18 SURFACE GEOLOGY. 
down from the ice-sheet by glacial rivers. At the time of their formation 
the adjoining valleys were probably still occupied by the unmelted ice. 
Nearer to the coast we_find in this situation beds of fine gravel, sand, or 
clay, sometimes enclosing marine shells and pine cones, and in several 
instances overlaid on their north-west side by coarse glacial drift or 
upper till a few feet in depth, giving evidence of a retreat and subsequent 
advance of the ice sheet. 
Submergence by the Sea. These marine deposits, which reach to about 
one hundred and fifty feet above the sea, afford the only certain proof 
found in our exploration of the modified drift in New Hampshire of any 
change in the relative heights of land and ocean. With the exception of 
the trunks, branches, and leaves of trees, which have been rarely found, 
all the rest of our modified drift is, so far as known, destitute of organic 
remains; and we have seen that the explanation of the thick deposits 
of the Champlain period, and of their present excavated and terraced 
condition, requires no submergence by the sea, nor change in the 
height and slope of the land. It seems quite probable that the sub- 
mergence in the glacial period, of which we have proof, amounting to 
fifty feet in southern New England, two hundred feet on the coast of 
Maine, and about five hundred feet in the valley of the St. Lawrence, was 
not caused by any downward and upward movement of the earth’s sur- 
face, but by the attraction of the immense masses of ice, which, as pointed 
out by Adhémar, would draw the ocean away from the equator towards 
the poles. The whole amount of water in the sea was diminished, but 
the accumulation of vast sheets of ice, several miles in thickness, would 
be sufficient to retain the ocean at its present height near their lower 
limits, while it would rise much higher than now about the poles, and at 
the equator would sink far below its present level. Such a rise of the 
sea, increasing in amount at high latitudes, is attested by the modified 
drift of both America and Europe; and coral islands afford proof of the 
corresponding depression of the ocean, succeeded by a gradual elevation 
to its present height, over large areas within the tropics. 
The two great continents appear to have existed, with somewhat the 
same outlines as now, from a very remote geological epoch. From the 
Silurian age to the glacial period we have no record that any part of New 
Hampshire was submerged beneath the ocean; and nearly all that we can 
