232 j. w. spencer — niagara limestone 



Concerning the Outlet of Lake Michigan 



The features of the outlet of this lake are similar to those of Lake 

 Huron, just mentioned, except that some higher Silurian strata overlie 

 the Niagara limestones, all of which have been penetrated by the chan- 

 nel, which has also been partly rilled with drift (D on map). The depth 

 of the buried valley is further indicated by borings just to the south (at 

 Cheboygan), which reach to more than a hundred feet below the drift- 

 covered floor of the straits. The submerged canyon of Grand Traverse 

 Bay, to the southwest, shows still better the depth of the preglacial 

 channels, to over 600 feet below lake level. Green Bay came very nearly 

 being a separate lake, due to the same geological formations. 



Outlet of Lake Superior 



The same belts of hard rocks separate Lake Superior from lakes Michi- 

 gan and Huron. These are penetrated by a channel, some two miles 

 wide, between Lake Superior and Lake Huron, as shown at E on the 

 map (Leverett). Thus it is found that even this lake owes part of its 

 configuration to the Niagara formation. 



Ontario-Saint Lawrence Valley 



The Saint Lawrence Valley is only a continuation of that of Lake 

 Ontario, where the southern highlands bounding the ancient broadened 

 channel still remains, although largely composed of shaly and soft strata. 

 Along one section of the waterway these strata were removed, exposing 

 for a few miles the underlying crystalline rocks. Among them, deep 

 channels had early been produced. These drift-filled and do not entirely 

 form the barrier to the lake, which is otherwise composed of the Calcifer- 

 ous formation, beginning only 66 miles below its outlet. Even here other 

 buried channels occur, so that the rocks which give rise to the rapids of 

 the Saint Lawrence are only spurs between the ancient courses of the 

 streams, now largely concealed beneath drift. 



Eesults 



Thus it is shown that Superior, Michigan, Huron, and Erie, with their 

 diverse configuration, owe the existence of their separate basins largely 

 to the distribution of the Niagara limestone underlaid by softer strata. 

 Georgia and Ontario are due to the softer Silurian and older rock forma- 

 tions without the presence of the hardest layers, so that their outlets are 

 characterized by broader valleys which are also of greater antiquity. 

 These observations relate to the valleys of the lakes and not simply to 

 their basins, which result from the obstruction of the ancient outlets by 

 the accumulation of drift and by the postglacial deformation due to 

 earth-movements. 



