8 IOWA STUDIES IN NATURAL HISTORY 
most part in rocky bluffs along the rivers through which the 
latter have cut their course, apparently going out of their way 
to avoid the more extensive, more easily occupied, and relatively 
uniform loess prairie land at the side. This peculiarity is ac- 
counted for by the indications that the Iowa River now follows 
in general an interglacial valley, which was probably 150 feet 
greater in depth than the one which it now occupies. 
TOPOGRAPHY 
Johnson County presents but little range in altitude, the vari- 
ations being but from approximately 665 to 800 feet above sea 
level. This variation is evident in the immediate vicinity of the 
Iowa River north of lowa City for some fifteen miles. Through- 
out this region the river valley is narrow, having been cut 
through the elevated rock formations; its valley walls rise ab- 
ruptly in many places in the form of bluffs, the tops of which 
may be a hundred feet above the river bed. 
From the entrance of the stream into the county to the point 
where it passes into the narrow, bluff-confined portion of its 
valley, in Section 22, Township 81 north, Range 7 west, only 
one side of the valley is abrupt, the north. Likewise, although 
to a less pronounced extent, below Iowa City the west slope of 
its valley is more abrupt than the east slope. 
Plate II is a photographic reproduction of those portions of 
the county which have been topographically surveyed by the 
United States Geological Survey. It.was obtained by piecing 
together those parts of Johnson County contained in the Amana, 
Oxford, Cedar Rapids, Mechanicsville, and West Liberty topo- 
graphical map sheets.* This plate shows the topography, in 
twenty-foot contour lines, of all. the county except the lower 
thirty-six sections of Fremont township, of which no survey has 
been made to date. -As shown by this map, the rough land lies 
for a few miles on each side of the Iowa River from Iowa City 
north. to the abrupt bend in that stream in Section 22, Township 
81, Range 7 west, continuing along the north edge of the valley 
through Jefferson and Monroe townships to the edge of the 
*Topographical Maps, published by the United States Geological Survey in codper- 
ation with the Iowa Geological Survey, Des Moines, Iowa. Cedar Rapids, Mechanies- 
ville, Iowa City, and West Liberty sheets were surveyed in 1887, Amana and Oxford 
sheets, in 1888. 
