Badlands National Park is a wonderful, feature-rich area for geologists. The strata here consist of the White River Group, a set of fairly young rocks geologically speaking (~35 Ma). These Oligocene rocks formed mainly by stream deposition and volcanic ash, and are quite soft. They are now being eroded away very quickly by streams cutting through the area.
The stops discussed below are all located on my Geology of Badlands Natl. Park map.
On Highway 240 at the Pinnacles Overlook, some spectacular examples of clastic dikes are present, one of the more unusual geologic features in the park. These are quite fine-grained, cutting through other layers, often vertically or at high angles. This particular dike is roughly a foot wide and stretches for many tens of feet in either direction.
At the Yellow Mounds Overlook, a terrific normal fault offsets the yellow and gray layers of the Yellow Mounds Paleosol and the Chadron Fm. The fault strikes ~N70W and dips ~60° SW. The Yellow Mounds Paleosol is a deeply weathered material found at the base of the Badlands stratigraphic sequence. This particular layer was weathered roughly 65-50 million years ago. The gray Chadron Formation formed ~50-40 million years ago. The fault occurred much later of course, but the timing of it is unknown.
A mixture of igneous & sedimentary processes were at work in this cross-bedded ash layer, located at the end of Door trail on the east end of the park. While the strata in the Badlands are dominantly sedimentary rocks deposited by ancient streams, active volcanoes at the time contributed significant amounts of ash. Here a layer of ash is cross-bedded.
A few more images from this beautiful park:
For the most part, the Badlands is only developed in the northeastern section of the park along Highway 240. There is, however, some development in the central section, along a dirt road called Sheep Mountain Rd. This takes you to the top of Sheep Mountain, a flat-topped butte. The first and primary overlook offers a view to the west toward the Black Hills, with Harney Peak visible in the distance. From there, the road gets really poor. This area of the park is not well developed; to continue from here to the end of Sheep Mtn. Rd. requires a vehicle with high clearance, and I'd recommend 4WD. Most visitors to the park stay on the much better developed northern area. Nonetheless the views up on Sheep Mountain are very rewarding.
All of these images and more are also present in my Geophotography Portfolio. For additional information, check out these references:
"Virtual Field Trips for Historical Geology: Badlands National Park", by Brett Dooley and Alton Dooley, Jr., available on iBooks.
"Geologic Formation", Badlands NPS website: https://www.nps.gov/badl/learn/nature/geologicformations.htm