414 Geology of Johnson County. 
The channel of the Iowa River, from Iowa City north, in this 
county, has been eroded to a depth of from twenty feet to upwards 
of one hundred feet into the hard Devonian limestone. As I have 
before intimated, it is believed that this valley was formed, to a 
great extent at least, by an old Devonian stream, and then sub- 
sequently filled by the sandstone during the Carboniferous age. 
The glacial drift in Johnson County, so far as it has been 
observed, is everywhere covered by a profound mantle of loess, so 
that it can be observed only along the borders of streams, and along 
the axis of surface drainage, where the overlying formation has 
been cut through, thus exposing the drift at the bottom. This 
formation, so far as can be made out, attains a thickness of from 
four feet to eighteen feet, and is composed of clay, sand, gravel, and 
boulders. The boulders, however, are rarely observed, except at 
the bottom of ravines, where they are sometimes quite numerous, 
and vary in weight from ten pounds to upwards of two thousand 
pounds. The drift, for the most part, shows but slight evidence of 
modification or rearrangement of its materials. In some places, 
however, it contains an abundance of ferruginous matter, so much 
indeed as to give it, at limited localities, a deep-red color. The 
gravel of this formation is made up of more or less rounded frag- 
ments of Devonian limestone, derived from the subjacent strata, 
smoothed and well rounded, often beautiful striated, pebbles of 
granite, quartz, trap, greenstone, and others of igneous origin, which 
have been derived from the north. The boulders are also of the 
same material and origin with the exception of those of Sioux 
quartzite, which were derived from the northwestern portion of the 
State. Devonian fossils (and rarely Carboniferoud, derived from 
the underlying rock, are also common. The old forest bed is also 
well represented, and occupies, as is usual throughout other portions 
of the State, a low horizon in the drift formation. 
While digging a well on the farm of Mr. Joseph Hedger, about 
five miles southeast from Iowa City, the forest bed was struck at a 
depth of about twenty-eight feet. This formation was represented 
by a dark-brown, slightly-combustible peat formation, which was 
overlayed by, and slightly mixed with, a layer of coniferous wood 
and twigs. This peat also contained abundant remains of plants, 
well-preserved seeds (apparently those of grass), and abundant 
remains of Coleopterous insects. This bed rests directly upon hard, 
