GLACIAL DRIFT, 



331 



moon produces two tides, one on the side next to it, and the other ex- 

 actly oioposite on the earth's surface. Now the specific gravity of ice, 

 compared with that of the earth as a whole, is about as one to six. 

 Therefore the ocean would be raised one mile at each pole, or nearly 

 a half mile above its present height. At the same time, because all 

 the ice was massed in high latitudes, it seems probable that everywhere 

 within the tropics the sea would fall, through the influence of gravita- 

 tion, below the depression of a half mile, which resulted from the removal 

 of water to form ice. 



These glacial sheets, when at their greatest extent and depth, caused 

 the sea to rise 200 feet higher than now at Long Island, as shown by ma- 

 rine shells. At a somewhat later date, when the ice-front was retreating, 

 the sea stood on the coast of New Hampshire and Maine 150 to 225 feet 

 above its present level. Probably at this time so much of the ice north- 

 ward had disappeared that this height does not correspond to that of 

 200 feet at Long Island ; for we have evidence (in the sub-marine chan- 

 nel of Hudson river) that after the ice began to retreat, the sea-level at 

 New York was depressed till it was at one time 600 feet or more below 

 its present height. When the sea was elevated 200 feet upon the coast 

 of Maine, or perhaps later, it stood 500 feet higher than now in the vicin- 

 ity of Montreal and along the St. Lawrence valley. This somewhat 

 greater height than we might expect seems to have resulted from prox- 

 imity to vast depths of ice resting upon the highlands of Canada and 

 Labrador. Near the middle of the Champlain period, when the ice-sheet 

 over the northern United States and the south part of British America 

 had principally melted away, but while immense ice-fields still lay farther 

 to the north, the amount of water restored to the ocean would not proba- 

 bly raise it more than half of the whole amount that it had been de- 

 pressed. At this time the tendency from gravitation to raise the sea- 

 level at the latitude of New York would be small, and no longer suffi- 

 cient, as when the ice-sheet had its greatest extent, to counterbalance 

 the depression, so that the sea might stand 600 feet or more lower than 

 now at New York, while the Hudson must form a channel now covered 

 by the sea. 



The testimony on this subject, which we have from Long Island and 

 the submarine channel of Hudson river, may be summed up as follows : 



