172 SURFACE GEOLOGY. 
of lowland lie on both sides. Excavations in the north-west 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 south-east, has only occasional layers 
of gravel, with the largest pebbles six inches in diameter, very irregu- 
larly stratified with sand, which is here four fifths of the whole deposit. 
The depth of modified drift forming the ridge is shown by wells to be 
from 50 to 90 feet. 
Portions of Seabrook and Salisbury are sandy plains, 25 to 50 feet 
above the sea. These are probably marine beds, deposited in a consider- 
able depth of water, but they are not known to contain organic remains. 
The most recent deposits of modified drift are the beaches and salt 
marshes bordering the ocean. Along much of our coast, at a distance 
varying from a quarter of a mile to one mile or more beyond the natural 
shore of hard land,—that is, of ledge, till, or ordinary modified drift,—we 
find a beach-ridge of quartzose sand, which becomes gravel or shingle 
near rocky shores. This ridge of loose material has been heaped up by 
the waves nearly to the highest point reached by them at high tide dur- 
ing storms; and when it is composed of sand, the wind piles it still 
higher in irregular hills, mounds, and ridges, which are constantly chang- 
ing inform. The beach-ridge of Plum island, at the mouth of Merrimack 
river, is thus blown into dunes 50 feet high. On the side away from the 
sea, this formation slopes somewhat steeply to the solid bottom, 10 to 40 
feet below the sea-level; towards the sea, it often slopes away very gently, 
with a wide area of hard sand between the lines of high and low tide. 
For a quarter of a mile out from these beaches the water is shallow, and 
the waves break upon shifting banks of sand. 
The area from the beach to where the land rises above the reach of 
the sea is usually occupied by salt marsh, which has a level surface two 
or three feet below the highest tides. This is composed of fine, clayey 
mud, brought in and deposited by the tide, which cuts channels for its 
flow and ebb. None of our forest trees can endure salt water; and the 
marshes are left to a rank growth of the grasses and sedges peculiar to 
the sea-coast. On the south side of Long Island, and thence southward, 
the beach is frequently divided from the shore by extensive sounds or 
shallow bays, which are partly filled with salt marsh. 
