292 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1912. 



the second was the epoch of land elevation, causing increase of alti- 

 tude and inaugurating erosion — the epoch of emergence, and the 

 third was the epoch of erosion or valley making — the destructional 

 epoch. These three epochs 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 producmg emergence of land areas occurred while in much of 

 the region of the lakes farther east, sedimentation was still going on 

 uninterrupted. Whatever land was then raised above the sea was 

 attacked by the forces of erosion. Thus, to some extent, sedimen- 

 tation, elevation, and erosion were all going on at one and the same 

 time. But the successive dominance of the three processes distin- 

 guishes fau'ly well the three phases of development 



OUTLINE OF GEOLOGICAL HISTORY OF GREAT LAKES REGION. 



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, excepting, perhaps, part of the Ai'chean area south of Lake 

 Superior and some parts of the plateau north of the Great Lakes, 

 was under tlie sea. The rocks that filled these basins have very dif- 

 ferent characters in different beds. There are conglomerates and 

 sandstones, shales and limestones, and in some places igneous rocks. 

 Each one 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 upon the rate and manner of disin- 

 tegration under the forces of erosion. The formation of the Great 

 Lakes basins has thus been dependent to a large degree upon the 

 character of the strata out of which they have been excavated — upon 

 their relative hardness, thickness, and arrangement. 



This was the constructional period m which nature was getting 

 ready for the subsequent making of the lake basins. The basins 

 themselves, however, did not begin to be made until another great 

 event in geological history had taken place — not until a change 

 occurred in the relative attitude of the land and sea. Beginning at 

 the close of the Paleozoic era there came an epoch of great earth 

 movements affecting all of the eastern part of North America, includ- 

 ing the whole of tlic Great Lakes region. In consequence of this the 

 land now occupied by the lakes was lifted out of the sea to an altitude 

 estimated by some to be relatively 2,000 or 3,000 feet higher than its 

 present altitude. This Was the time of the uplifting and folding of 

 the Appalachian Mountains. This process probably occupied some 

 thousands of years, but in a geological sense it was a relatively short 

 time. 



