An excerpt from Badlands: Its Life and Landscape (Joy Keve Hauk):

“The Brule Formation overlies the Chadron and represents the first half of the Oligocene Epoch. It is the one from which the fantastic shapes characteristic of the Badlands are carved. A notable feature of the Brule is the widespread color-banding, mostly yellow-beige and pinkish-red. Thin, discontinuous sheets of limestone from an inch to a foot or more in thickness show where the Chadron ends and the Brule begins. The limestone suggests there were shallow ponds at that time. Ten to 40 feet above the limestone sheets is the (Brule’s) lower nodular layer. Here are found many rocky nodules (concretions) and a rich deposit of fossils, especially of turtles and oreodonts. The climate must have been relatively mild and moist to grow food for the great numbers of oreodonts. The nodular layer is also called the red layer because it is reddish in color (…..) Some of the layers in the Brule (formation) are loose clay, some are firm, others are hard. These layers wash away at different rates, and it is this variation in weathering that makes the amazing rough landscape of the Badlands.”

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This sketch was done from the “bottom of the Wall” so there are formations in the foreground (pale white, softly eroding) representing the Chadron formation. There is a sod table (topped with dry yellow grass, middle right) which might be Chadron, or mingled sediments washed from upper layers of the wall. And behind these features is the lowest layer of the Brule, red layer, much more “upright” in structure, meaning it sheds water and has a greater slope than the Chadron level. It includes a nodular layer, above which it becomes red again, with pale yellow banding.

Note regarding oreodont fossils in the Brule formation. What is an oreodont, you ask? They were mammals with pig-like body structures, but they grazed and chewed cud like cows. There were a LOT of them, gauging by the sheer number of bones they left behind. They covered the plains in great herds from 40 million years ago until they became extinct (3 million years ago). No oreodont bones were disturbed in the sketching of this scene, but of course the eye looks for them everywhere…