566 TRANS. OP THE ACAD. OF SCIENCE. 



" Geological Survey of Missouri" (1855, pp. 69-78), in respect of the 

 line of separation between the fresh-water Loess and the marine Drift. 



Drift is a pretty comprehensive term in geology, and may be said 

 to embrace both fresh-water and marine drift. Geological Reports do 

 not always very carefully mark the difference, or they fail to define the 

 limits of the two kinds of deposit, either in area, or in the section. In 

 this respect, there appears to be some discrepancy between the Missouri 

 and the Illinois surveys. 



In the Missouri survey, the Jlluvial, the Bottom Prairie, and the Loess 

 appear to have been minutely observed and accurately described, but the 

 line of separation between the fresh water Loess and the marine Drift 

 does not seem to have been well defined, either in area or in section. 

 The Loess, containing fresh -water and land shells, and several species of 

 extinct mammalia, is made to comprise all the deposits between the Bot- 

 tom Prairie and the Drift, which is defined by Prof. Swallow as con- 

 sisting, first, of Altered Drift, composed of sand and pebbles, or the finer 

 materials of the Drift, removed and re-arranged by aqueous agency, 

 "subsequent to the Drift period and prior to the Loess"; and second, of 

 the Boulder formation, consisting of sand, gravel and boulders, or water- 

 worn fragments of the local rocks mixed with those of igneous and 

 metamorphic rocks, which were transported from the north; and third, 

 the Pipe Clay, lying directly below the Boulder deposit. The drift as 

 thus defined is said to have a range extending from the Osage and Mera- 

 mec rivers to the northern boundary of the State. In respect of range, 

 the Loess is said to cap all the bluffs of the Missouri, from Council 

 Bluff's to the mouth, and those of the Mississippi from the Des Moines 

 to the Ohio, "and forms the upper stratum beneath the soil of all the 

 high lands, both timber and prairie, of all the counties north of the Osage 

 and Missouri, and also St. Louis and the other Mississippi counties on 

 the south.'' 



It is apparent that this Loess is thus made to comprise all the yellowish 

 or brown clays, or clayey loam, which lies between the marine drift as 

 above defined and the Bottom Prairie; and both the range and the char- 

 acteristic fossils of the Loe3s are asserted of the whole: it is included 

 in the Loess, and the Loess is made to extend over all the high lands and 

 prairies of the northern part of the State, constituting the subsoil. The 

 Loess being a fresh-water deposit, if this were true, it would carry the 

 ancient expansion of the rivers, or inland lake, over all that wide area, 

 and even far beyond. Indeed the ftesh-water lake could scarcely have 

 found a limit and' a shore, short of covering all northern Missouri, and a 

 large part of Iowa, Kansas and Illinois, "reaching even into the Great 

 Lakes of the North. A comparison of the elevations from the mouth of 

 the Mississippi to Council Bluff's will show that such an inland lake 

 would imply, if real, a grpat change in the relative level of the surface 

 of the continent, since that day. That there has been a considerable 

 change of level, besides a cutting down of the barrier at Grand Tower on 

 the Mississippi, may be quite certain, but probably not by any means so 

 much as this theory would require. 



In the Illinois survey, the Loess is described as " mainly restricted to 

 the vicinity of our great river valleys," averaging in thickness "from 

 twenty to sixty feet in the river bluff's, and thinning out rapidly as we 

 recede from the river towards the summit level of the interior." This is 

 sufficiently indefinite; but it would seem to mean that the Loess follows 

 up the river valleys to the higher levels, and reaches not much beyond 

 the tops of the bluffs ; for the " Drift proper" (that is, the marine drift) 

 is, at the same time, described (beginning from the bottom) as consisting, 

 first, of blue plastic clay, containing small pebbles, fragments of wood 

 and trunks of trees; and second, of buff and yellow clays, gravel, and beds 

 of sand, with water-worn boulders of various sizes ; and thirdly, of reddish 

 brown clays, free from boulders, " and forming the subsoil of those por- 

 tions of the State remote from the streams, and where the Loess is want- 

 ing." This again is pretty vague; but the inference fairly to be drawn 



