\ 



ECOLOGICAL CROSS SECTION OF THE MISSISSIPPI. 143 



Stollc, but thins out towards the east. Worthen * deems it 

 probable *Hhat it was accumulated when the present valley 

 of the river was occupied by a chain of lakes, the waters 

 of which filled it nearly to the height of the adjacent highlands^ 

 and that it was originally spread over the whole surface of 

 the valley. A considerable portion of it was no doubt re- 

 moved during the drainage of the valley by the gradual ele- 

 vation of the surface and other portions have been subse- 

 quently removed by the action of river currents.^' Interest- - 

 ing in this connection is the opinion expressed by Ilersliey.f 

 3. Bouldery Drift. — The drift originally covered the high- 

 lands to a depth of 10 to 50 feet and was accumulated while 

 the whole surface was under water. It consists mainly of 

 yellow and brow^n clays intermingled with gravel, with here 

 and there an occasional boulder. Till, a blue, plastic clay, 

 ordinarily underlies the deposit, and betw^cen the blue and 

 the yellow clays there is sometimes a stratum of sandy 



material, a few feet in thickness. 



While ordinarily the loess is bedded on the drift and the 

 latter overlies the stratified rock, it sometimes occurs that the 

 drift apparently has been removed by currents previous to 

 the deposition of the loess, in which case the latter lies 

 immediately upon the rock. 



■ 



Carboniferous System. 



This, both in Madison and St. Clair counties, comprises the 

 lower portion of the coal measures and the sub-carboniferous 

 limestones. There is here a dip of the limestone which, dis- 

 appearing just below Alton, reappears about Pittsburg, 111. 

 These coal measures have a thickness of 300 feet or more. 

 They underlie nearly all the upland in St. Clair and Madison 

 counties, but are greatest in the eastern portion, thinning out 

 towards the river bluffs. They are composed of shales, 

 either sandy, calcareous or bituminous, and a few compara- 

 tively thin strata of fireclay, limestone and coal. The coal 

 seams vary from 6 inches to 4 feet in thickness. Here 



* he. ciL 298. 



t Hershey, O. H. TheLoessFormationof the Mississippi Valley. Science 



N. S. 6 : 768. 1897. 



