216 Taylor- —Highest Shore Line on Mackinac Island. 



least up to a level of 100 feet above the Algonquin beach as 

 seen on Mackinac Island, and 200 feet above it as seen at 

 Petoskey. 



It might be supposed that the small area of Mackinac Island 

 was submerged and re-elevated by movements so sudden that 

 the waves had no time for effective action at any horizon be- 

 tween 205 and 300 feet. But the difficulties in the way of 

 such a supposition are many and great, and when it becomes 

 necessary to extend the application of it to an area 100 miles 

 long and even to the whole area of the three upper Great 

 Lakes, the difficulties become insuperable. 



The most impressive feature of the old shore line which I 

 have described is the great strength of its development. The 

 time during which the water level stood at or within 35 feet of 

 its highest stage must have been of very long duration, for by 

 far the strongest development is within this limit. The ap- 

 pearance is just such as would be produced if the water had 

 risen slowly at a steadily decreasing rate, stood long at its 

 highest level, and then had fallen away again as slowly and at 

 a steadily increasing rate. The Iroquois beach of the Ontario 

 basin, described by Dr. G. K. Gilbert and Professor J. W. 

 Spencer, has the same strong development, except where lack 

 of drift gave little loose material for the waves to work upon. 



If the rise of the Algonquin beach on the east side of the 

 Georgian Bay continues northward, it must extend far above 

 the pass at Lake Nipissing. The Iroquois beach has the same 

 northward rise, and if its approximate mean plane, not count- 

 ing the exceptional high grade northeast of Watertown, 'N. Y. t 

 be produced northward, it too strikes much higher than the 

 Ki pissing pass. The two planes come as near together at 

 Nipissing as could be expected in view of the considerable 

 extend of irregular, local earth-warping. Considering this fact 

 and the general similarity of the two beaches, it seems proba- 

 ble that the Iroquois and Algonquin beaches are in reality one 

 continuous line. Terraces have been noted on the hills at 

 Nipissing, at an altitude of several hundred feet above the 

 pass and they are in all probability a part of the connecting 

 link. 



The generally accepted explanation of the origin of all the 

 old shore lines of the basins of the Great Lakes is that they 

 were made by bodies of water held as lakes in front of the 

 ice sheet as it retreated northward. But this theory does not 

 seem applicable to the Algonquin and Iroquois beaches. The 

 long duration of wave action at one horizon, as attested by the 

 strong development of these lines, is hardly reconcilable with 

 the idea of an ice dam, which must have been formed by an 



