462 PLEISTOCENE OF INDIANA AND MICHIGAN. 



plane of the Nipissing beach is only 6 feet above present lake surface 4 miles south of Bayfield 

 and that, declining southwestward, it intersects the surface at the head of Ashland Bay. A 

 line drawn from this intersection parallel with the isobases of the Nipissing beach (about N. 65° 

 W.) intersects the north shore about 15 miles east of Duluth. (See also p. 456.) 



The hinge line of the Algonquin beach at the west end of Lake Superior appears to be located 

 at an undetermined distance south of Duluth and does not appear to fall within the basin of 

 the lake unless the descent of the Algonquin plane southward is much more rapid than has 

 been supposed. The south slope of the Lake Superior basin south of Duluth is largely covered 

 with pebbleless lake clays which bear poor records of the beaches, showing only as wave-cut 

 benches with low cliffs and narrow sloping terraces. For this reason, and on account of the 

 forested condition of most of the country, the Algonquin beach has not yet been surely deter- 

 mined. If the beach runs westward from the region of the Keweenaw Peninsula without 

 notable turning (and such northward bending is not suggested by the isobases of the beaches of 

 Lake Duluth as determined by Mr. Leverett x ) then the Algonquin beach 12 to 15 miles south of 

 Duluth should have an altitude of something more than 700 feet above sea level or more than 

 100 feet above the lake, indicating a relatively wide separation of the hinge lines of the Algon- 

 quin and Nipissing beaches, amounting perhaps to as much as 100 miles. 



A characteristic of the Nipissing beach is the remarkable evenness of its plane over the 

 whole area over which it has bee.n uplifted and tilted. Its variations from a true plane inclined 

 slightly southwestward are, so far as now known, few and small. At North Bay the plane has 

 been uplifted 102 to 104 feet from the horizontal and at Peninsula Harbor, at the extreme 

 northeast angle of Lake Superior, 115 feet (aneroid); and the uplift has produced the effect of 

 simple tilting of a rigid plane and with little irregularity or warping. 



CORRELATIVES OF THE NIPISSING GREAT LAKES. 



LAKE ST. CLAIR. 



During the time of the Nipissing Great Lakes Lake St. Clair was at low stage; in fact, it 

 was almost if not wholly abandoned as a lake. Only a sluggish, relatively small stream with 

 locally expanded ponds or swampy parts remained. (See p. 496.) 



LAKE ERIE. 



The height of the water in a lake is in some degree dependent upon the volume of discharge 

 at its outlet, especially where the variation of volume is as great as 700 per cent. (See p. 496.) 

 The effect is also largely dependent upon the shape of the outlet; if this is broad and shallow 

 the effects of variation in volume will be slight, but if it is deep and narrow they will be rela- 

 tively large. Where Niagara River passes over the ledges of the Onondaga limestone at Black 

 Rock, its channel contains a narrow and rather deep passage which probably carried the whole 

 of the stream at the low stages, and this resulted hi a lowering of the lake level more than if 

 the rock sill had been broad and even. Lake Erie alone furnishes about 15 per cent (nearly 

 one-seventh) of the whole volume of Niagara River, and the conditions on the sill at the outlet 

 are such that a variation of 700 per cent in volume would markedly affect the level of the lake. 

 This variation is therefore, in all probability, the chief explanation of the drowning which 

 Moseley 2 found at Sandusky, Ohio, and on the islands in Lake Erie. He describes an old 

 land surface with stumps of trees and other vegetable remains now submerged 10 to 12 feet, 

 and also narrow creek beds which are sharply trenched into the bottom of Sandusky Bay and 

 are traceable across its whole width. 



Thus Lake Erie was at low stage during all the time of the Nipissing Great Lakes, and if, 

 as seems certain, the Fort Erie beach had been made previously (during the time of Lake Algon- 

 quin) with the waters at high stage, then the outlet, with the level of the lake at low stage, was 



1 Outline of history of the Great Lakes: Twelfth Rept. Michigan Acad. Sci., 1910, map on p. 27. 



- Moseley, E. L., Lake Erie enlarging; the islands separated from the mainland in recent times: Lakeside Magazine, Lakeside, Ohio, vol. 1, 

 1S98, pp. 14-17. Also Formation of Sandusky Bay and Cedar Point: Proc. Ohio State Acad. Sci., vol. 4, 1904, pt. 5, pp. 179-238. 



