352 MODIFICATION OF GREAT LAKES BY EARTH MOVEMENT. 



triple lake is known to us chiefly through the labors of F. B. Taylor, 

 who has made extensive studies of its shore line. This line, called the 

 Nipissing shore line, is not wholly submerged, like the old shores of 

 Lakes Erie and Ontario, but lies chiefly above the present water surfaces. 

 It has been recognized at many points about Lake Superior and the 

 northern parts of Lakes Huron and Michigan, and measurements of its 

 height shows that its plane has a remarkably uniform dip, at 7 inches 

 per mile, in a south-southwest direction, or, more exactly, S. 27° W. 

 As will be seen by the accompanying map, reproduced from Taylor, it 

 crosses the modern shore line of Lake Superior near its western end, 

 thereby passing beneath the water surface, and it similarly passes 

 below the surface of Lake Michigan near Green Bay and below the 

 surface of Lake Huron just north of Saginaw Bay. The southward 

 tilting of the land, involving the uplift of the point of outlet, increased 

 the capacity of the basin and the volume of the lake, gradually carry- 

 ing the coast line southward in Lake Huron and Lake Michigan, until 

 finally it reached the low pass at Port Huron and the water overflowed 

 via the St. Clair and Detroit channels to Lake Erie. The outlet by 

 way of the Ottawa was then abandoned, and a continuance of the 

 uplift caused the water to slowly recede from its northern shores. 

 This change after a time separated Lake Superior from the other lakes, 

 bringing the St. Marys Biver into existence, and eventually the present 

 condition was reached. 



These various changes are so intimately related to the history of the 

 Niagara Biver that the Niagara time estimates, based on the erosion 

 of the gorge by the cataract, can be applied to them. Lake Erie has 

 existed approximately as long as the Niagara Biver, and its age should 

 probably be reckoned in tens of thousands or hundreds of thousands 

 of years. Lake Ontario is much younger. All that can be said of the 

 beginning of Great Lake Nipissing is that it came long after the begin- 

 ning of Lake Erie, but the date of its ending, through the transfer of 

 outlet from the Mattawa to the St. Clair, is more definitely known. 

 That event is estimated by Taylor to have occurred between five 

 thousand and ten thousand years ago. 1 



The lake history thus briefly sketched is characterized by a pro- 

 gressive change in the attitude of the land, the northern and north- 

 eastern portions of the region becoming higher, so as to turn the waters 

 more and more toward the southwest. The latest change, from Great 

 Lake Nipissing to Great Lakes Superior, Michigan, and Huron, 

 involving an uplift at the north of more than 100 feet, has taken place 

 within so short a period that we are naturally led to inquire whether 

 it has yet ceased. Is it not probable that the land is still rising at the 

 north and the lakes are still encroaching on their southern shores'? 

 J. W. Spencer, who has been an active explorer of the shore lines of 



'Studies in Indiana Geography, X. A short history of the Great Lakes. Terre 

 Haute, 1897. 



