CHAPTER XII. 



OUTLINE OF GLACIAL AND POSTGLACIAL HISTORY OF THE GREAT LAKES 



REGION. 



By Frank B. Taylor. 



The Great Laurentian Lakes, or the Great Lakes, as they are commonly styled, are a group 

 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. 



PREGIACIAL HISTORY OF THE GREAT LAKES BASINS. 



STAGES OF DEVELOPMENT. 



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 basins were appar- 

 ently associated with the glacial epoch and are therefore of relatively recent date. It is the 

 later part of the Great Lakes history, comprising the glacial and postglacial stages, that has 

 most engaged the attention of students, 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 epoch and also a complex interglacial history. Only the main outlines of 

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

 briefly. 



The preglacial history of the Great Lakes is the geologic history of the region. For 

 convenience it may be divided into three stages, each dominantly though not exclusively char- 

 acterized by a particular phase of development. The first was the stage of sedimentation or 

 Paleozoic strata building — the constructional stage; the second was the epoch of land eleva- 

 tion, causing increase of altitude and starting erosion — -the stage of emergence ; and the third 

 was the stage of erosion or valley making — the destructional stage. These three stages are 

 not sharply and completely marked off from each other, although they may appear to be so in 

 some parts of the Great Lakes area. For example, in the northwestern part uplifts producing 

 emergence of land areas occurred while in much of the region of the lakes farther east sedi- 

 mentation was going on uninterruptedly. Whatever land was raised above the sea was attacked 

 by erosion. Thus, to some extent, sedimentation, elevation, and erosion were all going on at 

 one and the same time. But the successive dominance of the three processes distinguishes 

 fairly well the three phases of development. 



STAGE OF CONSTRUCTION. 



It is well known that the basins of the Great Lakes lie chiefly in depressions that were 

 formerly filled and completely occupied by Paleozoic strata. While these strata were being 

 laid down the whole region, except perhaps part of the Archean area south of Lake Superior 

 and some parts of the plateau north of the Great Lakes, was under the sea. The rocks that 

 filled these basins have very different characters in different beds. There are conglomerates 

 and sandstones, shales and limestones, and in some jilaces igneous rocks. Each of these classes 

 of rocks has many varieties with more or less variation in hardness and chemical properties, 

 and these qualities exercised an important influence on the rate and manner of disintegration 

 under the forces of erosion. The formation of the Great Lakes basins has thus been dependent 

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