[1] In the "mile high" plains in the center of the continent, the named layers preserve marine fossils from the Late Cretaceous Period.

in 1862 to the gray marine shales, often chalky in the middle layers, lying above the terrestrial Dakota Sandstone and usually below the massive limestones at the base of the Niobrara Chalk.

The name was taken from the type outcrop at Fort Benton, today a small city in Montana on the Upper Missouri River.

[5][6] In the lower Missouri River, west of Yankton, South Dakota, the distinction between the Benton and the Niobrara is very clear.

Iron sulfide in the bentonite seams converts to rust when exposed to air resulting in orange lines across exposures of Benton shale and chalk.