A Geological Tour of Denver, Golden, and Colorado's Front Range

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The Pierre Shale Formation

Pierre Shale on Rooney Road

Pierre Shale - View across Rooney Road of Dinosaur Ridge.

The Pierre Shale

On the western end of the Rooney Road exposure, where the road turns south through the valley of the Golden Fault, the Pierre Shale forms a gentle slope down to the roadbed.  Just across the road the Dakota Hogback forms the western side of the valley.  It would have been the shoreline of 100 million years ago.  The beginnings of the Cretaceous Sea, now known as the Benton Shale, can still be found along the hogback.  Missing, or at least partially buried under Rooney Road by more recent accumulations, is about 7,000 feet of the Pierre Formation.

Here, the distinctive layers of shale have become blurred.  Grey and yellow-brown lines seem to blend into an indistinguishable mud, slowly oozing off the slope.  Here and there, more solid formations stand out, as they resist the general erosion.