stanton.] IOWA SECTION. 21 



In western Iowa the Cretaceous beds are souiewbal better developed 

 and the exposures arc more satisfactory. The following description of 

 tlie genera] section from the base upward is condensed from Dr. White's 

 account in the Geology of Iowa. 1 



Iowa Cretaceous section. 



1. Nislinabotany sandstone: Feet. 



Rather coarse grained, friable, more or less ferruginous sandstone, witli 

 occasional thin, irregular layers of clay. Fossils: Fragments of angios- 

 permous leaves. Maximum thickness 100 



2. Woodbury sandstones and shales : 



Alternating thin beds of shale and sandstone, the former predominating. 

 The shales vary in character, and at some places a lignitic band is 

 shown. Fossils: Inoceramus labiatus and casts of a few other Lamelli- 

 branchs are occasionally found, and leaves of Salix meekii and Sassafras 

 cretaceum also occur. Maximum thickness 150 



3. Inoceramus beds : 



Impure chalky limestone and marls with Inoceramus labiatus, Osirea con- 

 gesta, and remains of several genera of fishes. Thickness 50 



A detailed section measured by Mr. O. H. St. John 2 along the Big 

 Sioux river above Sioux City, in which the basal sandstone is not 

 represented, shows 45 feet of the Inoceramus beds and 119 feet of the 

 underlying Woodbury sandstones and shales. 



The original section of Meek and Hayden was described from this 

 same neighborhood, and the divisions above given correspond in a 

 general way to the Dakota, Fort Benton, and Niobrara groups, though 

 it seems possible that a part of the Dakota is included in the Wood- 

 bury sandstones and shales. Evidently there is a gradual transition 

 fiom the one formation to the other at this point, and if marine fossils 

 were found to the bottom of the series Ave should expect them to belong- 

 to the same fauna as those in the upper beds. Four species of inver- 

 tebrates have been described by Meek and Hayden from the " Dakota 

 group at the mouth of Big Sioux river." Future studies may prove 

 that they belong to the Colorado fauna, but as I have not been able to 

 identify them satisfactorily with species that occur in the beds above 

 the Dakota elsewhere, it has seemed best to omit the description of 

 them. They are: 



Cyrena dakotensis. Mactra siouxensis. 



Trigonarca siouxensis. Arcopagclla? niacrodonta. 



Southwestward from this locality the Cretaceous occupies a broad 

 irregular belt through Nebraska and Kansas, lyiug unconformably on 

 the CarboniferDus, and farther south on the red beds that are usually 

 referred to the Jura-Trias. The strata dip gently to the northwest, and 

 in the western part of these states the Cretaceous is overlain by fresh- 

 water deposits of late Tertiary age. Since the beds are nearly horizon- 



J Rept. Geol. Sur. of Io-wn, vol. I, Doa Moines, 1870, pp. 285-29-1. 

 8 Kept. Geol. Sur. of Iowa, vol. II, pp. 196-199. 



