IOWA ACADEMY OF SCIENCE 
243 
sand material which is quicldy detected by the grating sound produced 
by the wagon wheels. Occasional boulders lie on the surface or have 
been gathered up and piled along the fences. They may he seen along 
the west line of section 32 southwest of East Sioux Falls, in sections 
22 and 15 east ofShindlar, and a pile of them may be seen from the 
railway train just southeast of the station at Shindlar. They are also 
frequent in sections 15 and 16 of township 100 north, range 50 west, 
and were seen at a number of places farther northwest (Fig. 3). 
The erosion valleys of this plain are narrow and steep sided. They 
are restricted to its eastern part near the Big Sioux valley and even 
here have determined the topography of only a small part of the- area 
that they drain. The usual relief features of the plain are low hills and 
broad swales interspersed with shallow undrained depressions. The 
broad swales are usually followed by streams, but these streams did not 
make the valleys which they occupy but only the narrow shallow chan- 
nels in which they flow. The low hills and ridges show by their posi- 
tion and form that they were not made by erosion, but that, like the 
broad winding depressions in which the streams flow, they are con- 
structural. Sufficient characters have now been given, to indicate the 
type of plain with which we are dealing. It is a glacial plain with 
very definite characters and is in decided contrast with the 1 erosional 
area to the north, and with the region on the Iowa side. 
The boundary between the erosional topography and the glacial topog- 
raphy is not always a sharp line, but within one fourth to one half 
mile the transition from one to the other takes place. The boundary is 
showrn on figure 3 as a heavy broken line. It leaves the Big Sioux val- 
ley about a mile south of East Sioux Falls and runs westward through 
south 29 and 30 along the south base of the ridge described above. 
Crossing into Sioux Falls township, it soon changes its course to 
southwest, and at a distance of one to one and one half miles from the 
Big Sioux, holds this course for about four miles, to northeast 16 of 
township 100 north, range 50 west. It then swings 'abruptly to the 
north and follows down a small creek valley through section 9, to the 
Big Sioux. To the southeast of this boundary is the slightly rolling 
glacial plain with its undrained depressions, boulders, and drift con- 
tinuing to the surface. To the northwest, the surface is rolling to 
rough, is entirely controlled by drainage lines, and the surface ma- 
terial is loess or pebbleless loam. 
From the mouth of the creek valley on the north line of section 9, the 
boundary is the edge of the Big Sioux flood-plain west and north to 
the union of Skunk Creek valley with the Big Sioux valley. The 
