128 N. H. WINOHELL GLACIAL LAKES OF iMINNESOTA 



It should be noted again that the western border of the lake Superior 

 ice-lobe, in the latitude of Saint Paul, crossed the Mississippi valley at 

 Saint Paul, deployed on the northern part of Dakota county, but barely 

 appears as such in Goodhue count3% and is not traceable in ^^^abasha nor 

 any of the valley counties farther southeast. If it ever existed in those 

 counties it has been buried by later modified drift, and it never filled 

 the valleys with till. But southward from Saint Paul, and especially 

 southward from Hastings, and also still more remarkably along the 

 Saint Croix valley northward from Hastings, gravel and sand terraces 

 are ver}" abundant. 



In order to explain these features, and also to account for the great 

 de})th and width of the gorge of the Mississippi, it may be supposed that 

 a long tongue of ice was continued southward in the Mississippi gorge 

 from the main ice-margin, and tlmt for a long time it subsisted after the 

 average limit of tlie ice had retreated farther north. Tliis tongue of ice 

 could be compared to those that occu{)ied the valleys of the finger lakes 

 of New York or the Cuyahoga valley in Ohio, or to the long slender 

 glaciers which still in northern latitudes fill the fiords which reach the 

 sea. The inevitable consequence of such a glacier in the Mississippi 

 gorge was to pond back the tributar}^ streams until the}^ were raised high 

 enough to tiow over the ice or along its margin. Such ponding formed 

 the lakes here called glacial lakes. 



It may be added, further, that this would have been the result for each 

 of the glacial epochs that have been identified in Minnesota, and that 

 the drift deposits of the different advances of the ice would be mingled 

 with those of the retreats in confused order, and that the drift of the 

 sei)arate epochs would overlap. Such features have been observed. 

 There are older and later gravel terraces, and each has its relation to the 

 flooded stages of the rivers, probably due to such cause. 



What agency such occupancy of the gorgeof theMississip})i by a narrow 

 glacier fed by the ice-fields farther north may have had in the production 

 of the greater lake which is sometimes su{)posed to have covered the 

 " driftless area,'' and hence of the loam which is spread everywhere in 

 that area, it might be interesting to inquire. Owing to the nature of the 

 supposed obstruction, such a lake could rarely, if ever, form a contin- 

 uous beach line. It must have mixed itsbeacli deposits with the earlier 

 till, or with the earlier loam, all about its edge. It must have retreated 

 slowly, and its effects as a lake would have been most marked in the 

 northern portion of its extension ; and finally It would have become 

 broken up and shrunken to the limits of the tributary gorges, and thus 

 would be at last wholly drained to the Mississippi as the ice-tongue in 

 recession o])ened one after the other of those valley's. 



