’s falls, four 
miles above the city, 5 feet at rapids a short distance above the mouth 
of Turkey river, and 20 feet at Garvin’s falls, three fourths of a mile 
below. 
In Boscawen and Canterbury, and through Concord, the lowest terrace 
for 12 miles occupies a wide area, of which a large part is overflowed by 
