Cretaceous Rocks of Iowa. — Calvin . 305 
City as at Ponca. At Sioux City, however, we have onl}- the 
attenuated edge of the Niobrara, but that fact in no way disquali- 
fies so much as is developed from being the stratigraphical equival- 
ent of the lower portion of the group as seen in greater force 
farther up the river. 
The three divisions of the Cretaceous recognized at and near Sioux 
City in reality represent continuous sedimentation over a gradually 
subsiding sea bottom. The sandstones and shales of the Dakota 
group with respect to their lower portions at least, were accumu- 
lated in a rather shallow land-locked sea. Currents swept the sand 
back and forth, sometimes building up, and again tearing down, 
previously constructed beds, and so produced the fine examples of 
cross bedding, or current structure, so well illustrated near Spring- 
dale a few miles northeast of Sioux City. The few molluscan 
species found in the lower part of the Dakota group indicate the 
presence of brackish water. The numerous vegetable remains 
which characterize the group imply that the large volumes of 
drainage water which maintained the conditions favorable to the 
existence of brackish water mollusks, carried not only sands, but 
swept in leaves and trunks of the willow, poplar, magnolia and 
other forest trees, from the adjacent lands. 
As the waters became graduall}' and progressively deeper owing 
to subsidence of the sea bottom, the conditionsfavoring the accum- 
ulation of sandstones and the existence of brackish water mollusks 
disappeared. The shore line was shifted farther to the east. The 
sediments of the region about Sioux City became finer and set- 
tled down in regular laj^ers beyond the reach of disturbing currents. 
The downward movement of the sea bottom seems not to have 
been altogether constant during the epoch of the Dakota group. 
There were occasional oscillations that from time to time permitted 
the formation of thin beds of sandstone, but before the close of 
the epoch the amount of sand that reached as far as Sioux Citj- 
was insignificant and fine clay shales greatly predominated. The 
shales of the Dakota group gradually merge into those of the Fort 
Benton. During the second epoch the subsidence had carried the 
shore line so far to the east that all coarse sands were deposited 
before reaching the area in question. Before the Fort Benton 
epoch began the brackish water estuary had long be(m trans- 
formed into a portion of a clear, open sea. At all events during 
that epoch true marine mollusks such as Inoceramus and Ostrea 
