I II The American Gtefllogist. September, 1804 
tion in our selected area during < 'retaceous times. The Benton 
shales are not very thick in this region, probably not more 
than 40 feet altogether. While they were accumulating the 
process of Subsidence went on. as before, at a rate more rapid 
than the upward growth of the sedimentary deposits. The 
lands, that during the Dakota stage had stood high above the 
sea, were by progressive subsidence reduced in altitude until 
finally base level was reached, and the sluggish drainage 
streams discharging into this part of the Cretaceous basin 
failed to carry even the liner clay and at last mechanical sedi- 
ments gave place to those of organo-chemical origin. When 
these conditions were reached, the region witnessed the be- 
ginning of the third phase of sedimentation in the (retaceous 
seas, and the Niobrara stage, with its peculiar beds of chalk, 
was inaugurated. Reviewing for a moment the conditions 
prevailing in that portion of the region that now lies west of 
the Sioux river, we may note that the water was clear, moder- 
ately deep, and unpolluted by mechanically derived earthy 
detritus; that the shore line, a portion of the time at least, 
was a hundred miles to the eastward; that the body of land 
contiguous to the shore was low and flat, and drained by 
streams with currents too feeble to bear any contributions 
from the land to the sea. It was in such a sea and under such 
conditions that the Niobrara chalk was deposited. 
Before the Niobrara age came to a close, the upward move- 
ment of the region began. Step by step the sea receded from 
its line of farthest advance, somewhere east of the middle of 
Iowa. Progressive elevation of the land quickened into life 
the practically dead streams of the Niobrara age, and mechan- 
ical sediments appeared once more in the Cretaceous sea and 
settled down upon the surface of the chalk to become the 
lower beds of the Fort Pierre shales. The Fort Pierre shales 
are found as far east as Yankton. South Dakota, and recently 
Keyes claims that they have been identified in northwestern 
Iowa.* In the latitude of Sioux City they may have origin- 
ally extended beyond the Sioux river. Our region during 
(retaceous times witnessed, therefore, a fourth phase of sedi- 
mentation. With the incursion of mud that inaugurated the 
work of building the strata of the Pierre group, the condi- 
*Proceedin'gs Iowa Acad. Sci., Vol. Part IV. \>. 25, 1894. 
