IOWA ACADEMY OF SCIENCES. H 



the condition of the Inoceramus-bearing lasers at Sioux City and the condition ob- 

 served in the basal parts of the Niobiara f^roup at St. Helena. Furthermore, the 

 beds referred by Hayden to tlie Dakota and Fort Benton groups are as well devel- 

 oped at Sioux City as at Ponca. At Sioux City, however, we have only the at- 

 tenuated edge of the Niobrara, but that fact in no way dii-qualifies so much as is 

 developed from being the stratigraphical equivalent 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 snales of the Dakota group, with respect to the lower portions 

 at least, were accumulated in a rather shallow land-locked sea. Currents swept 

 the sand back and forth, sometimes building up, and again tearing down, pre- 

 viously constructed beds, and so produced the fine examples of cross bedding or 

 current structure, so well illustrated near Springdale, 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 waters 

 which maintained the conditions favorable to the existence of brackish water mol- 

 lusks, carried not only sand but swept in leaves and trunks of the willow, poplar, 

 magnolia, and other forest trees, from the adjacent lands. 



As the waters became gradually and progressively deeper, owing to subsidence 

 of the sea bottom, the conditions favoring the acummulations 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 settled down in regular layers beyond the reach of disturbing currents. The 

 downward movement of the sea bottom seems not to have been altogether con- 

 stant 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 City was in- 

 significant, and fine clay shales greatly predominated. The shales of the Dakota 

 group gradually merge into those of 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 been transformed into a portion of a 

 clear open sea. At all events, during the epoch true marine mollusks, such as 

 Inoceramus and Qstrea. had supplanted Cyrena and Margaritana, while marine saur- 

 ians and teleost fishes multiplied and became the dominating types of the oceanic 

 realm. 



The soft limestone and softer chalk of the Niobrara group are indicative of still 

 deeper waters and remoter shores. During this epoch no gross sediments from the 

 land reached as tar as Sioux City. Not since the earlier part of the Dakota group 

 had it been possible for leaves and twigs of forest trees to be carried into the 

 region. It was during this Niobrara epoch that the subsidence reached its max- 

 imum, and the maximum extension eastward of the Cretaceous sea was attained. 



Toward the close of this epoch the upward movement of the land began, the 

 sea withdrew, and shales of the Fort Pierre group were deposited above the chalk 

 from Yankton westward. 



When we recall the fact that the three groups recognized at Sioux City and Ponca 

 represent the effect of continuous sedimentation over a subsiding sea bottom it 

 will be seen that the question of dividing the sediments into distinct groups 



