BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 VOL. 24, PP. 229-232 JUNE 7, 1913 



RELATIONSHIP OF THE GREAT LAKE BASINS TO THE 

 NIAGARA LIMESTONE 1 



BY J. W. SPENCER 



{Presented before the Society in abstract December SI, 1912) 



CONTENTS 



Page 



Niagara limestone barriers to the lakes 229 



Concerning the preglacial Erie outlet " 231 



Huron and Georgia outlets 231 



Concerning the outlet of Lake Michigan 232 



Outlet of Lake Superior 232 



Ontario-Saint Lawrence Valley 232 



Results '. 232 



Niagara Limestone Barriers to the Lakes 



The dependence of diverse topographic forms on geological structure 

 is nowhere better illustrated than in our Great Lakes. The valleys are 

 excavated in shales, soft and impure limestones and sandstones, with 

 their development retarded by the occurrence of overlying beds of hard 

 rock. Such strata, consisting of the Niagara limestone, have given rise 

 to the separation of the basins of five of the Great Lakes, that of Geor- 

 gian Bay being distinct from the basin of Lake Huron. This feature is 

 true, although the lakes trend in every direction, as is shown on the ac- 

 companying map. 



Underlying the superficial drift the Niagara limestone forms the rock- 

 floor of the country for a breadth of 5 or 6 miles or perhaps even 20 

 miles. This formation constitutes a barrier to Lake Erie, separates 

 Georgian Bay from Lake Huron, and Green Bay from Lake Michigan, 

 and finally forms a wall between the southeastern part of Lake Superior 

 and lakes Michigan and Huron. 



In the evolution of the valleys of the lakes the less durable strata were 

 removed from a broad extent of country, but the drainage from each of 

 these was forced to cross the hard barriers, which thus gave rise to their 

 relatively narrow outlet. At the same time this zone of durable lime- 

 stone became a more and more prominenl tableland as the valleys wore 

 being more easily deepened. But the rains and the small streams grad- 



1 Manuscript received by the Secretarj of the Society March :;. L918. 



(229) 



