COMPLETE SUCCESSION OF IOWAN CRETACIC TERRANES. 
BY CHARLES KEYES. 
For several reasons the. section of Cretacic strata in northwestern 
Iowa is of especial interest. It was in this section, near the mouth of 
the Big Sioux river, that for the first time in the New World the strati- 
graphic equivalents of the English Cretacic formations were clearly 
recognized. In this section true chalk, made up of countless myriads 
of microscopic organic remains, was first discovered in this country. 
This section is also a part of the classic locality for the Mid-Cretacic 
formations of America. 
Notwithstanding the fact that the Cretacic terranes constitute the 
surface rocks over a very large part of the state of Iowa their strati- 
graphic relationships were never clearly made. For a period of 
50 years, from the time of Meek and Hayden’s visit to the Upper Mis- 
souri country, in 1858, the exact geologic position of the several terranes 
was largely misunderstood. In all this time only a single correlation 
of any of the formations appeared which was eventually proved to be 
correct. This was my own reference in 1894 of the chalk beds in the 
extreme northwest corner of the state ' and the neighboring portions of 
South Dakota to the Niobrara horizon, those chalk-exposures farther 
southward, along the Big Sioux river, belonging not to the Niobrara 
as generally regarded but to a, lower part of the section. The relations 
of White’s Nishnabotna sandstone to the rest of the Iowa Cretacic sec- 
tion remained from the first enigmatical. Correlation, and naming of 
the several members displayed above Sioux City, with the southeastern 
Colorado sequences, 400 miles away, without a single intervening ex- 
posure of the rocks seemed unwise. 
Quite recently the logs of certain well-sections obtained near Sioux 
City have disclosed the long missing data for establishing a complete 
section of the Cretacic rocks of Iowa. This section may be expressed 
as follows: 
Coloradan Series, 
Dakotan Series 
7. Niobrara limestones 
6. Hawarden shales 
5. Crill limestones 
.4. Woodbury shales 
3. Ponca sandstone 
2. Sergeant shales 
.1. Nishnabotna sandstones 
.150 feet. 
.125 feet. 
.100 feet. 
.150 feet. 
. 25 feet. 
, 75 feet. 
200 feet. 
