IOWA ACADEMY OF SCIENCES. ng 



The outcrop along Onion creek is an almost exact dupli- 

 cate of the Walnut creek section. The drift mantle is thinner, 

 and from two to five feet of loess has been stained to a 

 yellowish buff, and loess concretions are more in evidence, thus 

 attesting to the greater progress made in leaching. Here, 

 again, the upper portion is distinctly jointed, while lower 

 the deposit is apparently structureless. Gasterpod shells 

 abound throughout, but only two species, not listed previously, 

 appear — 



Helicodiscus lineatus, (Say) Morse, and 



Pfanorbis bicarinatus, Say, 

 both of which are terrestrial forms. 



In connection with these deposits of buried loess, certain 

 arenaceous to silty gray-brown deposits, remarkably homogen- 

 eous and devoid of pepples and bowlders, border some of 

 the larger streams and are perhaps worthy of special mention. 

 They are discussed here with the hope that they may throw 

 some light on the process of loess accumulation. These highly- 

 siliceous deposits flank the Skunk and the Squaw; are notice- 

 ably present along the lower coarse of Indian creek, but 

 are more in evidence along the eastern margin of the Skunk 

 river valley, below Bloomington. The deposits attain a max- 

 imum thickness of from three to five feet on the brow of 

 the bluffs, thin rapidly inland and are scarcely recognizable 

 more than a mile from the bluff scarp. These deposits are 

 responsible for the heavy, sandy roads along so many of 

 the streams in the Mississippi valley and are shunned alike by 

 the teamster and the bicyclist. They are often known, locally, 

 as "White Oak Soils," because that very well known and desir- 

 able species of oak finds in them a congenial host. The 

 deposits are thoroughly oxidized and leached and appear to be 

 wholly devoid of structural or bedding planes. The coarsest 

 materials which enter into their composition are found nearest 

 the flood plain, and the size of the grain diminishes gradually 

 as the deposit feathers out away from the river. The source of 

 the materials and the transporting agent are not difficult to 

 apprehend. The process of accumulation is going on to-day. 

 The wind, sweeping across the broad flood plain, gathers up 

 such material as can be transported and moves it toward the 

 restraining bluffs. Perhaps only the very finest materials are 

 given continuous passage for any considerable distance. But 

 through successive short excursions, the coarser silt-particles 



