146 Sioux City Academy of Science and Letters. 
Dr. Bain@ gives a most complete historical summary 
of the work in this region prior to 1895 and of the famous 
discussion through which the age of the rocks became 
agreed upon. 
Although so much has been written regarding the 
general geology of the region and the broader strati- 
graphical relations have been so long understood, no de- 
tailed mapping of the formations throughout Dakota 
County appears to have been done heretofore. It is there- 
fore thought best to summarize the general relations 
together with features observed in the field, although 
such summary may add nothing to the facts already 
known. 
STRATIGRAPHY. 
The exposures of rock best available for study in 
Dakota County occur along the escarpment which 
traverses the county from southeast to northwest. This 
escarpment presents for the most part, however, steep 
slopes covered with loess and grassed over. On such 
slopes are many small steps or minute fault terraces 
caused by the loess settling and slipping down to lower 
levels after some of its calcareous content has been 
leached by solution. At both the lower and upper ends 
of the county the river has cut into the bluff so recently 
as to undermine the upper strata which have broken off 
in precipitous cliffs sometimes slightly overhanging. 
Usually the base of the escarpment is concealed by a 
heavy talus. 
The indurated rocks underlying Dakota County are 
of upper Cretaceous age. The oldest formation expose:l 
is the Dakota, which comprises sandstones, clays, and 
shales, with interbedded seams of lignite. Above the 
Dakota lie the basal shales of the Benton Group, which 
also contain a thin seam of lignite. Above this shale 
which is here provisionally called the Graneros, as sug- 
gested by Mr. N. H. Darton, of the U. S. Geological Sur- 
vey, is a limestone member very probably correspond- 
ing to the Greenhorn limestone of the Benton group in 
Colorado.” 
aBain, H. F., Geology of Woodbury County: Iowa Geol. Survey, 
Vol. 5, 1895, pp. 265-273. 
bGilbert, G. K., Underground waters of the Arkansas Valley in 
eastern Colorado: Seventeenth Ann. Rept. U. S. Geol. Survey, pt. 2, 
1896, p. 564. 
