BLUFF SECTIONS 141 



water shells from the silt. The writer collected the following species of 

 mollusks in this stratum: 



Planorhis hicarinatns. Amnicola 



Planorhis dilatotiis. Pisidirnn 



^egrn en t in a a nnujera. Sph (vrl mn s u lea t ion . 



Physa (jyrinu. Uiiio (?) fragments. 



Lymnwa rcfleaa. Pyramidula striatella, 1 specimen. 



Lymnwa humilis Say (?). Strobilops, fragment. 



Valvata tricarlnata. 



The last two are terrestrial and were evidently washed in. 



This fauna is such as occurs in the Aftonian elsewhere, and this, 

 together with the stratigraphic position, indicates that the silt, and 

 probably the gravel, are Aftonian. Moreover, this, with the other bluff 

 sections described in this paper, is only a part of the series of sections 

 which may be traced southward along the bluffs of the Big Sioux and the 

 Missouri rivers to Sioux City and southward. All of these sections show 

 the same characteristics as those which were determined by the writer in 

 Harrison and Monona counties.^^ 



SECTIONS AT SIOUX FALLS 



Several interesting sections occur within the limits of the city of Sioux 

 Falls. They are of special interest because they lie within the so-called 

 Altamont moraine. A brief discussion of several of these sections fol- 

 lows : 



13. Illinois Central Railway section east of Sioux Falls. — This includes 

 the first two contiguous cuts along the Illinois Central Eailway 1 mile 

 east of Sioux Falls. These are the cuts described by Todd and Wilder. 



The thickness and character of the strata in these sections are some- 

 what variable, l)ut the first or west cut shows a complete section on the 

 northeast side as follows : 



Sandy, somewhat loess-like stratum, possibly a^olian, '2, to 5 feet. 

 Gray, weathered drift, probably Kansan, 7 to 9 feet. 

 Gravel, sand, and silt : 



Sand, gravel, and boulders, 1 to 2 feet. 



Silt, with sand and pebbles, ferruginous, 2 to 1 feet. 



Sand, gravel, and boulders, 5 to 6 feet. 

 Weathered gray drift, probably Kansan, 2 feet ex])osed. 



The southwest side of the cut is similar, but the upper Kansan ( ?) is 

 thicker, tougher, and less weathered. 



13 See Harrison and Monona counties report, Iowa Geological Survey, vol. xx, 1911. 



