The sediments that make up the Niobrara Formation (or Niobrara Chalk) were deposited in the Western Interior Seaway during the Late Cretaceous between 82 and 87 million years ago.
The sediments that make up the Niobrara Formation (or Niobrara Chalk) were deposited in the Western Interior Seaway during the Late Cretaceous between 82 and 87 million years ago.