MODIFIED DRIFT ALONG MERRIMACK RIVER. 95 
similar stratum of clay about 25 feet thick, overlaid by 15 feet of sand, is 
worked near Bow Mills. A large alluvial area, including clay-beds, bor- 
ders Suncook river from one to two miles above its mouth. 
These deposits of clay were accumulated in very quiet water. The 
rivers which flowed from the melting ice-sheet appear to have brought 
down a greater depth of modified drift in the south part of Hooksett than 
was supplied along the distance in which the clay-beds occur. A consid- 
erable depth of nearly still water was thus held back in this part of the 
valley during the deposition of the extensive plains of Concord; and the 
same floods which deposited fine gravel and sand in Concord, carried for- 
ward clay to Pembroke and Hooksett. When this channel was nearly 
filled, and the river began to excavate the barrier below, the current be- 
came so rapid that a layer of sand was formed above the clay. 
No very extensive tracts of interval are found on the Merrimack south 
of Concord. The low terrace, lying slightly above the freshets, but hav- 
ing nearly the fertility of an interval, sometimes occupies large areas; 
but its average extent is much less than that of the high terraces or 
plains, which often come nearly to the river. From the mouth of the 
Soucook river to Hooksett the low terrace is well developed on the west 
side, averaging a quarter of a mile wide. On the east side it is narrow, 
but attains considerable width between Suncook and Hooksett, where it 
is interval. Southward to Manchester it is nowhere wide, but nearly all 
the way occupies a narrow margin on each side of the river. The Con- 
cord Railroad, along its entire length, is built on this terrace. 
7 5 Amoskeag ¢ 5 
3 
Kames. Pond. g & dam, 179. 8 8 
w, te E. 
150 ft. 
above sea. 
We seem mm eens we me ew 
Fig. 21.—SECTION IN MANCHESTER. Length, 2 miles. 
At Amoskeag falls the alluvium is two miles wide, and it averages 
thus for three miles, the city of Manchester lying at the middle of this 
distance on the east side. The largest part of this area consists of high 
sandy or gravelly plains, whose barrenness made this township, under its 
former name of Derryfield, proverbial for poverty. The Amoskeag falls 
were then utilized only as a fishing-place. The river here descends 56 
feet, from 179 to 123 feet above the sea; and its water-power has within 
