482 GEOLOGY. 



The shore lines of the Great Lakes have been similarly warped. Thus 

 the shore lines of Lake Iroquois, 1 the ancestor of Lake Ontario, decline 

 from the northeast to the southwest at the average rate of three and 

 a half feet per mile, the slope being steeper to the north and gentler 

 to the south. The old shore lines east of the east end of Lake Ontario, 

 are about 400 feet higher than those at the southwest end. The beaches 

 of Lake Algonquin 2 (Fig. 521) are 25 feet above the present lake 

 at Port Huron, and 635 feet above the lake at North Bay, Ontario. 

 The shore lines of the Michigan lobe of Lake Algonquin are 205 feet, 

 above the lake at Mackinac, and are estimated to be 100 feet below 

 the lake at Chicago. Similar figures might be cited for other localities. 



The shores of the Nipissing lakes (Fig. 522) show a similar 

 though lesser, deformation. Since the Nipissing lakes were later 

 than the preceding, their shore lines show that the deformation was 

 in progress while the ice was retreating. 3 The import of all these 

 data is the same, namely, that the land or the water surface has been 

 warped since the ice melted, and the change has been greatest toward 

 the centers of glaciation, and that it began before the lakes had attained 

 their present dimensions. A part of the change is undoubtedly due 

 to the effect of the attraction of the ice on the water. 4 This, how' 

 ever, leaves a large residuum to be otherwise explained. The history 

 of many small lakes affords data of the same sort. 5 



Along the Atlantic coast south of the area of glaciation there have 

 perhaps been complex movements, but of no great range, in the Pleis- 

 tocene period. On the whole, elevation (relative) appears to have 

 exceeded the depression, but the latest movement (present) appears 

 to have been one of depression, as the drowned ends of the valleys 

 between Long Island and Carolina, and numerous other minor 



1 Gilbert, 18th Ann. Rept., U. S. Geol. Surv. 



2 Taylor, A Short History of the Great Lakes, published in " Studies in Indiana 

 Geography "; also Am. Jour. Sci., Vol. LXIX (1895), pp. 69, 249. 



3 Other references relating to post-glacial deformation are the following: Spencer, 

 J. W., Am. Jour. Sci., Vol. XL (1890), p. 443; Vol. XLI (1891), p. 12; Vol. XLII 

 (1891), p. 201; DeGeer, Proc. Bos. Soc. Nat. Hist., Vol. XXV (1892); Upham, Jour. 

 G., Vol. II (1894), p. 383; Taylor, Am. Geol., Vol. XIII (1894), pp. 316 and 365; 

 Am. Jour. Sci. r Vol. XLIX (1895), pp. 69, 249; Bull. Am. Jour. Sci., ser. 4, Vol. I, 

 pp. 219, 228, 1896; Coleman, Bull. Geol. Soc. Am., Vol. X (1S98), p. 165 et seq.; 

 Fairchild, Bull. Geol. Soc. Am., Vol. X (1898), p. 27 et-seq. 



4 Woodward, Bull. 48, U. S. Geol. Surv. 



6 Lake Passaic, Geol. Surv. of N. J., 1893. 



