94 SURFACE GEOLOGY. 
The description of the ordinary alluvial terraces and plains of the Mer- 
rimack was interrupted at the mouth of Soucook river. Thence nearly to 
Manchester the average width of the alluvial area is about one mile. Its 
narrowest place is at Hooksett, where ledgy hills rise close to the river 
on both sides. The Pinnacle, a sharp peak of white quartz on the west, 
350 feet above the river, affords a very beautiful view of the valley north 
and south, including several villages. The most interesting portion of the 
alluvium is its highest terraces. These are from 100 to 125 feet above 
the river, and are usually well shown upon both sides. Their similarity 
in height on opposite sides and their very steep escarpments facing each 
other, as already frequently pointed out, indicate their formerly continuous 
extent. These upper terraces form wide plains in Bow, which have been 
partly eroded by Moore’s brook, and in- Hooksett, south of the Pinnacle 
and north from Martin’s ferry. The last of these areas extends back at 
-the north one mile from the river. ‘The greater part of its material is 
coarse gravel, and its origin seems to have been from the north-east at 
the time of the departure of the ice, differing from the ordinary fine allu- 
vium, which was slowly deposited from the floods of the main valley. 
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 continuous stratum, which has a thick- 
ness of from 20 to 30 feet, with its top about 100 feet above the river. 
It is overlaid by a few feet of sand. The upper part of this stratum con- 
sists of a hard and compact gray clay. At a depth of 10 to 15 feet this 
is usually separated, by a thin layer of sand one fourth of an inch to three 
inches thick, from the underlying d/we clay, which is soft and plastic when 
dug from the bank. A gradual transition from the gray to the blue clay 
is rarely seen. These divisions are nearly equal in amount, but in some 
of the brick-yards only the upper gray clay is exposed. Except the lower 
part of the blue clay, which is of inferior quality, both layers are well 
adapted for brick-making. Deposits of the same gray and blue clay, the 
latter always below the former, are frequently found in the south-east 
part of the state, near the coast, and along the Hudson river and Lake 
Champlain.* Two miles above the most northerly of these brick-yards, a 
* Natural History of New Vork, Mather's Geology of the First District, p. 128, etc.,and Hitchcock’s Geology 
of Vermont, pp. 157, 160, etc. 
