302 The Americcm Geologist. May, isas 
and Hayden, Dr. White applies to all the strata at Sioux City 
lying below the chalk the name of The Woodbury Sandstones and 
Shales, while the calcareous deposits composed of chalk and soft 
Inoceramus-bearing limestone he calls the Inocaramus beds. 
Below the mouth of Iowa creek, about three miles nearlj' east of 
Ponca, Nebraska, the Missouri river washes the foot of a high 
bluff in which Cretaceous strata, identical in all essential respects 
with those seen in Iowa above the mouth of the Big Sioux, are 
exposed to a hight of more than a hundred feet. The several 
beds of the preceding section, from 2 to 8 inclusive, are easily 
recognized, and at the summit of the section, cropping out from 
beneath the thick mantle of loess, are indications of the chalky 
beds of number 9. Farther up the river, almost directly north of 
Ponca, there is another splendid natural section which is more 
than a mile in length and at least 150 feet in hight. At the base 
of the section are the beds seen below the mouth of Iowa creek, 
while away above all the sandstones and shales lie from twentj'-five 
to thirty feet of rather hard chalk and Inoceramus-heaiTing lime- 
stone. There can be no doubt that the beds near Ponca, 
Nebraska, are the exact equivalents of beds in Iowa. Indeed 
one may look away from the exposure at the bend east of Ponca, 
across the plain which is here the combined valley of the Mis- 
souri and Big Sioux, for a distance of onl}^ ten or twelve miles 
to the corresponding exposures in Iowa. In the two bluffs that 
look toward each other from opposite sides of the plain, you may 
trace the same succession of strata that, but for the erosion of 
the two great streams, would still be continuous across the inter- 
vening space. Furthermore the beds are about equall}' well 
developed on both sides of the valley. 
Now the exposure at the bend of the Missouri, three miles be- 
low Ponca, Nebraska, is described in detail by Hayden in the 
First Annual Report of the United 'States Geological Survey of 
the Territories, 1867, pp. 47 and 48. The chalky, marly or 
calcareous beds, which are the exact equivalent of the Inoceramus 
beds of Iowa are referred to the Niobrara group. The dark col- 
ored shale, identical with number 8 of the preceding section is 
called the Fort Benton Group, while all the complex mass of 
alternating sandstones and shales in the basal part of the ex- 
posure is recognized as belonging to the Dakota group. 
Between Ponca and St. James, about thirty miles in a direct 
