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

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Geologic Time Period

The Pierre Shale Formation

Pierre Shale on Rooney Road

Pierre Shale - Close-up of an ancient seabed. Mud and sand coming off an eroding shale layer is suggestive of the ocean floor as it existed some 70 million years ago.

An ancient site...

It has all the appearance of of an ancient shipwreck, the protruding ribs all that remain of a ship lost in some sudden storm or disaster. Momentarily caught in the gaze of some deep-sea camera sweeping the floor for long-lost treasures or potential archeological sites, the sea floor casts shadows across an eerily silent world.

If the camera and lighting combine to create a world of light and darkness, what is revealed is not a shipwreck, but an ancient seabed  What appear to be the ribs of a sunken ship are something else - a hardened layer of seabed protruding from a hillside, somehow resistant to the elements which are destroying the surrounding stone. The stone, which time and pressure created, is now returning to its origins, the tiny particles of clay, silt, and sand which drifted down and settled on the ocean floor some 70 million years ago.

Shale is primarily composed of clay fragments, technically defined as particles having a diameter of less than 1/256th mm.  Almost too small to be considered "particles," they would have turned an otherwise clear blue Cretaceous Sea to an opaque cloud, not blocking all light, but limiting vision to a few hundred feet.

In a world routinely patrolled by giant predators, such as sharks, mosasaurs, and plesiosaurs, the murky waters may have provided some measure of invisibility.  Yet, the defense was something of a two-edged sword.  If predators had trouble seeing their prey in the murky waters, they could themselves approach unseen until the final few feet of an attack.  The predators were indeed giants. Carcharadon megalodon, possibly an ancestor of the modern-day great white shark, may have reached a length of 15 meters, or 50 feet, and weighed over 50 tons, bigger than a killer whale, and almost the size of a sperm whale.  Mosasaurs, with a crocodile-like head, a lizard-like body, and a 50-foot length, were another large hunter.  The plesiosaur, another 50-foot monster, usually had a long snake-like neck protruding from a plump body and equipped with four flippers for propulsion, somewhat resembling the images shown of the Loch Ness monster of Scotland.

Whatever role the murky waters played in hunting, their geological legacy is a mixture of rock layers. Sand and organic matter mixed with the clay, sometimes intermixed, sometimes creating separate layers of shale and sandstone.  Storms, waves,  and currents combined with animal or plant activity to stir up the sediment.  Sand and pebbles, eroding off distant hillsides, might be deposited as an alluvial fan at the mouth of a river or stream.  The occasional or periodic flood would move the larger grains further and re-deposit them.  The lighter mud and clay would remain suspended for a longer period, to be carried a greater distance, to settle as a separate layer over the sand.

The relatively thick layers suggest that conditions may have remained relatively stable for long periods of time.