158 MISSOURI BOTANICAL GARDEN 



'^A considerable portion of this bottom is below the high 

 water level of the Mississippi, and is therefore subject to 

 periodical inundations from the annual overflow of the river, 

 while other portions are above high water mark and owe their 

 origin to some other cause than the existing river. It seems 

 probable indeed that a considerable portion of the area now 

 comprised in this bottom was originally occupied by deposits 

 of drift clay and loess, that were deposited after the valley 

 was scooped out of the sub-carboniferous limestones, filling 

 the valley to the height of 50-0^ feet above its present level, 



iod 



subsequent 



epoch. In proof of this there are niany elevations scattered 

 over the surface of this bottom, locally known as "mounds/' 

 the formation of which have generally been referred to human 

 agencies- These elevations vary in height from ten to sixty 

 feet or more above the level of the surrounding bottom and 

 when carefully examined are found to consist of drift clay 

 and loess, remaining in situ just as they appear along the 



bluffs 



been formed 



same way by the removal of the surrounding strata by cur- 

 rents of water. We had an opportunity of seeing a good sec- 

 tion of the large mound in the upper part of the city of St. 



Louis 



mound 



for material to be used in filling adjacent lots. It was found 



common 



drift clay at the base, which was overlaid by thirty feet or 

 more of the ash-colored marly sands of the loess, the line of 

 separation between the two deposits remaining as distinct 

 and well defined as they usually arc in good artificial sections 

 in the railroad cuts through these deposits. 



"Hence we infer that these mounds are not artificial ele- 

 vations raised by the aboriginal inhabitants of the country, 



but 



and 



remained as originally d(^posited, while the surrounding 

 contemporaneous strata were swept away by denuding forces. 

 They are not found to occupy any fixed relative position in 

 relation to each other, or to have any regularity of size or 



