QUATERNARY LAKES IN THE MISSISSIPPI BASIN 483 



increase in altitude up the Ohio. Thus, although the deposit along 

 each tributary and its branches is usually isolated and lies at a 

 different altitude from that on every other stream, the different 

 bodies have such a regular arrangement and have so many char- 

 acters in common that there can be little question but that they are 

 closely related, and they appear to be in large part lake deposits, 

 but in smaller part stream deposits, so that they may be referred 

 to as fluvio-lacustrine. 



Fig. i. — Lake Muddy, in southern Illinois. One of a series of lakes, now extinct, 

 caused by a rapidly growing valley filling on the Mississippi and certain other streams, 

 the filling forming a dam across the mouths of tributaries. The lakes stood at dif- 

 ferent altitudes, being controlled by the altitude of the Mississippi at their various 

 outlets; each was in a continual state of fluctuation, the position of its surface at any 

 moment being controlled by the stage of the Mississippi, and for a part of the time 

 each was intermittent. The narrow part of Lake Muddy near the outlet was in a 

 narrow, high-walled part of the valley, due to uplifted hard rocks. With the approach 

 of every flood on the Mississippi water gushed up through the narrow part of the lake 

 to the broader inland, a part carrying with it fine sand which, with interbedded lake 

 silt, formed a delta at the lower end of the lake, fronting toward the head of the lake. 



