When I first laid eyes on the Badlands of South Dakota, I remember thinking to myself “This is pretty cool”. Now to be fair, I’ve said this many times during my trips to National Parks like this one. But when you see something like this up close, you can’t help but just stand in awe. What makes something like this even more amazing is a little knowledge of how it came to be.
The Badlands are situated in southwestern South Dakota between the Cheyenne River to the north and White River to the south. Sediments (sands, silts and clays) deposited over the region over the late Cretaceous through the Oligocene form the majority of the sedimentary stratigraphy in the park. Nearby volcanism deposited intermittent ash layers. The National Park itself covers just under 1,000 square kilometers. While today the region is pretty arid, during the Oligocene (40-25 mya) it was warmer and rich with life. Before that, about 68 mya, the badlands were at the bottom of a shallow sea (which is represented by the fossil-rich Pierre Shale). This area is known for having one of the largest and most complete assemblages of Eocene and Oligocene mammal fossils in the world!
A topographic profile (N-S trending) shows the Badlands are situated on the edge of a topographic high to the north, known as the uplands. This results in “The Wall”, which is aptly named due to the high relief between the uplands on the north side of the badlands and the base of the wall. This wall is where the town of Wall and its very successful tourist attraction, Wall Drug, get their names. If you’ve ever driven on I-90 through South Dakota, you know what I’m talking about (and most likely stopped there just to see what all the signs were about).
The wall began as a scarp on the northern side of the White River. As erosion chewed away, the wall migrated northward, and continues to do so. This leads to an important point: the Badlands are disappearing. The ridges, pinnacles, and ravines that make up the Badlands are formed by water and wind erosion over time following regional uplift which occurred about 5 mya. The erosional processes that created these features are the very same that are tearing them down. Erosion here occurs at a rate of about one inch per year. Runoff water carries sediments from the easily weathered sedimentary units and volcanic ash to the White River. Eventually, the Badlands will erode away… completely.