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  SD Fox Hills Formation

october 2016
Last Modified:
october 2016

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Location description

The Fox Hills Formation is a Cretaceous (Maastrichtian) geological formation with outcrops in the central-northern part of South Dakota and in the central-southern part of North Dakota. The Fox Hills Formation consists of marginal marine, yellow sandstone with shale interbeds. It was deposited as a regressive sequence during the retreat of the Western Interior Seaway in Late Cretaceous time. It is underlain by the marine Pierre Shale in the US and by the equivalent Bearpaw Formation in Canada. The Fox Hills is overlain by the continental sediments of the Laramie Formation

The marine parts of Fox Hills Formation are characterized by successive layers of fossiliferous concretions that are dominated by great numbers of one or two molluscan species.

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The Fox Hills Formation is moslty exposed West of the Missouri River. In South Dakota predominantly in Pennington, Haakon, Meade, Butte, Ziebach, Dewey and Corson County. In North Dakota mostly in Sioux, Emmons en Morton County. 

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The fossil biota of the Fox Hills Formation is enormously diverse, and includes plant fossils, sharks, rays and land and aquatic reptiles. These include turtles, crocodiles and mosaraurs, as well as several species of dinosaurs. The vertabrates from the formation are, however, mainly known from teeth and disarticulated bones, as articulated fossils are rarely found. 

Dispite the faunal diversity, the Fox Hills Formation is most famous for its incredibly beautiful ammonite fossils. The high degree of preservation of their natural mother-of-pearl shell makes them appear to be preserved as opal, often called ammolite. 

Fox Hills Formation ammonites normally occur in concretions, which are cracked open to reveal the fossils


There are 1 photographs of fossils from this location in our Fossil ID System.
Go to the SD Fox Hills Formation Species List to identify your own finds!

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