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 - Erosion-resistant layer of an ancient seabed juts out from an exposure of Pierre Shale along Rooney Road.

The Pierre Shale

Sand dunes and seabeds...

By Jack Barkstrom

Coming east on I-70 out of the mountains, the narrow off-ramp to I-470 can be nerve-wracking at the normal exit speed of 60 miles an hour. With Green Mountain looming ahead, it requires an intense concentration to stay within the concrete barriers on either side - in addition to tapping the brakes. Sometimes noticeable on the right, just before the Rooney Road overpass, is a curious white formation. It can be mistaken for a snow bank in winter, particularly at night.  But its white surface reflects brightly even in summer.

What appears to be a "snow bank" is actually what is left of an ancient sand dune. In time, it can be placed somewhere between the retreat of the great inland sea, some 70 million years ago and the dinosaur extinction of 65 million years ago.  Ocean depths had been replaced by rivers, plains, and river deltas, as well as swamps, with rocks and debris, including coal, deposited in giant flood plains, as the sea was pushed back into what is now Kansas.  It had become dry enough to sustain pockets of sand, which, over time, accumulated into sand dunes, much like the Lyons Formation sand dunes of 280 million years ago, or the Great Sand Dunes of southeastern Colorado today. The grains of sand turned to sandstone.

The white sandstone at the Rooney Road roadcut, part of the Laramie Formation, may be the most eye-catching roadside feature, but it extends for just a few hundred feet or so. For a quarter of a mile, as Rooney Road bends to the southwest, Laramie sandstone blends into the yellow-brown sandstone and shale of the Fox Hills Formation, then to the grey and brown sandstone and shale of the Pierre Shale formation, a time span of perhaps five to ten million years.

The Cretaceous Western Interior Sea

Sometime after the Iguanodon herds moved along the seashore of Dinosaur Ridge, leaving their tracks about 100 million years ago, the inland sea began advancing west, beyond the Colorado border, to cover the eastern half of Utah. It was a relatively shallow sea, as seas go, although at its greatest depth in the Denver or Colorado region it was 600 feet deep.  The ocean which existed here has been called by several names, from the Cretaceous Western Interior Sea to the Western Interior Seaway to just the Cretaceous Sea.  It extended from the Arctic to the Gulf of Mexico, covering nearly all of Colorado and Kansas, as well as eastern Utah. It would exist, as a sea, until around 70 million years ago, when it receded.

The geological result of 30 million years of oceanic existence is the Pierre Shale, a marine deposit of 8,000 feet of hardened ocean-bottom ooze and mud, the thickest geological formation in the Denver region. For all that, the exposure at Rooney Road is only about a thousand feet.

Viewed from the road, the layers of ocean floor appear nearly vertical, the result of the uplift of the Rockies, beginning around 72 million years ago. Grey, alternating with yellow-brown and brown, the layers are relatively thin, some little more than parallel lines or traces up and down the slope. In some places they seem to bend or swirl, testament to an uneven ocean floor, or evidence of unknown geological pressure zones brought to bear on a particular region.