244 
IOWA ACADEMY OF SCIENCE 
border is then the south edge of the Skunk Greek flat and runs north- 
west through the center of this township. The part of this township 
to the south of Skunk creek is a, glacial plain while to the north the 
topography is maturely dissected and the surface exposures are of 
loess. The contract of the topography on opposite sides of this, valley 
is very pronounced and an excellent example of glacial versus erosional 
topography. 
In this entire distance along the Wisconsin margin from the Big 
Sioux opposite the state line to the center of township 101 north, range 
50 west, a distance of 15 to 17 miles, there, is not a single hill that might, 
be called a terminal moraine hummock. The marginal part of the 
glacial plain is not even more uneven than that farther back, except 
for irregularities due to recent erosion or to incomplete obliteration of 
pre-Wisconsin surface features. 
Professor Todd traced the course of the “ outer moraine” across this 
area; in the following words: 5 “Beginning on the west side of the Big 
Sioux, about a mile north of the northern boundary of Iowa, a high 
massive ridge begins to extend westward and southwestward around the 
Great Bend of the Big Sioux, and. continues its westerly course to near 
the southwest corner of township 101, range 51.” >Tear the Big Sioux 
valley south and southwest of East Sioux Falls this ridge is fairly 
prominent but it becomes less prominent westward and in southeastern 
Sioux Falls township is represented only by disconnected hills. These 
features, apparently taken by Professor Todd as morainic, are all on 
the Kansan just beyond the actual Wisconsin margin and are not 
morainic but erosional. However, the contrast between the glacial plain 
to the south and the erosional topography to the north was detected 
and its true significance realized. Westward from thje Great 
Bend, Professor Todd states that this ridge “continues its westerly 
course to near the southwest corner of township 101, range 51. ” A 
broad ridge-like elevation does continue westward along the county 
line, from, the Great Bend, but this elevation does not mark the Wis- 
consin margin, for as noted above the southern part of township 101 
north, range 50 west, south of Skunk creek, is a glacial plain. 
This separation of the Kansan and Wisconsin drift-plains, is based 
upon physiographic features, although the boulders of the Wisconsin 
plain and the loess-covering of the Kansan areas, are accordant strati- 
graphic lines of evidence. The Wisconsin drift is very hard to dis- 
tinguish from the Kansan, at least in the marginal parts of the Wis- 
consin area, or else it is almost entirely lacking. In a few places the 
5 Todd, J. E., The Moraines of Southeastern South Dakota, Bull. 158 U. S. Geol. 
Survey, 1899, p. 35. 
