464 THE ILLINOIS GLACIAL LOBE. 



consist of a series of rock barriers, called " chains," separated by pools or 

 stretches of slack water. The slope also is greatest in the lower portion, 

 about half the fall occurring in the last 5 miles. The narrowness of the 

 valley in this new course is well shown in PI. XVIII. It averages scarcely 

 1 mile. 



The uplands immediately bordering the narrow portion of the Missis- 

 sippi Valley stand 100 to 120 feet above the stream. They carry a capping 

 of loess 30 or 40 feet in average depth, beneath which there is a sheet of 

 drift ranging in depth from a mere trace to 75 feet or more. In the portion 

 below the bend, just south of Leclaire, the drift extends down about to the 

 level of the present river bed, but from Leclaire northward the rock rises 

 50 to 75 feet above the river. The rock cutting accomplished by the river 

 is even less than these figures would indicate, for there appears to have 

 been a small preglacial stream leading northward from the vicinity of 

 Leclaire, whose valley may have been widened but little by the river, and 

 whose rock floor extends below the level of the present stream. The rock 

 excavation is, therefore, confined mainly to a short section 2 or 3 miles in 

 length, in the vicinity of Leclaire, and even here the cutting has reached a 

 depth of scarcely 75 feet. 



One of the abandoned drainage lines above referred to, known as 

 "Cattail .Slough," leads southeastward from a point in the Mississippi Valley 

 opposite Clinton, as shown in PI. XVIII, to join Rock River Valley at Erie, 

 Illinois. Another, Meredosia Slough, leaves the Mississippi opposite the 

 mouth of the AVapsipinicon River, a few miles below Clinton, and leads 

 southeastward to Rock River Valley below Erie. Each of these channels 

 is so low that it may be utilized either by Rock River or by the Mississippi 

 at flood stages of those rivers. From Erie (see PI. XVIII) two channels 

 lead westward to the Mississippi, one along the present course of Rock 

 River, the other along Pleasant Valley, which is nearly parallel with it on 

 the north. 



The distance along the present course of the Mississippi from the north- 

 west end of Meredosia Slough to the point where Pleasant Valley connects 

 with the river is only 12 miles, while the course by way of the slough and 

 valley is fully twice that distance. The course by way of Cattail Slough 

 and Rock River is also fully double that by the present Mississippi. The 

 narrowness of the valley of the present stream from Cordova to Hampton, 



