QUATERNARY LAKES IN THE MISSISSIPPI BASIN 489 



more or less concave. For the resulting topographic feature, 

 the bottom of Muddy River Valley may be taken as a type and 

 Muddy may be an acceptable name for it, referring, as the name 

 does, both to a particular type and to a principal character of 

 the deposit, and the streams which flow over it, and also to the 

 general character of the country where the feature is developed. 

 On the other hand, in case aggradation keeps pace with the 

 growth of the dam the material is in general coarser and the upper 

 surface rises up stream, though at a less rate than the original 

 stream channel. For this topographic feature the surface of the 

 deposit forming a low terrace along Big Sandy River in eastern 

 Kentucky may be taken as a type and called a Sandy. Perhaps 

 also it will be found desirable to speak of the island-like hills sur- 

 rounded by the deposit as Island Hills, and the hill bearing the 

 town of Island, in Kentucky, may be taken as a type. 



Summarizing. — The inferred history of the lake deposit reads 

 about as follows: In middle or late glacial time the rivers were 

 flowing on beds about 100 feet below their present ones. Whether 

 this great depth was attained in an interglacial epoch by a regional 

 uplift or was reached through the deep scouring of glacial floods 

 has not yet been determined. The tributaries entered the flood- 

 plains of the Mississippi and Ohio on channel bottoms only about 

 40 feet lower than those in use today and their flood-plains were 

 near the position of their present channel bottoms, these positions 

 being controlled by low- and high-water stages on the master 

 streams. As at present, at low-water stage there was no standing 

 water in the tributaries, but at high water the deep channels were 

 filled by back water from the rivers, thus forming long, narrow 

 winding lakes. When aggradation began on the Mississippi and 

 Ohio, both low- and high-water marks on them and on the tribu- 

 taries rose. At low water there were embryo, perennial lakes in the 

 channels of the tributaries at their mouths and at high water the 

 flood-plains were covered more deeply than before. The area 

 covered both at low- and high-water stages gradually extended until 

 low- water stage reached the altitude of the former flood-plain. 

 From this time on there were perennial bodies of quiet water of 

 considerable size on each tributary, and wedge-shaped masses of 



