WHOA, NICE CEPHALOPODS!

Coiled cephalopod, Hoploscaphites sp., in concretion.  This same concretion contains Baculites, gastropods, and bivalves.

 South Dakota has an amazing variety of geological features present on the landscape and an interesting array of formations cropping out along the roads.  I have a particular love for the state since in 1965 I left my home in central Kansas and traveled north for additional education.  It was a long trip, 325 miles, for a small town kid sort of striking out on his own (my undergraduate education was less than 100 miles away from home, and in the same state).  In fact, it was downright scary!  

The first thing that I noticed upon my arrival in eastern South Dakota was the fact that most of the rock exposures were covered by what we lovingly called “glacial crap”.  The second was that in some of the stream valleys, where a few exposures existed, there were old friends from western Kansas—the Niobrara Formation, Carlile Formation, Dakota Formation, etc.  These rocks made me feel more “at home”!

But the real excitement came from our field trips to “West River” and the badlands and the Black Hills.  I had visited these areas the previous summer on a road trip with a couple of college buddies but at age 20 our minds were mostly on other “things”.  We poked around at the rocks, collected a few, did some hiking but always kept our eyes out for the other “things”.  I knew that I wanted to return to South Dakota with a knowledgeable guide.  And so I did.

In the summer of 1966 I started a thesis project West River and to break up the driving from Chamberlain (my work station) I often stopped at the little community of Wasta.  Today Wasta is located at Exit 98 on high speed (75 mph legal but often faster) I-90.  In the “old days” Wasta was just a little store on U.S. 16 where one could purchase a cold soda, and explore the Pierre Shale.

I had previously examined a few outcrops of the Pierre in extreme western Kansas (Wallace County), mostly near McAllaster Buttes, and found the formation to be rather unfossiliferous (a few nondescript baculites) and kind of “ugly”.  The Pierre represents deposition in the deepest waters of the Western Interior Seaway during the latest Cretaceous, and many tens/hundreds of miles from the nearest shoreline.  I sort of put the formation in the back recesses of my mind---until I explored South Dakota!

The Missouri River Trench, as South Dakota geologists call the feature, exposes the Pierre from near the South Dakota—Nebraska border (Fort Randle in the south central part of the state) all the way north into North Dakota.  In fact, there are even exposures along the Missouri River near Yankton in the south eastern section.  Geologists from South Dakota Tech have been studying the unit for years and have described a number of new vertebrate critters.  In addition, invertebrate fossils are numerous.

From the Missouri River Trench west to the Black Hills (and beyond) the Pierre crops out at numerous localities but especially along the major streams. I found that special exposures were located along the Cheyenne River near Wasta.  Suddenly the Pierre lost its ugliness (my earlier opinion) and became exciting!

Exposures of the Pierre along the Cheyenne River yield concretions and these concretions are often fossiliferous.  Most fossils are mollusks including the pelecypods Inoceramus sp. and Pteria sp., the cephalopod ammonites Baculites sp. and Hoploscaphites nodus, the gastropods Margaritella flexistriata, Amauropsis sp., Drepanochilus sp., Anisomyon sexculcatus, the scaphopod Dentalium gracile, as well as the coral Micrabacia americana (Sharman, 2008).  But most collectors are not interested in the snails but in the beautiful straight-shelled cephalopods (Baculites sp) and the coiled cephalopod Hoploscaphites.  These ammonites often have mother-of-pearl present and the suture patterns on the shells are quite distinct.  They are really spectacular fossils and collectors will notice South Dakota specimens in museums around the world.

Baculites sp. in concretion.

 Today, as I understand the situation, the private land is very difficult to access and collect.  Therefore, if you plan on collecting I would suggest contacting the various rock and mineral clubs or rock stores located in or near the Black Hills.  I have found them to be a wealth of information.

It is not always fun experiencing the aging process but---the alternative is not very appealing either!  What I do have are my memories and I often think of those days where collectors could wander around South Dakota, visit with the ranch owners in a friendly fashion, and bash open concretions looking for those ammonites.

REFERENCES CITED

Sharman, G. R., 2008, Petrology, Geochemistry and Paleontology

of Fossiliferous Concretions from the Cretaceous Pierre Shale, South Dakota [abs.]: Geological Society of America Abstracts with Programs, v. 40, no. 5, p. 85.

mike