MODIFIED DRIFT ALONG MERRIMACK RIVER. IOI 
vial area on the west is narrow, consisting principally of the low terrace, 
which is about 25 feet above the river. Plains of considerable extent 
occur on the opposite side, 50 to 75 feet above the river. At Tyngsbor- 
ough the alluvium is wholly cut off on the west, and nearly so on the 
east, by hills of ledge or till. 
Five miles south of the state line, the Merrimack 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, about thirty- 
five miles to its mouth three miles east of Newburyport. South and 
south-east from its bend are extensive low alluvial plains. These were 
deposited by the floods from the melting ice-sheet in New Hampshire, 
which kept their course south-east to Massachusetts bay. These plains 
form the very low water-shed between Lowell and Boston, and are the 
continuation of the slowly descending ancient flood-plain, which we have 
traced in the upper terraces of Merrimack river through New Hampshire. 
When these extraordinary floods abated, the river found a lower channel, 
which had been mainly sheltered from the deposition of modified drift by 
its crookedness and closely bordering hills. 
The area here crossed by the river is remarkable for peculiar accumu- 
lations of till, which forms steep, smoothly rounded oblong hills 100 to 
200 feet in height. These are set almost as thickly as possible over an 
otherwise nearly level country. Their prevailing trend, especially north 
of the Merrimack, as in South Hampton and Kensington, is north-west 
to south-east, or approximately parallel to the motion of the ice-sheet, 
which must have heaped them up beneath its mass, and left them at its 
melting in their present form. The next chapter will contain a full de- 
scription of these hills, which occur occasionally in many portions of 
New Hampshire. 
At the mouth of Merrimack river a ridge of sand, 25 to 50 feet high 
and 10 to 40 rods wide, extends several miles both to north and south, 
facing the ocean. Its gentle east slope forms the beaches of Salisbury 
and Plum island. This portion of the sand brought down by the river 
has been swept back again by the waves, and lifted above their reach by 
the wind. Marshes a mile wide lie on the west side of this ridge. These 
recent deposits will be described, with those of the coast northward, in a 
later portion of this chapter. 
