Niobrara Chalk - Bloomington, NE
Posted by: 
The Snowdog
N 40° 03.950 W 099° 02.240
14T E 496816 N 4435064
The Niobrara Chalk Formation underlies most of the Great Plains of the US and Canada; this outcropping is south of Bloomington, Nebraska.
Waymark Code: WM13NHJ
Location: Nebraska, United States
Date Posted: 01/16/2021
Views: 4
The Niobrara Chalk Formation was deposited between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian stages of the Late Cretaceous. It is composed of two structural units, the Smoky Hill Chalk Member overlying the Fort Hays Limestone Member. The chalk formed from the accumulation of coccoliths from microorganisms living in what was once the Western Interior Seaway, an inland sea that divided the continent of North America during much of the Cretaceous. It underlies much of the Great Plains of the US and Canada. Evidence of vertebrate life is common throughout the formation and includes specimens of plesiosaurs, mosasaurs, and pterosaurs as well as several primitive aquatic birds. The type locality for the Niobrara Chalk is the Niobrara River in Knox County in northeastern Nebraska. The formation is a rich source of fossil vertebrate remains and has been extensively explored since its discovery in 1870.
This outcropping of Niobrara Chalk is on a bluff on the north side of the road. There is not much room to park, so pull over to the side of the road and use your flashers. Do not attempt to climb this bluff, the rock and chalk is soft and unstable. This chalk is actually a "marl" which is a mixture of chalk and clay. The grey color is due to organic material within the chalk. As the organic material is weathered by exposure, it becomes white. This chalk has a bit of iron in it, which provides orange and red highlights in the rock.
Additional Sources:
https://geokansas.ku.edu/chalk
Nebraska Geology Cretaceous Webpage