The Brule Formation was deposited between 33 and 30 million years ago, roughly the Rupelian age (Oligocene).

It is a sequence of fine grained clastic rocks (claystones, mudstones, siltstones) interbedded with freshwater carbonates, volcanic ash (tuff), and sandstone.

[1] The formation was named by N. H. Darton "for the Brule Indians, who once roamed over Pine Rldge Ind.

They expand considerably and include, at top, beds which appear not to be represented in the typical regions.

I have Introduced the designation Brule clay and separated the underlying Titanotherium beds as Chadron fm.

[4] Notable among the local fauna are Bathornithid birds, ranging from the highly varied wetland-dwelling Bathornis species to the gigantic Paracrax.

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