MODIFIED DRIFT ALONG MERRIMACK RIVER. 97 
In Litchfield and Merrimack the high sandy plains have a larger devel- 
opment than in any other portion of this valley excepting Concord. On 
the east side we find the modified drift occupying almost the entire town- 
ship of Litchfield. An area from one fourth to three fourths of a mile 
wide next to the river is the low fertile terrace-—which is partly interval, 
as opposite the mouth of Souhegan river, but mostly lies somewhat above 
high water. East of this is the plain, about 100 feet above the river, co- 
inciding in its eastern boundary nearly with that of the township. Its 
greatest width is opposite Thornton’s Ferry, where it extends three miles 
back from the river. Its surface is in general very level ; a depression is 
partly occupied by Darrah, Halfmoon, and other ponds. This wide 
alluvial area becomes narrowed to two thirds of a mile after entering 
Hudson, but again expands about Otternic pond, which is surrounded by 
plains. Two miles farther south, below Nashua, this area is contracted 
to only one fourth of a mile at each side. 
The plains of Merrimack extend five miles southward from Reed’s 
Ferry, having the same height as on the east side, and extending back 
nearly two miles from the Merrimack at their widest portion, which is 
along Souhegan river to Burnap bridge. Below this the alluvial area 
averages a mile wide nearly to Pennichuck brook, on whose north side it 
is interrupted by till which extends almost to the river. More than half 
of this width is occupied by the plains, which are mostly very level, with 
scarcely any elevations above the general surface, but having occasional 
hollows that often enclose small ponds. A considerable portion of the 
plain at one mile south from Thornton’s Ferry has undergone erosion to 
the amount of 25 feet, now remaining 75 feet above the river, At the 
south-west part of this terrace clay deposits, which have been used for 
brick-making, occur near two small ponds. A single low terrace, one 
third of a mile wide, lies between the southern extension of these plains 
and the river. 
From Reed’s to Thornton’s Ferry, two terraces are well shown below 
the plains. The north part of Souhegan village, and the road farther 
south, lie upon the higher of these, which is about 60 feet above the 
river. A succession of five terraces was observed south of Naticook 
brook at Thornton’s Ferry. The river here is 100 feet above the sea, 
and the terraces are 20, 35, 50, 75, and 105 feet above the river, the first 
VOL. Ill, 13 
