THE ANCIENT OUTLET OF LAKE MICHIGAN. 219 



Upham's observations on the shore lines of Lake Agassiz have 

 fully demonstrated that in northern Minnesota and Dakota, and 

 in Manitoba farther north, the land was depressed, not elevated, 

 at the time of the overflow of river Warren from Lake Agassiz ; 

 and hence, there as in the St. Lawrence Valley, the cause of over- 

 flow must be looked for in the retreating front of the Pleistocene 

 ice sheet. In the Bulletin of the Geological Society of America 

 (vol. ii, 1891, pp. 243-276) Upham has described a number of glacial 

 lakes associated with large river channels, north of our boundary 

 in Canada. The channels are now deserted by the great streams 

 that carved them, and are occupied only by smaller streams, 

 which are frequently " laked " by the alluvial deposits brought in 

 by lateral tributaries, as will be referred to again further on. 



In 1885 Gilbert traced out the shore lines of the expanded 

 waters of Lake Ontario, afterward named Lake Iroquois by 

 Spencer, and showed that they converged to the southeast, and at 

 Rome, N. Y., an outlet was found through what is now called the 

 valley of the Mohawk. Only a brief mention of the attractive 

 problem offered by this locality has yet been published. Spencer 

 has called attention to the probable former discharge of Lake 

 Huron and Georgian Bay across the province of Ontario by way 

 of the river Trent (Proceedings of American Association, xxxvii, 

 1888, p. 198) ; and Gilbert has suggested that at an earlier stage 

 there was another outlet farther north, by way of Lake Nipissing 

 and the Ottawa River the account of this being found in his ex- 

 cellent History of the Niagara River, published by the Commis- 

 sioners of the State Reservation at Niagara, in their sixth annual 

 report. The reading of this history will greatly increase the pleas- 

 ure of an intelligent visit to the great cataract. It was of the 

 outlet by way of the Ottawa River that Wright gave an account 

 in the New York Nation for September 22, 1892. 



The abandoned channel of overflow of the ancient Lake Bonne- 

 ville at Red Rock Pass in northern Utah, and the "old river bed" 

 leading from Sevier Desert to Great Salt Lake, well known from 

 Gilbert's monograph, are analogous to the old channels here con- 

 sidered, although the overflows there were not produced by gla- 

 cial barriers. 



All these abandoned channels have certain features in com- 

 mon. At their upper end, where they trench across a divide of 

 greater or less distinctness, they open out upon lacustrine plains 

 of greater or less extent and distinctness, whose converging shore 

 lines may be traced to the point of discharge. The breadth of the 

 abandoned channel is relatively constant throughout a great part 

 of its length ; from which we may infer that the volume of water 

 received from the lake at its head was large in comparison with 

 that received from the tributaries lower down in its course. None 



