Dinopedia
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The Three Forks Group is a stratigraphical unit of the Famennian age in Montana, USA.  The Famennian age was the last age/stage of the late Devonian.  It was at that time that the first tetrapods appeared in the seas.  Major minerals found in this formation are dolomite, mudstone, and shale.

Dolomite

Dolomite

Mudstone

Mudstone

Shale

Shale

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