ECOLOGICAL CROSS SECTION OF THE MISSISSIPPI. 153 



tially filled. The alluvial valley of the Mississippi, near 

 St. Louis, varies in width from four to eight miles, and at 

 no point are its waters circumscribed by the approach of 

 limestone bluffs on either side, though at some places on the 

 Missouri shore the bluffs almost overhang the river. 



loess. 



during a period when it was covered by 

 loving, bodies of water, was overlaid by 

 ing after the glacial period, as shown by 



drift 



lowed by a general uplifting, after which it became subjected 

 to the influences which continue to the present day. The 

 plateau, where not protected by vegetation, was partially 

 denuded from loess, and this, with such soil as might have 

 been formed, carried down the slopes. Smaller streams, 

 finding their origin in precipitation or in local springs, further 

 served to furrow the upper plateau, 

 side evidences of such action are not s( 



While on the Missouri 



different where the Illinois section is concerned. Here 

 broad, fan-shaped slopes border the hills. This difference is 

 in part due to the geological formation and in part to the fact 

 that the Mississippi in later times hugged the Missouri shore. 

 The Missouri plateau lies at from 500 to 600 feet above 

 sea-level. It is of a rolling nature, numerous smaller valleys 

 intersecting the land. Here and there are found points of 

 greater elevation than that of the surrounding country, but 

 these arc usually of circumscribed area. The surface is partly 

 wooded, partly open and covered by herbaceous growth, 



maj 



limited 



extent and always surrounded by woodland, arc always 

 found where the surface is flat or nearly so. A feature com 



mon to the woodland and 



Sink- 

 diameter 



filled 



of from 10 to 50 yards, originally formed entranci 

 Frequently, however, they have become partly 

 sediment so that the entrances of the caves have been closed, 

 resulting in the formation of ponds (plate 12,1). James* 



* James, E. Account of an Expedition from Pittsburgh to the Rocky 

 Mountains. (Thwaites, R. G. Early Western Travels 1748-1846. 14 : 111. 

 Cleveland 1905.) 



