300 The American Geologist. November. 1902 
THE CARBONIFEROUS FORMATIONS OF 
HUMBOLDT, IOWA. 
F. \V. Sardeson, Minneapolis. 
PLATE XVII. 
During the past summer, the writer spent several days 
in Humboldt county, Iowa, and found an opportunity to be- 
come more nearly acquainted with that somewhat isolated 
geologic field, one which few geologists are known to^ have 
visited. White described it briefly long ago and MacBride 
has recently again describedf it. Their accounts do not near- 
ly exhaust the readily obtainable geologic information, espec- 
ially of the Carboniferous formations, considerable additional 
being at once evident. Nor does the matter herein presented 
complete that which may be learned there. It is the purpose 
of this article to indicate some remaining fertile spots in that 
geologic field. 
Humboldt county, it may be repeated here, lies in the 
prairie region of northern Iowa, midway between the Alis- 
sissippi and Missouri rivers. It covers the confluence of the 
East and West Des Moines rivers, upon a glacial plain which 
slopes in general from northwest to« southeast, somewhat 
more than 100 feet in 30 miles. MacBride says that "the 
highest elevation within its limits- rises not more th<in 30 or 
40 feet above the general level." The Des Moines rivers "oc- 
cupy comparatively narrow channels, cut down from 20 to 70 
feet below the level." 
The geologic formations are nearly horizontal or dip in 
the same direction and in nearly the same degree as the plain. 
The exposed formations are accordingly few. The cliffs and 
wooded hills along- the rivers are doubly attractive by reason 
of contrast to the broad face of cultivated prairie and they are 
also the prime source of geologic information. The approx- 
imate northern limit of coal measures and the limit here of 
natural exposures of stratified formations beneath the glacial 
drift, in extension toward jNIinnesota are scientifically worthy 
of special note. The peculiar method of draining the prairie 
land with its numerous shallow glacial basins, by means of 
natural and artificial sinks is economically of interest, and as 
* Iowa Geol. Sur., vol. ix, pp. 117-147, 1898. 
