986 HISTORICAL GEOLOGY. 



would make all tlie barrier needed. The pitch of the land about Nelson 

 River is now eastward, and the rate about two feet a mile. 



Upham sets aside the idea of this change of level, and makes the lake and 

 the southward discharge the result of a damming by the ice-sheet along the 

 northeast border of the lake. For details of his observations and his view 

 of the events of the period, the reader is referred to his elaborate report 

 already mentioned. On the slopes leading down to James Bay, the southern 

 extremity of Hudson Bay, marine deposits occur up to a height of 450 feet 

 above the level of Hudson Bay, indicating that the Hudson Bay region finally 

 lost all its elevation, and became, further, much depressed. This is part of 

 the evidence presented by Upham to prove that the ice-dam was required. 

 But there is doubt whether the retreating ice would have long remained a 

 barrier under the warm climate of the Charaplain period. 



5. The region of the Great Lakes. — Lake Ontario, now 247 feet above the 

 sea, was in Champlain time at sea level, at the head of the long St. Lawrence 

 Bay, as already explained. But the northern and southern shore-lines are 

 widely different in height, owing to the warping of the surface in the later 

 reelevation. North of the middle of tiie lake, the height above the water 

 surface of the prominent shore-line or beach (called the Iroquois beach by 

 Spencer, who mapped its position) is 355 feet, while south it is 189 feet, 

 whence the increase in height northward is 166 feet in a distance of 60 

 miles. At the east end of the lake depression, the corresponding heights 

 are 483 feet at Watertown and 194 near Rome, an increase northward of 289 

 feet in 60 miles. The depth of the lake at the time was nearly 1000 feet — 

 equal to the present depth, 740 feet, plus the mean height of the opposite 

 shore-lines. (The positions of these upper shore-lines are given on the map.) 



Westward along the lake, the height of the upper shore-line decreases, 

 and at the west end, 200 miles distant from the east, it is only 116 feet — a 

 diminution from Watertown of 367 feet in 200 miles. 



Lake Erie is now 326 feet above Lake Ontario, or 573 feet above sea 

 level. The height of its upper shore-line south of the lake, at Cleveland, 

 is 213 feet; and that of the upper, north of it (the Ridgeway beach of 

 Spencer), is 273 to 351 feet. The heights increase eastward. The upper 

 at the west end, near Fort Wayne, is 207 feet, and toward the east end, 261 

 feet. The mean height of the upper line south of the lake is about 200 feet, 

 and the same is true as regards the southern shore-line of Lake Ontario. 

 The fact suggests the inference that the heights of the two lakes may have 

 had the same difference then as now. Through the subsidence the lake lost 

 its outlet to the Ohio — the Wabash River, which had served this purpose, 

 becoming a tributary to the lake. 



Lake Superior is now 602 feet above sea level, and Michigan and Huron, 

 582 feet. The latter lakes are but nine feet above Lake Erie. 



On Lake Superior, the upper shore-line of the north side has a height 

 above the lake at Josephine Mountain — 50 miles west of Thunder Bay — 

 according to A. C. Lawson, of 509 to 607 feet ; at Duluth, the west end, of 534 



