NOTES ON THE GEOLOGICAL SECTION OF IOWA 577 



shales and sandstones of the Des Moines stage in old drainage 

 channels, and over the surface generally, as far to the northeast 

 as Delaware and Jackson Counties. In the counties last named 

 remnants of coal- measure strata are found in troughs cut in Silurian, 

 and even Ordovician, beds, and similar remnants occur in old river 

 channels cut in the Devonian limestones of Muscatine, Linn, and 

 Johnson Counties. The extreme advance of the coal-measure sea 

 was of comparatively short duration. For the greater part of the 

 Des Moines stage, so far as it is represented in Iowa, the shore-hne 

 oscillated back and forth over the area now occupied by the valleys 

 of the Des Moines and the Skunk Rivers. Within this area there 

 are records of numerous slight movements of elevation and sub- 

 sidence. 



The sediments referred to the Missourian stage follow those of 

 the Des Moines without break. The crustal oscillations seem to 

 have been less numerous; the waters were clearer; the chmate 

 was less humid; arenaceous deposits are scarce; limestones and 

 shales make up the bulk of the deposits of this stage; progress was 

 made toward the more arid conditions of the Permian. 



The Permian. — The gypsum beds in Webster County, together 

 with the associated red shales and sandstones, have been referred 

 by Professor Wilder to the Permian system. By some writers 

 they have been referred to the Triassic, by some to the Cretaceous. 

 These beds contain no fossils, and their stratigraphic relations are 

 such as to lend no aid in determining their exact position in the 

 geological column. They lie unconformably on deposits of the 

 Des Moines stage; in some places they rest on St. Louis hmestone, 

 for erosion had cut through the whole thickness of the Des Moines 

 sediments before conditions favoring the deposition of gypsum began. 



The Cretaceous system. — The Dakota and Colorado stages of 

 the Upper Cretaceous are represented in northwestern Iowa by a 

 series of sandstones, shales, and chalky limestones. In his report 

 on the Geology of Iowa, published in 1870, White divides the Iowa 

 Cretaceous into the " Nishnabotany sandstone," the ''Woodbury 

 sandstones and shales," and the "Inoceramus beds." The sand- 

 stones along the Nishnabotna River, as well as those at Sergeants 

 Bluff and Sioux City in Woodbury County, together with some 



