240 J. \V. GOLDTHWAIT ALGONQUIN AND IROQUOIS BEACHES 



side differential depression. It is interesting to see^ however, that De 

 Geer, in employing ^'isobase for zero/^ seems to have thought of it as a 

 hinge line. In his remarks on the deformation of the shorelines of 

 lakes Agassiz and Dakota, already quoted, he makes ^'isobase for zero" 

 equivalent to '^'the limit for the uplifted region," and although he dwells 

 upon the uplift north of the line, he says nothing to suggest depression 

 south of it. The conception of a "hinge line" is, therefore, no new^ one. 

 Upham, De Geer, and others long ago recognized that successive up- 

 lifts probably did not hinge on the same line.^^ This appears to be true 

 in a measure in the case of the upwarpings of water-planes of the Lake 

 Michigan basin, for the northern limit of horizontality of the beaches of 

 Lake Chicago is near the latitude of Milwaukee, Wisconsin, and Grand 

 Haven, Michigan, while the corresponding limit for the Algonquin plane 

 is about 100 miles farther north. Measurements of altitude of the plane 

 of the later Mpissing shoreline, however, indicate that all post- Algonquin 

 deformations hinged on a single line.^^ 



THE ALGONQUIN PLANE AS A DATUM PLANE 



It has been stated that south of the "hinge line" the Algonquin beach 

 varies at most but 17 feet, and probably not more than 7 feet in altitude 

 for 350 miles from east to west and 200 miles from north to south. 

 Although this region may possibly have been raised or depressed from 

 its original position so uniformly as not to warp the beach perceptibly, 

 such a movement seems hardly probable. A variation of at least 20 or 

 30 feet might be looked for in such long distances. While recognizing 

 the possibility of an even uplift or depression, therefore, we may reason- 

 ably favor the idea that in the region of horizontality the Algonquin 

 beach stands today very nearh^, if not precisely, where it stood when 

 made. 



Since the beach itself in this region has an average altitude of 605-607 

 feet above sealevel, the calm-water level of Lake Algonquin at this stage 

 may be placed at 600 feet. This figure is open to certain corrections, 

 however, for conditions peculiar to the latter part of the glacial period, 

 chief among which is the lowered sealevel which resulted from the stor- 

 ing up of water in the ice-sheet.^^ According to Upham, the sealevel 

 during the stage of greatest glaciation may have been 100 or 150 feet 

 below its normal position. ^^ By the time the ice-sheet had melted back 



18 See passages just referred to, and especially Upham, op. cit., p. 481. 

 1" Goldthwait : .Tournal of Geology, vol. 16, 1908, pp. 473-474. 



2" R. S. Woodward : On the form and position of the sealevel. Bulletin of the U. S. 

 Geological Survey, no. 78, 1888, p. 60, 

 ^ Upham : op. cit., pp, 515-516, 



