1877.] Surface Geology of the Merrimack Valley. 525 
slightly east of south. The upper and lower portions of the ` 
river which occupies this valley are known by different names. 
For more than fifty miles from its source, this river is called 
Pemigewasset ; and the name Merrimack is applied to it only 
from the confluence of the Winnipiseogee River with the Pemi- 
gewasset at Franklin. ; 
After entering Massachusetts the river turns to the east at 
North Chelmsford, and thence pursues a devious east and north- 
east course at right angles to its valley in New Hampshire. It 
here threads its. way among hills, with no distinct, wide valley ; 
and only low water-sheds divide it from adjoining basins on the 
south. - 
In considering the surface geology of Merrimack Valley, wè 
will begin at its head and describe first the modified drift 
which forms conspicuous plains, terraces, and intervals or bot- 
tom-lands along its course in New Hampshire, and occurs in 
gravel ridges, similar to the Kames of Scotland, well shown at 
many places along the whole course of this river ; next, the prom- 
inent rounded hills of coarse glacial drift or till, which are finely 
displayed along this river in Massachusetts; and, last, the 
marshes and beaches at its mouth. After this, we will inquire 
what these deposits teach in regard to the history of this valley 
in the glacial age, during the melting of the great northern ice- 
sheet, and since that time. j 
The modified drift of the upper part of the Merrimack, called 
Pemigewasset River, is usually one half mile to one mile wide, 
and is bordered on both sides by high hills or mountains. Be- 
low Franklin the modified drift is usually one to two miles wide ; 
its greatest development is in Concord, in Merrimack, and in 
Litchfield, where it has a width of nearly four miles. The hills 
Which border this part of the valley rise with comparatively gen- 
tle slopes, and the lowest points of its eastern water-shed are only 
three hundred and fifty to six hundred and fifty feet above the 
Sea, 
On Pemigewasset River modified drift occurs first in Lincoln, 
five miles from Profile Lake. This is very coarse, water-worn 
gravel, containing pebbles six inches to one and a half feet in 
diameter, or sometimes larger. It has an irregularly smoothed 
surface, imperfectly terraced, with its outer margin twenty feet 
above the stream. From this point modified drift is continuous 
on one or both sides of the river for thirty miles. In the first 
Seven miles, to Woodstock village, it consists wholly of gravel of 
