MAP COMPARISONS 253 



as soon as the weight of the ice-cap was effectively diminished at the cen- 

 ter of snow accumulation. 



Comparison of Maps 



The reader interested in this study should compare the published maps 

 of Pleistocene deformation of the Great Lakes area. The earliest was by 

 De Geer, in number 67 of the bibliographic list, and reproduced by Salis- 

 bury in his Glacial Geology of New Jersey (97, page 201). Coleman 

 did not give a map of isobases. Goldthwait has published several maps, 

 in number 108, pages 465-469 ; 109, plate 5, facing page 233. In this 

 study the papers by Coleman and Goldthwait should be specially con- 

 sidered. 



If De Geer's zero and 200-feet isobases had been drawn parallel with 

 his 400-feet line, they would all represent fairly the deformation west of 

 the 75th meridian. The lines curve too sharply at this meridian to rep- 

 resent facts as now interpreted. 



Goldthwait' s map (108, page 465) places the hinge line of the early 

 glacial lakes across the Lake Erie basin precisely as in our map ; and his 

 latest map (109, plate 5) is in general accordance for the Ontario basin 

 and Canada, when it is understood that his isobases represent only the 

 deformation of the later beaches, taking a line through Hamilton, On- 

 tario, as the hinge line. The accompanying table shows that Hamilton 

 was lifted 98 feet before the beaches were tilted at all ; so that 100 feet 

 is to be added to his isobases to make them accord with the present map. 



In our small map, plate 12, the isobases of the large New York map 

 have been extended, with the same curvature, across New England on the 

 east and across the Michigan-Huron area on the west. In broken lines 

 are inserted, for comparison, three of Goldthwaifs isobases, from his 

 figure 1, in title 108. "A" is his hinge line for lakes Chicago, Maumee, 

 and Whittlesey, which joins our zero line. "B" is his hinge line for the 

 deformed Algonquin beach, which cuts obliquely our 100-feet isobase. 

 "C" is his isobase of 835-feet altitude of the Algonquin plane, or 239 

 feet of tilting, which lies over our 400-feet isobase. The Algonquin hinge 

 line admittedly had previously received some uplift. If that was as much 

 as 100 feet, it would practically harmonize the maps. 



Somewhere, both east and west, the isobases will curve decidedly, to 

 surround the northern center of uplift. Over New England the present 

 lines indicate the maximum of uplift, as any northward bending will 

 reduce the amount of deformation. If the northward convex curvature 

 of Goldthwaifs lines is correct, then our New York isobases can not 

 bend much to the north and still connect with his. The northward con- 



