ECOLOGICAL CROSS SECTION OF THE MISSISSIPPI. 



151 



suiting 



limestone 



influence of vegetation and the elements, and partly colluvial, 



amounts of loess being: carried 



Not everywhere 



same 



be shown later. From StoUe southward, the soil partakes 

 far less of the loess character than it does immediately north 

 of this. This is intimately associated with the physiography 

 of the region and the interrelation existing between the 

 course of the river in preglacial times and the position of the 

 Pcnnsylvanian series. 

 The plateau back of the bluff is covered with a thick layer 



much lime 



advanta 



colored 



Hilgard.f 

 Worthcn,]: 



Illinois, gives 



upland 



"The Quaternary deposits, which include the alluvium, 

 loess and drift, are of the greatest economical importance, 

 because the soil is everywhere based upon one or other of 

 these subdivisions, and owes its peculiar features to the 



and from 



organic matters have been derived 

 of the pulverized remains of pre 



ist mamly 

 and their 



lity depends as much upon the mechanical 

 material as upon its chemical compositioi 



The con- 



ditions under which the drift clays were accumulated were 

 such as to reduce the various rocks from which the material 

 was derived to the condition of a fine sediment that was 

 deposited at the end of that period in the form of a finely 

 pulverized clay, intermingled with silica in the form of sand, 

 magnesia and Hme, in such proportions as to form a soil of in- 

 exhaustible fertility, under a judicious system of cultivation. 

 To these mineral ingredients have since been added the or- 

 ganic matter derived from the successive growths of animal 

 and vegetable life, that have from year to year matured, and 

 decayed upon the surface, all of which have entered into and 



1^ ■ * 



* Bowman, ibid. 53. 



t Soils. 285. 



i Worthen, A. H. Geol. Surv. lUinois. 1 : 315. 1866. 



