Glacial Drift of the Basin of Iowa. 973 
of the streams of this area are usually broad, with one side more 
or less well defined, while the opposite side gradually emerges into 
the adjoining upland. 
By far the greater portion of the surface of the area is 
occupied by the drift formation, which varies much in thick- 
ness at different localities. The difference seems to be par- 
tially due to original deposition, and partially to subsequent erosion. 
A line drawn from Waterloo, in Black Hawk county, in a north- 
west direction to Mason City, in Cerro Gordo county, and another 
line drawn from Waterloo to Osage, in Mitchell county, would 
include within their boundary the area of the thinnest drift of the 
region, and, indeed, of Iowa. In all parts of this attenuated drift 
area, the underlying rock strata is exposed at the surface in very 
numerous places. 
Present evidence seems to indicate that the superficial geo- 
logical formation of this entire region shows two main Glacia 
epochs, each of which is characterized by several subdivisions. , 
As belonging to the earlier Glacial epoch, there are recognized 
two drift sheets. Between these occur, at numerous points in the 
area, and other portions of the State, vegetal accumulations, indi- 
cating a non-glacial interval. This horizon is usually referred to 
as the “Old Forest Bed.” The material of the lower and upper 
drift sheets is (so far as observed) essentially the same, being com- 
posed for the most part of a heavy, dark blue clay, containing a 
greater or less number of transported deangulated erratics and pe 
bles, numbers of them being in a partially decomposed condition. 
At some points the clay of these beds is distinctly stratified, and 
as readily separated into layers as ordinary limestone. 
The vegetal accumulation which occupies a horizon between these 
drift sheets is composed of coniferous wood, branches and twigs, 
which is very often associated with ancient peat, and more or less 
mixed with the drift. In digging wells in this division, natural gas 
is often obtained,! this originating from the organic accumulation. 
Gathering evidence seems to demonstrate that the vegetal accu- 
mulation of this region, as well as that of some other places in 
the State, are referable to a single horizon. 
t In an article on “The Geology of Johnson county, Iowa,” which 
appeared in the May number of this Journal for 1888, mention was 
made, by the writer, of wells of this character in Linn county, Iowa. 
