IOWA ACADEMY OF SCIENCE 
201 
200 feet. It rests directly upon Carbonic limestones. In Nebraska the 
Missouri bluffs are composed of this rock for many miles. By means 
of well-sections this rock is easily traceable far to the east, southeast and 
south, and there becomes the typical Nishnabotna sandstone — the basal 
member of the Cretacic section of the state. It everywhere rests in 
marked unconformity upon the older rocks. 
In Bain’s general geological section of Woodbury county ( ( Iowa Geol. 
Surv., vol. Y, p. 256) No. I is the Nishnabotna sandstone, Nos. 2 and 3 
the Sergeant shales, No. 4 the Ponca sandstone, Nos. 5 to 11 the Wood- 
bury shales, and No. 12 the Crill chalk. 
Iowa’s Mesozoic section was formed in Mid-Cretacic times. It com- 
prises no less than seven well defined members. It has a maximum 
thickness of over 800 feet. 
