Taylor — Highest Shore Line on Mackinac Island. 217 



ice sheet which we know to have retreated with a halting, 

 fluctuating movement. 



But there is positive evidence of marine post-glacial sub- 

 mergence in the St. Lawrence valley. On the mountain at 

 Montreal marine shell beds occur at an altitude of 520 feet 

 above the sea, and in the Pleistocene deposits of Vermont 

 near Lake Champlain the skeleton of a whale was found. In 

 the depths of Lake Superior a marine or brackish water 

 organism is found living today. These facts are all well 

 known and their prima facie significance is unmistakable. 

 Yet many geologists put them aside for the ice-dam theory 

 which seems more complete.' The idea of the ice dam 

 accounts for the submergence which made the shore lines. 

 But the facts proving marine submergence do not explain the 

 cause of that submergence. There is no proof that the waves 

 which washed the Iroquois beach were of fresh water. They 

 may have been salt, but more probably were brackish. If 

 there were no evidence of marine post-glacial submergence in 

 the St. Lawrence valley, then, on purely negative evidence, we 

 might remain in reasonable doubt. But the marine shells at 

 Montreal and the Champlain whale are positive evidence of 

 marine submergence of those localities, and they are not more 

 than 150 miles from the Iroquois beach at Watertown, IN". Y., 

 and 100 miles from its position at Fine. No adequate reason 

 can be given why we may not make the almost obvious infer- 

 ence that these marine beds are in the same area of post-glacial 

 submergence as the Iroquois beach. We know that salt water 

 stood more than 520 feet deep at Montreal, and we do not 

 know that the Iroquois waves were of fresh water. It seems 

 more pertinent, therefore, to ask what barrier could have held 

 the salt water out of the Ontario basin, than what could have 

 held a supposable body of fresh water in. 



Higher shore lines exist in the Ontario and Erie basins, but 

 they are generally comparatively weak in development and 

 many of them are known to be of a distinctly fragmentary 

 character, suggesting isolated areas of submergence and less 

 duration of time in their formation. All those which have 

 been thoroughly identified as shore lines are in hydrographic 

 basins so situated with respect to the several lobes of the 

 retreating ice sheet and certain old disused river beds as to 

 strongly suggest the operation of ice dams. The longest 

 beaches yet found at the higher levels are those of the Erie 

 basin. The Maumee beach, which is the highest, extends east- 

 ward from Fort Wayne and ends at Cleveland, and Dr. Gil- 

 bert found the termini of others of the same series in western 

 New York. All these termini are in comparatively open 



Am. Jour. Sci— Third Series, Vol. XLIII, No. 255.— March, 1892. 

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