732 ABSTRACTS OF PAPERS 



SYSTEM OF QUATERNARY LAKES IN THE MISSISSIPPI BASIN 

 BY E. W. SHAW* 



(Abstract) 



In the northern part of the Mississippi basin there are certain thick bodies 

 of clay and certain physiographic features which indicate a great system of 

 extinct lakes extending from southern Wisconsin to eastern Kentucky. The 

 clay lies at low and concordant altitudes and occupies an aggregate area of 

 several thousand square miles. The physical character, horizontal attitude of 

 the surface, shore features and fossils show that the material is of lacustrine 

 origin, and it appears that the lakes were formed through the rapid develop- 

 ment of valley trains on the Mississippi and Ohio rivers in late Glacial time, 

 the debris damming the lower ends of tributary valleys. The lake deposits 

 are thus "valley fillings," ranging in thickness up to over 100 feet. In each 

 valley the surface of the fill is practically horizontal, but the altitude of the 

 surface varies from valley to valley. The height increases regularly from 

 Cairo up the Mississippi and from the same point up the Ohio. The height, 

 and hence the extent of the water in each lake, being controlled by the river, 

 fluctuated as the river rose and fell ; but the lakes served as reservoirs, so that 

 the range between high and low water was not so great as it would otherwise 

 have been. Shore features were generally poorly developed, but in places, as 

 near Madisonville, Kentucky, there are unmistakable beach ridges. 



Discussion 



Prof. R. D. Salisbury: Deposits of the sort referred to by. Mr. Shaw have 

 been known at various points along the Mississippi for many years, but they 

 have not been interpreted generally as certainly lacustrine. While some of 

 the deposits bear the marks of lacustrine origin, others bear the marks of a 

 sluggish fluviatile origin, and lacustrine and river phases of deposition alternate 

 frequently in the same valley. In some of the valleys with which I am famil- 

 iar, lacustrine, swamp and river conditions of deposition seem to have alter- 

 nated. Great caution is necessary, therefore, in classing deposits of the sorts 

 referred to as wholly lacustrine. On the other hand, the fact that conditions 

 of deposition changed from time to time in some valleys, is no proof that the 

 deposits referred to by Mr. Shaw are not strictly lacustrine. 



Mr. Shaw replied as follows : In reply to Professor Salisbury, I wish to say 

 that we do find both stream and lake deposits and every stage of gradation 

 between. I touched on this point but briefly in order to avoid confusion. The 

 Monongahela here at Pittsburgh and other streams — for example, a small one 

 in the extreme northwestern corner of Illinois, in the area of which Professor 

 Salisbury speaks — were not ponded, but were able to build up as rapidly as 

 the overloaded master streams. In these cases the deposits contain coarse 

 material, show irregular stratification, and the origin is not so clear, for some 

 streams with low gradients carry only very fine material, and when they 

 aggrade they form a deposit with an almost, though not quite, horizontal 

 upper surface. But the deposits described in the present paper seem to have 



Introduced by David White. 



