166 FULLER AXD CLAPP MARL-LOESS OF LOWER WABASH VALLEY 



as has been pointed out, as well be the result of wind accumulation as 

 of water. The fossils are now found up to the oOO-foot level, but never 

 above, a fact which appears to be explainable by the relatively oxidized 

 and leached condition of the silts above this level, the fossils and other 

 lime constituents which the latter must have originall}^ contained having 

 been largely or entirely removed by solution. 



To a student of physiography the terraces and silt plains are significant 

 features. In character they are entirely unlike the normal topography 

 of the loess in the Ohio- Wabash basin, and are at variance with any 

 known form of wind accumulation on a similar scale. On the contrary, 

 they are of a form which is typical of aqueous deposition. Their alti- 

 tude, limited as it is by the 500-foot contour, is again suggestive, if not 

 conclusive evidence of aqueous conditions. 



Origin of the Marl-loess 



manner of deposition 



The conclusions reached by the writers from the evidences noted in 

 the field, the more important features of which have been presented in 

 the foregoing discussion, is that the marl-loess is an aqueous deposit, 

 consisting of silts deposited in expansions of the White and Wabash 

 rivers. 



The upper limit of the marl-loess, as determined by a large number of 

 accordant observations, is 500 feet above sealevel, or approximately 120 

 feet above the Wabash fiat. The terraces are an original constructional 

 feature, whose differing levels and slopes indicate that the deposition did 

 not reach up to anything like a uniform plain of the altitude mentioned. 

 Strictly fluviatile origin is therefore ruled out as an explanation of the 

 marl-loess as a whole, though it is not at all improbable that some of 

 the lower grayish silts are of this origin. No criterion for the differen- 

 tiation of the strictly fluviatile deposits, if they occur, was found. 



The hypothesis which is accepted as best fulfilling the requirements of 

 the evidence postulates a fluvio-lacustrine condition. The valleys of the 

 lower Wabash and Ohio rivers, with their tributaries, are believed to 

 have been occupied by water standing at the level indicated. This body 

 of water seems to have covered large areas in southern Indiana and 

 Illinois and in northern Kentucky, and appears to be recorded through- 

 out the region by marl-loess flats and silted divides at the 500-foot level. 

 Although the waters possessed a lake-like expanse, it is probable that 

 moderate, if not fairly strong, currents existed, and determined the dis- 

 tribution of the loess and the character of its fossil contents. 



The fluvio-lacustrine body, like all lakes of its class, was probably 



