172 FULLER AND CLAPP MARL-LOESS OF LOWER WABASH VALLEY 



least in part, indirectly through the agency of wind, which picked up 

 the finer materials during fluctuations of water level from the deposits 

 along the Wabash valley and transported them eastward. 



PROBABLE EOLIAN DERIVATION OF THE MAIN LOESS ACCUMULATIONS 



With the exceptions of the belt of the marl-loess and that of the sup- 

 posed aqueous division of the loess of the common type, both of which 

 are of extremely restricted area as compared with the loess sheet as a 

 whole, the great mass of the silts are lacking in features suggestive of 

 aqueous origin. The presence of these silts at all altitudes up to the 

 highest the region affords (640 feet), the absence of fossils, the absence 

 of stratification, and the lack of definite topographic forms all point 

 to an accumulation through an agency other than that which governed 

 the deposition of the marl-loess. This agency is believed to have been 

 the winds. 



An examination of eastern Edwards county, Illinois, shows that be- 

 yond a distance of 15 miles there is a general lack of silts of the loess 

 type, showing the source of the material could not have been to the 

 westward; neither can the material have come from the Wabash itself 

 under conditions in any way approximating those existing at present, 

 for under such conditions the maximum accumulations would be along 

 the immediate borders of the valley. The fact that the common loess 

 is sometimes entirely absent along the immediate tops of the blufis 

 facing the Wabash on the east side, but begins to appear within a short 

 distance, and for several miles rapidly increases in thickness, and then 

 slowly but persistently decreases, suggests that the marl-loess along the 

 Wabash and White rivers was the source of the material. The agency 

 which brought about the accumulation we believe to have been the 

 westerly winds blowing across the marl-loess beds which were exposed 

 during periodic fluctuations of the water level. 



As has been pointed out, the common and marl loess types are essen- 

 tially contemporaneous, and for that reason it has been assumed that 

 the marked degree of leaching and oxidation exhibited by the former as 

 compared with the latter type at similar depths below the surface is due 

 to weathering processes acting during its accumulation. Under the pos- 

 tulated conditions of wind accumulation, ample time and opportunity 

 for the weathering of the material and the removal of the shells by leach- 

 ing is afforded. The surface was doubtless covered by vegetation, which 

 would not only aid in these processes, but would, in the case of the finer 

 silts in question, tend to counteract any tendency toward lamination 

 during the deposition. It would not, however, prevent the broader 



