THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS xliii 



In the Morris basin the shale bottom has been eroded in places 

 by the current and the hollows have been filled with sand, but from 

 the Morris basin to the bend of the river the rock floor is swept 

 clean. 



The old preglacial valley through which the lower Illinois flows, 

 and where rock bed lies many feet below the bottom of the present 

 river, seems to have been so imperfectly filled by glacial deposits 

 that throughout nearly its entire length the stream is re-established 

 in its old course. The valley ranges in width from two and a half to 

 fully fifteen miles. Its greatest width is reached just above the 

 mouth of the Sangamon. The valley is also very broad at the bend 

 of the Illinois. The narrowest portions are a short section near 

 Peoria, where it passes through the Shelbyville moraine, and a sec- 

 tion embracing the lower 60 miles, where it traverses the subcar- 

 boniferous and the Silurian limestones. 



The Illinois River bottom-lands are covered with patches of tim- 

 ber, sand banks, mud-flats, and meadows. A good deal of this area 

 is too low and marshy for cultivation, full of swamps, bogs, bayous, 

 and lagoons, many of the latter being parts of old channels of the 

 stream which have been cut off and filled up at both ends as a conse- 

 quence of local changes in the course of the stream ; but w^here the 

 elevation is sufficient the soil is a rich sandy loam. An example of 

 this is found in the "Crow" Meadows" in Marshall county. This tract 

 of land is a broad table-land or second bottom extending from the 

 north line of the county down to Sparland, widening near Henry to 

 eight or nine miles between the river and the low bluff-line on the 

 west. It is beyond the reach of inundations, and is of unsurpassed 

 fertility, although it contains much sand. The bluffs rise on each 

 side of the bottom-lands very abruptly in most places, and to a 

 height reaching at times 125 to 150 feet, cut into sharp ridges by the 

 valleys of the small streams that drain the adjacent regions. They 

 are all thickly timbered. 



The current of the Illinois from La Salle to its mouth is not suffi- 

 cient to carry off the material brought in from the upper portion of 

 the stream, and therefore it is in the process of silting up. During 

 the interglacial period when the land-slope was much less, this part 

 of the river became so filled that now the rock bottom lies about 100 

 feet below the present bed of the river. 



The principal tributaries of the Illinois are the Fox, Vermilion, 

 Mackinaw, Sangamon, and Spoon rivers, and Macoupin, Crooked, 

 and Apple creeks. 



