530 Surface Geology of the Merrimack Valley. [September, 
yearly from one bank and adding to the side opposite, leave no 
doubt that the river has flowed at the foot of the bluffs along 
their whole extent, occasionally making a deep excavation beyond 
its usual bounds, as on the east side south of Sugarball Bluff; 
that the high plain originally filled the whole valley ; and that 
the river has swept many times from side to side over the space 
occupied by its lower terraces and interval. 
Valuable beds of clay, extensively used for brick-making, 
occur in the highest terrace for four miles north from Hooksett 
upon the east side. This clay appears to form a nearly continu- 
ous stratum, which has a thickness of twenty to thirty feet, with 
its top about one hundred feet above the river. It is overlaid by 
a few feet of sand. The upper half of this stratum consists of a 
hard and compact gray clay. Ata depth of ten to fifteen feet 
this is frequently separated by a thin layer of sand, one fourth 
of an inch to three inches thick, from the underlying blue clay, 
which is soft and plastic when dug from the bank. These divis- 
ions are nearly equal in amount, but in some of the brick-yards 
only the upper gray clay is exposed. The same gray and blue 
clay, the latter always below the former, are frequently found in 
the southeast part of New Hampshire and along Hudson River 
and Lake Champlain. 
At Amoskeag Falls the alluvium is two miles wide, and it 
averages thus for three miles below, the city of Manchester lying 
at the middle of this distance on the east side. The greater part 
of this area consists of high sandy or gravelly plains, whose bar- 
renness made this township, under its former name of Derryfield, 
proverbial for poverty. The falls were then utilized only as 
a fishing place. The river here descends fifty-six feet, and its 
water-power has within fifty years built up the largest city ™ 
the State. 
In Merrimack and Litchfield the high sandy plains have æ 
larger development than in any other portion of this valley, 
excepting Concord. On the east side the modified drift occu- 
pies almost the entire township of Litchfield. An area one 
fourth to three fourths of a mile wide next to the river is the 
fertile low terrace, which is partly interval, as opposite the mouta 
of Souhegan River, but lies mostly somewhat above high water 
East of this is the plain, about one hundred feet above the river, 
having the same height as in Merrimack on the west. __ a 
The sand and gravel of the plain between Nashua River an 
Salmon Brook, on which the principal part of the city of Nashua 
