22 TWELFTH REPORT. 



Whether Spencer's interpretation is correct on the main proposi- 

 tion of an eastward drainage to the Gulf of St. Lawrence remains to be 

 determined. It will be necessary to learn how much the level of the 

 old rock beds have been reduced by ice weighting and differential depres- 

 sion and by ice erosion. The beds certainly have been sufficiently loAvered 

 to render it impossible to run the glacial drainage to the ocean over 

 a rock floor sloping continually seaward. The amount of erosion and 

 depression seems to decrease southAvestward or toward the peripheral 

 portion of the ice sheet. It will not be surprising, therefore, if the 

 proi>er evaluation of these factors will throw the balance of probabilities 

 in favor of preglacial discharge of much of the Great Lakes region to the 

 Gulf of Mexico. 



The data thus far collected in central Michigan favor this interpre- 

 tation. The borings along the line of the buried channel that runs 

 across Michigan from Saginaw Bay to Lake Michigan indicate, not only a 

 fall in that direction (which is the reverse of the course outlined by 

 Spencer), but also a widening of the channel. Deep borings in western 

 Indiana between the head of Lake Michigan and the Wabash show a 

 lower rock floor than any yet discovered across Illinois and suggest a 

 preglacial drainage to the Wabash and Ohio. 



EFFECTS OF GLACIATIOX IX GREAT LAKES REGION. 



The Great Lakes region has been modified by glaciation in various 

 ways, chief among which are (1) drift deposition, (2) glacial erosion, 

 and (3) probably weighting and depression by the ice. On the first of 

 these modifications, drift deposition, there is oi)portunity for definite 

 knowledge, and no differences of opinion are held by geologists. On the 

 second and third there are debatable questions, and in consequence a 

 lack of uniformity of opinion. 



EFFECT OF DRIFT DEPOSITION. 



The drift has so completely filled the valleys which connected the 

 several basins that the ])Osition of these valleys is known only through 

 data from borings. Their beds lie far beloAv the present surface of the 

 lakes. Consequently were the drift removed from these channels the 

 lakes would stand much lower than at present. The basin of Lake Erie, 

 it should be noted, is rendered shallow by a thick accumulation of drift. 

 The greatest depth of water is about 210 feet, but borings at Cleveland, 

 near the mouth of Cuyahoga River, show the drift to extend down to. 

 within lUO feet of sea level, or more than 470 feet below lake level. 



The present divide between the Great Lakes or St. Lawrence drainage 

 and the Mississippi drainage is determined very largely by moraines and 

 thick drift deposits. The drift has been heaped up in greatest amount 

 at the ends and along Ihe sides of the tongues of ice that passed over 

 the lowlands now occu])ied by the Great Lakes. There is also in the 

 southern jieninsula of ^Michigan a remarkable amount of drift, perhaps 

 a thicker deposit than in any other area of equal size in America. This 

 apparently resulted from the convergence of ice movement occasioned 

 by the trend of the bordering Great Lakes basins, there being southward 

 movement on the west side and southAvestward on the east. The northern 



