344 G. K. Gilbert on certain Glacial jyhejioynena. 



Marys contribatecl tlieir waters. Their mouths were morr 

 fifty feet higher than now, and tlie flood-plains of graves 

 sand, which they then formed, now flank their valleys uc^ U'l- 

 races, and can be traced for fofty miles toward their sourco^ 

 When, by the retiring of the lake, they were united, but slight 

 cause was needed to turn them eastward along the level bot- 

 tom of the deserted channel, and they have now cut their beds 

 so deeply in the drift, that the highest freshets do not connect 

 them with the Little river. 



In addition to its general interest, the fact of- this ancient. 

 southwesterly lake-discharge is an important element in the 

 study of the nature of the changes, in virtue of which the 

 lakes have stood at so many different levels since the ice- 

 period. The idea suggested by Dr. Newberry (in a paper betbr:- 

 the Boston Society of Natural History in 1862), that they may 

 have been due to local, rather than continental, upheaval an I 

 depression, receives here strong confirmation. At the point 

 where Lake Michigan once found outlet southward through the 

 Desplaines river,* the rocky barrier is but four feet above the 

 present level of that Lake, and seventeen feet above Lake Erie. 

 A barrier of drift may have existed one hundred feet higher. 

 At the Wabash outlet the rocky rim is 170 feet above Lake 

 Erie, and was probably covered by fifty feet of drift. And at 

 Lewiston, where the Niagara commenced its work, the eleva- 

 tion of the rim is 88 feet higher than the present Lakaf 



The order in which these channels were opened and deserteii. 

 is not yet known, but whatever sequence is supposed, it ' 

 equally evident that the changes that produced it must have 

 involved the tilting and warping of the land. At whatever 

 time the Wabash valley received the discharge, the barner>. 

 east and west, must have been relatively much higher than at 

 present. To restore now the old water level and current at 

 Fort Wayne, we would need, not merely to fill the gorge of the 

 Niagara, and renew the escarpment at Lewiston, but to con- 

 struct on that escarpment a retaining wall 170 feet high and 

 many miles in length ; and, after filling the valley of the Pes- 

 plaines to the height— one hundred feet— of the adjacent drilt 

 hills, another hundred feet would be needed to complete tlie 

 dam. Just what has been the warping of the basin to produce 

 tliis contortion of the rim, I am not prepared to say, but tne 



American Lakes considered as chronometers of post-glacial time" (Trans. Chic^ 



Acad. ScL, vol ii, p. 14); and the additional data here given in regard to n « 



kindly furnished me, by the siime gentleman, in a private letter. „ ^^ r, '^'^i 



+ Xat. Hist, of Xew York, Geology of Fourth District. By James Hall, p. ^- 



It is interesting to note, in passing, that the containing rock was m e^ ^^-j^ 



because there alone was an underlying softer rock exposed. 



