336 



lost to history- The ku'ger vaHeys generally reiimiu sufficiently unobscured 

 to enable geologists to trace their courses, either continuously or at inter- 

 vals close enough together to enable a safe inference to be made concerning 

 their previous courses. The larger the valley the better chajice it had in 

 general to leave behind itself traces of its former course, for, occupying the 

 lowest part of the surface and carrying great quantities of water, it was 

 automatically kept open by drainage from the melting ice. Yet even the 

 largest river trenches were in imminent danger of defacement. Such an 

 instance is found in Jay and Adams counties, Indiana, where there are 

 signs of a huge valley whose bottom is buried beneath nearly 400 feet of 

 drift and no traces left of its existence on the surface. Another case is 

 that of the preglacial Mississippi where it turns southeastward to the 

 Illinois valley n'ust below Clinton, Iowa. 



The map shows large hiatuses wherein there are no preglacial streams 

 indicated, but they certainly exist buried in several hundred feet of drift. 

 West and south of the basin of Lake Michigan and between that basin and 

 the Lake Erie depression in northern Indiana and Michigan no details are 

 shown, and only a few larger courses suggest probabilities of preglacial 

 existence. Q'hc depth of tlie drift and the absence of deep-seated natural 

 resources do not encourage the digging of a sufficiently large number of 

 deep wells to permit the construction of a topographic map of the preglacial 

 surface. Enough, however, is known to assure us that the ancient drainage 

 lines were quite different in many details from the present systems. 



Without further preliminaries we shall discuss the pros and cons re- 

 garding the claims of the streams shown on the maps to a preglacial an- 

 cestry. For the sake of convenience of treatment, the area is divided 

 according to the several smaller drainage basins which make up the 

 greater Mississippi basin. This will be found convenient because there 

 are wide elements of correspondence between the present and the pre- 

 glacial drainage basins, as a glance at the generalized map will show. The 

 basin of the Great Lakes, which seems to cut out a portion of the Mis- 

 sissippi basin, and which is separated by a very low primary divide, over 

 which the lakes drained in the Ice Age. is discussed briefly. 



THE PREGLACIAL DRAINAGE OF THE UPPER MISSISSIPPI 



BASIN. 



The preglacial divide of the northern side of the Upper Mississippi 

 basin is not definitely determined. It can. be pretty definitely located at 



