152 Sioux City Academy of Science and Letters. 
The loess overlies the drift where it is present or else 
lies directly on the Cretaceous strata. Its thickness 
varies from a few feet to nearly 100 feet. In texture the 
loess resembles a deposit of fine river silt. It shows no 
stratification, but rather a vertical striation on fresh 
faces. The loess contains many small, irregular-shaped 
concretations nearly white, composed principally of cal- 
cium carbonate, and shows the presence of calcium car- 
bonate upon treatment with hydrochloric acid. The 
greater part of the material is, however, insoluble. 
To an oxide of iron is probably due the characteristic 
buff color. 
Although small shells, usually of land forms not yet 
extinct are found in the loess there are many facts which 
indicate that it is of aqueous origin. This point will be 
again considered. 
Alluvial deposits make up the great Missouri River 
flood plain and are accumulating in all the valleys that 
have been graded. Alluvial cones or fans have been 
built on the Missouri flood plain at the mouths of some 
of the smaller gullies, notably south of Hubbard. 
Talus deposits consisting of loess and rock debris 
line the escarpment throughout the county. 
STRUCTURE. 
The strata lie nearly horizontal, but the slight dip to 
the northwest, averaging 6 feet to the mile, is sufficient, 
together with the upstream slope of the Missouri in the 
same direction, to carry in succession the Dakota sand- 
stone, the Graneros shale, and the Greenhorn limestone 
below the water within a distance of about 35 miles 
from the south line of Dakota County. The rocks have 
been but little disturbed since deposition. A succession 
of broad undulations may be traced in the escarpment 
section from southeast to northwest through the county. 
(See fig. 6.) This structure probably corresponds to the 
gentle synclines and anticlines that have been observed 
in Woodbury County, Iowa, and, if so, the axes trend 
in a northeast-southwest direction or at right angles to 
the seaward direction from which the pressure probably 
came. Aside from these faint folds the slight northwest- 
ward inclination of the strata might be regarded as their 
initial dip. 
