Geology of Johnson County. 417 
less rapidly ascending sides, and rising to a height of from ten feet 
to upward of sixty feet above the intervening depressions. 
The valleys of the Iowa River and its tributaries are relatively 
narrow and deep, and bordered by more or less steep acclivities, 
and flanked at frequent intervals by deep but narrow and rapidly 
ascending ravines, and these again often ramify in shallow depres- 
sions which draw the water quite effectually from nearly all portions 
of the upland. f 
The immediate valley of the Iowa River, as I have before inti- 
mated, attains an average width of one-half of a mile, and a depth 
below its immediate borders of from sixty feet to upward of one 
hundred and seventy-five feet. Well-defined terraces are sometimes 
observed along this stream, and they have been produced by the 
deepening of the valley by the action of the waters of the stream. 
As has been before stated, the surface of nearly, if not quite all 
of this county, is occupied by a loess formation, which effectually 
conceals the underlying drift formation, except along the valley- 
sides and the axis of surface drainage, where the overlying deposit 
has been cut through. This deposit attains a greater thickness 
along the streams than adjacent to them, and consists, for the most 
part, of an exceedingly finely comminuted yellow or buff-colored 
clayey earth, with an admixture of humus in favorable situations, 
as in the beds of drainage depressions and in the valleys, as well as 
most usually the more level surface of the upland, which gives to 
it a color varying from a light-ash to a deep-black. Upon the 
higher points, however, the soil usually contains comparatively 
little humus, for the reason that it is swept down by the rains as 
fast as it is accumulated by the decay of vegetation, and deposited 
in the beds of ravines and the slight valley bottoms. 
The following section, taken at the “brick-yard,” in the north- 
east portion of Iowa City, gave the following result :— 
_ 1. Very fine brownish “loamy” soil, containing humus—three 
inches, 
2. Very fine and homogeneous yellow-clayey earth—fifteen feet. 
3. Very fine and homogeneous bluish-grey, clayey earth, having 
more or less numerous brownish-drab streaks running through it, 
and containing numerous fossils, many of which were in a crushed 
condition—fiye feet. Entire thickness unknown. = 
The different divisions of this section pass into each other by 
