248 
IOWA ACADEMY OF SCIENCE 
which all the slopes and surface features have been determined by stream 
erosion. The writer has studied quite thoroughly thei major portion 
of O’Brien and Osceola counties and is very positive in making the 
statement that in all the Kansan area of O’Brien and surrounding coun- 
ties there is not a single un drained depression of the type that is com- 
mon on the Dakota plain. 
The Chicago, Milwaukee and St. Paul Kailway, running south from 
Sioux Falls, comes out onto the glacial plain just north of the county 
line, and continues southward across this plain through Harrisburg to 
Canton. At a number of places along this road swamps may be seen 
and boulders lie on the surface, ft is evident that if the identity of 
the Wisconsin plain is established farther north it should continue 
south to Canton. The writer has not seen the region southwest of Can- 
ton, but from the topographic map of the area it seems evident that 
thJe southeast border of this plain is approximately as given by Profes- 
sor Todd, running from the point of the upland south of Canton, south 
by southwest through Beresford. 
It has already been noted: that the loess is absent over the Wisconsin 
plain, but the matter is of such importance that a more complete state- 
ment is justified. The rugged region of the Iowa, side isi loess- covered, 
with numerous exposures in the road cuts. The: area within the east 
loop of the Great Bend between Sioux Falls and East Sioux Falls, the 
area within the west loop of the Great Bend, and that west of the Big 
Sioux and north of Skunk creek, are all loess-covered, as well as the 
rugged area south of Canton. In contrast with this loess-covered, rugged 
area the Dakota plain is free from loess. On figure 3 twenty exposures 
of loess are mapped in the area north of the Wisconsin drift-plain to 
the east and west of Sioux Falls, nineteen exposures" are mapped on 
the Iowa side and eight in the rugged area south of Canton. Many of 
these loess exposures are taken from the map published by Professor 
Shimek on page 131 of volume 23 of the Bulletin of the Geological So- 
ciety of America. On this map by Professor Shimek only one ex- 
posure of loess is mapped within the area, which is included in the Wis- 
consin drift-plain of figure 3, and this exposure is just at the edge of 
the Wisconsin plain, in or near the west bluff of the Big Sioux val- 
ley. On the other hand there are twelve exposures of loess mapped 
within the east loop of the Big Sioux, sixteen exposures along the Iowa 
upland between the state line and a point opposite Canton, and eleven 
exposures in the upland south of Canton. The plotting of these loess 
exposures brings out the fact that the loess-covered area is identical with 
the area of erosional topography, while the area without loess is identi- 
cal with that having a glacial topography. 
