IOWA ACADEMY OP SCIENCES. 
119 
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 Uneatus, (Say) Morse, and 
Flanorbis 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 fiank the Skunk and the Squaw; are notice- 
ably present along the lower course 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 
