This mile-thick layer of dark gray mudstone and shale accumulated at the bottom of the Cretaceous Western Interior Seaway. Clams, ammonites, and baculites are the most common fossils. Their distribution tells that for millions of years, this seaway connected the Gulf of Mexico to the Arctic Ocean and covered all of what today is Colorado.

A) Fossil ammonite.
B) Collecting fossils in the Pierre Shale.
C) The Pierre Shale was deposits at the bottom of a shallow sea.
D) Outcrop on Rooney Road near Golden.

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