1877.] Surface Geology of the Merrimack Valley. 535 
Upper Green. Here it is interrupted for a little distance, be- 
yond which it lies on the northeast side of this street, extending 
to within a half mile of Old Town Hill. The entire length of 
this ridge is six miles. No other high deposits of modified drift 
are found in this vicinity, and wide areas of lowland border it on 
both sides. Excavations in the northwest part of the city show 
the ridge there to be composed mainly of water-worn gravel, with 
the largest pebbles about a foot in diameter. A railroad cut, 
known as March’s Hill, two miles farther southeast, has only 
occasional layers of gravel, with the largest pebbles six inches in 
diameter, very irregularly interstratified with sand, which is here 
four fifths of the whole deposit. The depth of modified drift form- 
ing this ridge is shown by wells to be from fifty to ninety feet. 
At the mouth of Merrimack River a ridge of sand, twenty-five 
to fifty feet high and ten to forty rods wide, extends for several 
miles both to the north and south, facing the ocean. Marshes a 
mile wide, with their surface two or three feet below the highest 
tides, lie on the west side of this ridge. Its gentle eastern slope 
forms the beaches of Salisbury and Plum Island. For a quarter 
of a mile or more out from these beaches the water is shallow, 
and the waves break upon shifting banks of sand. The ridge is 
built up or washed away by the same cause, and is also chan- 
neled and heaped into mounds by the winds, which are con- 
stantly changing its form. 
The mouth of the river has varied much during the past 
sixty-five years. A fort built in 1812 at the north end of Plum 
Island remained at one time three fourths of a mile north from 
the river’s mouth on Salisbury Beach. Subsequent changes have 
brought the river back, so that now it flows out to sea at nearly 
the same point as in 1812. 
We having now examined the recent geological records of this. 
valley, it remains for us to seek their order and meaning. The 
edges of the lenticular hills of till are overlaid by the kames, 
and these are in turn partly covered by the alluvium of the plains 
and terraces. The till is, therefore, the oldest of these deposits. 
This extraordinary formation, and the rounded form and stria- 
tion of exposed ledges, observed in all countries where till is 
found, presented one of the most difficult problems of geology, 
Which has been solved and made clear by a theory too wonderful 
ever to have been conjectured, were we not led to it by abundant 
_ and. undeniable testimony. This theory was first brought out 
prominently by Agassiz in 1840, and was based upon his studies 
