Sioux City Academy of Science and Letters. 155 
With the subsidence of the land surface during Mes- 
ozoic times the sea began to advance from the west over 
eastern Nebraska and South Dakota into western Iowa. 
The first deposits appear to have been sediments of Da- 
kota time. These, being mainly sands, were deposited 
on beaches and in estuaries, with also deposits of clays 
formed in deeper and quieter waters. The sands were 
doubtless derived largely from the disintegration of the 
quartzite along the shore, and the clays came from the 
limestones and soils of the land area to the east. 
An interesting discussion relating to the age of the 
Dakota sandstone was waged among distinguished 
American and European scientists from 1855 to 1870 
_ with also a controversy regarding its origin—whether it 
is a fresh water, or brackish water, or brackish water and 
marine deposit. The great areal extent of the beds, from 
north to south and east to west, practically coextensive 
with the interior Cretaceous sea, the presence of marine 
forms, the predominance of sandstones and their cross- 
bedded character, the presence of lignites and the grad- 
ual transition upward into marine beds all are indicative 
of marine shore conditions, and it is now generally con- 
sidered that such were the conditions of its deposition. 
At the close of Dakota time the sea overspread the 
region as far as eastern Iowa and southeastern Minne- 
sota and in the deeper waters deposition of the Benton 
shales began. Shallower waters may have prevailed for 
short periods during which local layers of sand were car- 
ried seaward by currents and spread out over the clays, 
and at one period near the middle of the Benton, the 
waters were deep and clear and contained abundant life. 
It was at this time that the fossiliferous Greenhorn lime- 
stone was deposited. 
The Greenhorn limestone beds are the topmost beds 
of the Cretaceous present in Dakota County, the Carlile 
shale appearing above them in eastern Dixon County, the 
Niobrara beds above the Carlile shale in northwestern 
Dixon County and the Pierre shale still farther to the 
northwest and west. 
From the evidence it seems uncertain whether these 
later formations once overlaid this area and were re- 
moved by the pre-Glacial erosion or whether their present 
boundaries represent approximately their eastern limit 
