2i8 THU POPULAR SCIiJNCE MONTHLY, 



The broad channel now followed by the Minnesota River is 

 analogous to that across the Maumee-Wabash divide, although 

 the lake from which the river flowed to cut the channel has 

 shrunk away so far as to withdraw its waters beyond our north- 

 ern boundary. The former occupation of the Minnesota channel 

 by a large river was first pointed out by General G. K. Warren, 

 in the Annual Report of the Chief of Engineers, 1868, page 307 ; 

 and a fuller account of it was published in the American Journal 

 of Science for December, 1878. Warren looked to a northward 

 elevation of the land as a reason for the former southward direc- 

 tion of drainage, as Gilbert had done in Ohio ; and this view 

 generally obtained until the region was carefully studied out by 

 Warren Upham, whose reports are found in the annual volumes 

 of the Minnesota Geological Survey, and by whom a special ac- 

 count of the valley is given in the Proceedings of the American 

 Association for 1883. Here the name of river Warren is pro- 

 posed for the ancient stream by which this great trough was ex- 

 cavated : in earlier papers Upham had given the name of Agassiz 

 to the lake from which the river issued. Further account of the 

 ancient lake and river is found in the first two volumes of the 

 final report on the Geology of Minnesota, now in progress. 



The same observer has described a southwest overflow from 

 Lake Superior, when the greater part of its basin was presum- 

 ably occupied by retreating ice, and its waters rose about five 

 hundred feet above their present level. The overflow took place 

 across the pass between the valleys of the Bois Bruld and the St. 

 Croix Rivers in northwestern Wisconsin ; the channel across the 

 pass being about a thousand feet wide and nearly a hundred 

 feet deep, although its depth is now somewhat decreased by a 

 marshy filling from which the headwaters of the Bois Brul(? run 

 back to the present lake (Geological and Natural History Survey 

 of Minnesota, ii, 1888, p. 642). The lower course of the St. Croix 

 follows a " great valley," whose fuller history is deferred to later 

 volumes of the Minnesota Survey ; its lakelike expansion above 

 its confluence with the Mississippi is said to be due to alluvial 

 obstruction by tributaries (ibid., pp. 377, 643). These studies by 

 Upham in the Northwest were only continuations of the work 

 that he had begun in New Hampshire several years before, where 

 he recognized the shore lines of a small glacial lake in the south- 

 ern part of the north-sloping Contoocook Valley, with an overflow 

 to the southeast at Greenfield, N. H. (Geological Survey of New 

 Hampshire, iii, 1878, pp. 116-119). A few years ago, in company 

 with Mr. C. L. Whittle, I traced out a number of deltas and shore 

 lines on the slopes of this picturesque valley, but have not since 

 then been able to complete the attractive study of mapping and 

 restoring the old lake. 



