THE GLACIAL AND POSTGLACIAL LAKES OF THE GREAT 



LAKES REGION.! 



By Frank B. Taylor. 



GENERAL STATEMENT. 



The Great Laurentian Lakes, or the Great Lakes, as they are com- 

 monly styled, are a gi'oup of valleys which have been turned into 

 lakes. Geologically speaking, the lakes themselves are new and 

 youthful forms, although the valleys in which they lie are much older. 



The basins of the Great Lakes were once valleys with free drainage 

 and no lakes, like the Ohio Valley of to-day. The events which 

 changed them into water-filled basiiis were apparently associated 

 with the glacial period, and are therefore of relatively recent date. 

 It is the later part of the Great Lakes history, comprising the glacial 

 and postglacial epochs, that has engaged the attention of students 

 most, because the facts relating to that part are the newest and most 

 numerous. But in any comprehensive view, the fact should not be 

 overlooked that the Great Lakes, or rather the basins in which they 

 lie, had a long and complicated history before the glacial period and 

 also a complex interglacial history. Only the main outlines of the 

 earlier epochs are known at the present time, and it will suffice to 

 enumerate tlu^in liere briefly. 



PREGLACIAL HISTORY OF THE VALLEYS OF THE GREAT LAKES. 

 PHASES OF DEVELOPMENT. 



Tlie preglacial history of the Gre<it Jjakes is simply the geological 

 history of the region. For convenience it may be divided into three 

 epochs, each one dominantly, though not exclusively, characterized 

 by a particular phase of development. The firet was the epoch of 

 sedimentation or Paleozoic strata building — the constructional epoch ; 



» Published by permission of Director of the U. S. Geological Survey. This paper is au advance publi- 

 cation of Chapter XII of a monograph on Pleistocene deposits and glacial lakes of Indiana and Michigan, 

 Iiy Frank Lovorett and Fnuik B. Taylor, U. S. Gcol. Surv. Mon., vol. — , in press. This was not orig- 

 inally intended for separate publication, and inasmuch as it is merely an outline of the lake history, full 

 and specific credit is not given here for tlic work of each of the several geologists who have materially con- 

 tributed to the elucidation of this complex history. Such due acknowladgment will l^e found included 

 with the complete discussion of the phenomena in the monograi)h. 



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