THE END: FOX HILLS FORMATION

EXPOSURE OF FOX HILLS SANDSTONE, NORTHWEST COLORADO SPRINGS

 

Most rockhounds in the Colorado Springs area are somewhat familiar with the several rock units deposited during the Cretaceous (~144 to ~65 Ma) in the Western Interior Seaway (WIS).  The most recognizable unit may be the complex of transgressive near-shore marine, beach, deltaic, and estuarine sandstones (mostly) of the Dakota Group (or Dakota Formation and also including, locally, the Purgatoire Formation). 
 

These rocks may be best known as the “Dakota Hogback”, a prominent topographic feature along much of the eastern flank of the Colorado Front Range.  The Graneros Shale, overlying the Dakota, is a dark-colored shale representing deepening waters (transgressing seas) and deposits of offshore mud.   As the seaway continued to deepen, the limy deposits of the Greenhorn Limestone, Carlile Shale, and Niobrara Formations were deposited.  These limey muds were followed by deposition of thousands of feet of marine muds laid down many miles from the shoreline.  This mud became known as the Pierre Shale and is present under nearly all of eastern Colorado.  Most of these Cretaceous rocks described above are well exposed in or near Garden of the Gods Park and Red Rock Canyon Open Space.  The Pierre Shale can readily be observed in the road cuts along Uintah Street leading west from I-25.

Perhaps 70 Ma the early Rocky Mountains begin to appear (Laramide Orogeny) and the WIS started its retreat from Colorado.  The near-shore marine, lagoon, and beach sands of this regressive sea are known as the Fox Hills Sandstone but are not well-exposed in the Colorado Springs area; however, there is a cut along an abandoned road near north Centennial Blvd., just south of where it joins West Woodman Road.  At this locality the Fox Hills is composed of a brownish-orange, poorly cemented sandstone and sandy shale.  If not for the road excavations the unit would be covered with vegetation.

I have examined the known local exposures of the Fox Hills for a number of reasons.  One, the unit sort of brings a closure to “what comes around goes around”---the Dakota Formation represents the beginning of the vast interior seaway while the Fox Hill signifies the end of the WIS, closure if you will.  Two, numerous vertebrate fossils, including dinosaurs, have been collected from other states where the Fox Hills is exposed.  And third, the Fox Hills in South Dakota has produced some of the most beautiful ammonoid fossils that I have observed.  Many are covered with iridescent mother-of-pearl that shines and reflects like opal.  In fact, collectors value the specimens for this mineral termed “ammolite”, and use it in jewelry construction.

I first ran into these fantastic specimens ~45 years ago while rummaging around northwestern South Dakota during a field trip.  We were breaking open concretions and these wonderful specimens, mostly of Discoscaphites, were abundant.  It seems as if most were nabbed and turned over to a museum!  This locality is one of those places where I always wanted to return for a second time; however, thus far it has not been in the cards.

About the only fossil that I have collected from around here are numerous specimens of a small bivalve termed Cymbophora. 

To complete the Cretaceous story, the Laramie Formation, so well-exposed in Ute Valley Park (and described in other articles), overlies the Fox Hills while the Dawson Formation spans the Cretaceous-Tertiary boundary.

 

Discoscaphites sp. COURTESY OF www.fossilmuseum.net

Cymbophora sp.FOX HILLS SANDSTONE