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| 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.
mike