DEPARTURE OF ICE-SHEET FROM THE LAURENTIAN LAKES. 25 



low land of the eastern part of the INIichijjjan upper peninsula, the Western Su])e- 

 rior waters fell 50 feet or more below their former outlet to the Saint Croix and 

 ISIississippi rivers, and thenceforward the outlet of lake AVarren past Cliioago to 

 the Mississippi, by way of the Des Plaines and Illinois rivers, carried away the 

 drainage from the glacial melting and rainfall of the lake Superior basin. 



Lake Wakren 



At a time that was probably somewhat later than the end of the Western Supe- 

 rior lake, its analogue, the Western Erie glacial lake, which had outflowed past 

 Fort AVayne, Indiana, to the AVabash, Ohio and ^Mississippi rivers, became likewise 

 lowered and merged in lake AA^'arren, which in its soon ensuing maximum stage 

 stretched from the south end of lake Michigan to the north side of lake Superior, 

 northeast to lake Xipissing, and eastward to the east end of lake Erie and the 

 southwestern limits of the lake Ontario basin. The river outflowing from lake 

 AVarren probably cut down its channel 50 feet or more into drift which had been 

 deposited in the rock valley of the Des Plaines river in the vicinity of AVillow 

 Springs and Lemont, between 15 and 30 miles southwest of Chicago. The mouth 

 of the lake was thus reduced in height and transferred upstream, until, at the end 

 of the duration of this outflow and of the glacial lake AVarren, the drift-covered 

 divide in the old channel, situated near Summit and tlie elbow of the Des Plaines, 

 about 10 miles southwest from the shore of lake ^Michigan at Chicago,'^ was only 

 seven or eight feet above the present mean lake level. AVhile the outlet continued 

 here, all the northern part of the area of lake AVarren, extending about 600 miles 

 from Duluth to lake Nipissing, was uplifted about 350 to 400 feet. 



Lake Algonquin 



AA^'hen the glacial melting and retreat at length j^ermitted an outflow from the 

 Saint Lawrence basin over a lower pass, which was through central New York to 

 the Mohawk and Hudson, the water surftice in the basins of lakes ^Michigan, Huron 

 and Superior fell only some 50 or 75 feet from the latest and lowest stage of lake 

 AA''arren to its short-lived successor, lake Algonquin. This lake was ice-dammed 

 only at low places on its east end, as at or near the heads of the Trent and Mattawa 

 rivers, lying reipectively east of lakes Simcoe and Nipissing, wdiere otherwise its 

 waters must have been somewhat further lowered to outflow by those i)asses. A 

 careful study of tlie late glacial or Champlain epeirogenic uplifting of all portions 

 of the Saint Lawrence drainage area, as known })y the present inclinations of its 

 many shorelines, convinces me tliat CJilbert f and AA^'right J have overestimated the 

 imj)ortance of the outflow, if any such took place, from lake Algonquin past the 

 present lake Nipissing to the Mattawa and Ottawa rivers. Professor Spencer's 

 Algonquin beach is very clearly the Nipissing beach of Mr Taylor, and this earliets 



♦For information concerning this locality, and for a map and profiles of the canal now beinR 

 constructed past it with continuous descent away from tlio lake, I am indebted to Mr Ossian 

 Guthrie, of Chicago, who has bestowed much study on the glacial drift and Pleistocene history of 

 that region. 



t" History of the Niagara River," Sixth An. Rep. of the Commissicners of the .State Reservation 

 at Niagara, fur the year I88(i, pp. 01-84, with eight plates (also in tiie SmithscHiian Annual R<-i)ort 

 for IHiXJ). 



X Bulletin Geol. Society of America, vol. iv, 189."., ■is.'.-l.tl . 



IV— Bull. GKor,. Soc. Am., Vol. r,, isoi. 



