20 TWELFTH REPORT. 



iug. The history is so complex that it can only be sketched in outline in 

 the brief time allotted for this address.^ 



It is primarily to glaciation that we owe tlie presence of these gTeat 

 lake basins, and a sketch of their histoiy naturally includes an outline 

 of the leading events of the Ice Age. Accompanying the glaciation there 

 have been deformations of the earths form that seem attributable in 

 some measure at least to the great ice sheets that covered the laud. 

 The region occupied by the Great Lakes appears to have been weighted 

 down and depressed during the height of glaciation, and to have risen 

 somewhat upon the melting of the ice, yet not to its preglacial altitude. 

 These earth movements are thus important factors in the history of ths 

 lakes, though it is not improbable that other forces independent of 

 the weighting and disappearance of the ice have been intiuential in 

 causing the changes of level here displayed. 



GREAT ALTITUDE PRECEDING THE ICE AGE. 



That the ice age was preceded by higher altitude than the present in 

 eastern Xorth America is shown by the presence of deep valleys, now 

 submerged, in the gulfs and bays along the Atlantic coast. 



From the Hudson valley northeastward the deep valleys are filled with 

 the oldest drift of that region. Borings in the Hudson river valley 

 near West Point, made by the Xew York Water Supply Commission, 

 show that the valley floor at that place is 450 feet or more below sea 

 level. Wells and borings in the vicinity of Boston show a rock floor 

 200 to 400 feet below sea level and there are similar deeply filled val- 

 leys on the coast of Maine. The coast line at that time was some dis- 

 tance farther out than now and much of the area of the Gulf of St. 

 LaAvrence was above sea level. The old valley of the St. Lawrence can 

 ]»e traced through the Gulf and out beyond the banks of NeAvfoundlaud. 

 The ice did not cover it far below Quebec and it remains unfilled. It is 

 SOO to 1200 feet below sea level, the depth increasing as one passes doAvn 

 the old channel. 



The high altitude, which is easily demonstrable on the coast, is more 

 difficult to prove in the interior of North America around the Great 

 Lakes, for the drainage from that region had a long course to the sea. 

 and the lakes are probably near the headwaters of the old drainage 

 lines. It seems entirely probable, however, that the high altitude of 

 eastern North America would have affected the Great Lakes region and 

 given the beds of the present lakes an altitude high enough to have 

 afforded free drainage to the sea. 



It is thought that before the glacial period there were no bodies of 

 water where the Great Lakes now stand, but instead broad lowlands 

 bordered bv belts of higher land. The condition seems likelv to have 

 been about like central and. eastern Tennessee and Kentucky and south- 

 ern Indiana, where the weak rock formations are marked by lowlands, 

 and the more resistant by highlands. 



^A much more complete discussion of the subject is now in preparation, for a mono- 

 graph of the United States Geological Survey, by mv associate in glacial investigations, Mr. 

 F. B. Taylor. 



