|Discoscaphites collected from Fox Hills Formation, South Dakota.|
There are a number of quite interesting highways traversing east-west across northern South Dakota; however, my favorite is the somewhat lonely SD 20 heading east from Reva Gap to Mobridge on the Missouri River. Actually the road starts at the Wyoming-South Dakota state line at Camp Crook. This route crosses over a variety of rocks that are somewhat unfamiliar to readers in Colorado Springs. Above the ubiquitous Pierre Shale are the, in ascending order, Fox Hills Formation, Hell Creek Formation, Ludlow Formation, Cannonball Formation, and Tongue River Formation.
The Fox Hills, a unit that is also present around Colorado Springs, grades upward from the marine shales of the Pierre into marine and then into marine and brackish water sandstones. The unit represents the end of the great Western Interior Seaway and rocks are essentially shoreline deposits of the receding waters. Although the Fox Hills near Colorado Springs contains a few fossils, mostly small clams, there are parts of the formation in north-central South Dakota that are extremely fossiliferous. Some concretion layers have produced literally thousands, perhaps tens of thousands, of beautiful ammonites (and a plethora of other invertebrates and vertebrates), especially of the genera Discoscaphites , Sphenodiscus, and Hoploscaphites. They represent some of the youngest ammonites in the U. S. (remember the ammonites became extinct at the end of the Cretaceous).
Somewhere around the town of Timber Lake, a “long time ago”, I was able to collect some of these concretions with most specimens going to a museum. However, I was able to hold on to a single specimen. Perhaps individuals may still collect on private lands.