1 8 SURFACE GEOLOGY. 



clown 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 trunk.s, 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 Adhemar, 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 



