108 THE MICHIGAN ACADEMY OF SCIENCE. 



ORIGIN OF THE GREAT LAKES. 



The origin of the basins or depressions tliat contain the Great Lakes 

 of North America is a matter upon which much has been written. It 

 is generally thought that these basins did not hold great bodies of fresh 

 water in preglacial times, and it is considered donbtful if any of them 

 were occupied by salt water connected by a strait with the ocean. On 

 the contrary it is commonly snj)])Osed that before the Glacial epoch large 

 river valleys were discharging through the tracts now covered by the 

 Great Lakes either southward to the Gulf of Mexico or eastward to the 

 Gulf of St. Lawrence. A difficulty in the acceptance of this interpre- 

 tation is found in the very low altitude reached by some of the lake 

 basins. The Lake Superior basin drops in its deepest part to 400 feet 

 below sea level, the Lake Michigan basin to nearly 300 feet, the Lake 

 Huron basin to 150 feet, and the Lake Ontario basin to nearly 500 feet. 

 Evidently river drainage alone did not give these l)asins their present 

 low altitude, and supplementary ageni^ies must be called in. It is 

 thought by most geologists who have given attention to this subject that 

 the low altitude was produced in part by a removal of material by the 

 ice fields which are found to have utilized these basins as lines of axial 

 movement in one or more of the great invasions. The large amount of 

 drift carried from each basin into the bordering districts certainly 

 strongly supports this vicAV. It is held by several glacialists that the 

 Great Lakes region and country to the north stood much higher before 

 the glacial epoch than since, and some hold that the weight of the ice 

 sheet caused much of the depression. Gilbert's studies indicate that the 

 region is now working toward a higher elevation, and if the uplift is 

 continued long enough it will bring the beds of the Great Lakes above 

 sea level. The explanation of the very low altitude of parts of the basins 

 of the Great Lakes seems likely, therefore, to be found in a combination 

 of glacial erosion and a depression of the region either by the weight of 

 the ice or as a result of internal stresses. It should be borne in mind 

 that the contours or shapes of the basins of the Great Lakes have been 

 greatly modified by drift accumulations so that they bear but little 

 resemblance to a river valley. 



The tributary preglacial drainage lines can in some cases be located 

 by means of the deep borings and traced toward the basins of the Great 

 Lakes through tracts where there is no surface indication of their 

 presence. By means of these borings Dr. A. C. Lane, the State Geolo- 

 gist, has reached the conclusion that much of the drainage of the south- 

 ern penisula of Michigan led westward into the Lake Michigan basin. The 

 preglacial outlet from that basin toward the ^Mississippi has not yet 

 been discovered. 



BIBLIOGRAPHY. 



The following list of reports and papers is designed to include such 

 as add materially to the knowledge of the Pleistocene features and de- 

 posits of Michigan. To list every publication that touches upon these 

 features and deposits, to say nothing of outlining the scope of each 

 paper, would involve more space than this review can take. The litera- 

 ture of the Great Lakes alone includes more than 200 titles. * 



