142 Sioux City Academy of Science and Letters. 
fall of 250 feet in 1,000. The drainage away from the 
river is not precipitous, but a traverse from the point 
mentioned south to the creek in the south half of sec. 
22 reveals a descent of over 200 feet in 14 miles. The 
opposite side of this creek has a steeper slope. The 
drainage of this locality is typical. (See fig. 2.) It 
reaches the Missouri via. Elk Creek after a course of 
over 16 miles, whereas a direct course across the flood 
plain to the river would be less than one-eighth that dis- 
tance. Structural causes for this system of drainage are 
hard to discover. As is explained later the locally under- 
lying rocks dip slightly toward the northwest, so that a 
much simpler form of drainage would have been devel- 
oped had the dip of the strata influenced the surface flow 
of waters. 
The divide between the upland streams and the ra- 
vines opening on the flood plain was formerly farther 
from the escarpment than it now is. On the unstable 
surface a continuous stream such as Elk Creek is able 
rapidly to enlarge its basin. Though developed on a 
loess-covered plain underlain by almost horizontal rocks 
which do not differ greatly in hardness each small creek 
or wet-weather stream with its parallel branches join- 
ing it nearly at right angles forms a trellis or grapevine 
drainage system. The lateral branches of Elk Creek 
where it parallels the Missouri flood plain are continu- 
ously cutting backward up their slopes and pushing the 
divide toward the escarpment. The ravines from the 
flood plain are themselves cutting backward, though 
more slowly, and the next stage of topography that will 
result will be a more ragged escarpment line marked by 
many such outliers or prominences as Prospect Hill in 
Sioux City. 
From the point on the bluffs, previously mentioned, 
may be observed the whole course of the Missouri bor- 
dering Dakota County. As the eye follows the escarp- 
ment on the Nebraska side of the river beginning at a 
point above the north county line where the southeast- 
erly course of the river changes to due east the river is 
seen to be there cutting into the bluffs from which it 
swings out into the lowland, making a series of loops in 
its broad alluvial valley until it reaches the bluffs at the 
mouth of Big Sioux River on the Iowa side. It comes 
