Under the Rock Pile: How old are the Colorado Rockies | SummitDaily.com

Under the Rock Pile: How old are the Colorado Rockies

Alexander Gunow
Under the Rock Pile
Alexander Gunow
Special to the Daily |

What is it about Colorado that gives us some of the best skiing, riding, cycling and hiking in the world? It’s the mountains, of course! This elevated land provides us with great terrain and vistas to explore as well as an exhilarating climate for exercise and recreation.

If you are lucky enough to be a resident of Summit or Eagle counties, or if you are an occasional visitor, it won’t take long to wonder how these mountains came into existence or to ask how old they are.

For the age of the Colorado Rockies, the short answer is between 70 to 80 million years old (MY).

How do we know this?

It may be hard to imagine, but, just 80 MY ago, Colorado was at or beneath sea level. This is confirmed by the existence of 80 MY old oceanic sediments (shale) or shoreline deposits (sandstone) found throughout the state.

If we could go back 80 MY ago, we would witness an ancient, shallow sea with a maximum depth of about 700 feet. Based on the compilation of the fossil record and age dates for sedimentary rocks, here is how this area of Colorado appeared 80 MY ago:

This huge inland seaway, including most of Colorado, is known as the Cretaceous Seaway. The climate in Colorado was balmy, with nearly tropical temperatures. The animal life within the seaway was inhabited by ammonites (akin to the nautilus), sea turtles, numerous species of fish and the swimming reptile, Plesiosaur, which grew up to 70 feet long. As erosion took place from the highlands to the west and east, the bottom of this seaway accumulated thick sequences of mud.

At about 70 MY ago, we would observe the seaway beginning to recede, and islands emerging from the muddy blanket of the shrinking seaway. As the western U.S. began to rise and the ocean level began to recede to the northeast, thick sediments underlying this shallow ocean were pushed up by tectonic forces from the west; the uppermost layers of the crust began to fold as they rose. Deep within the crust, guided by an ancient northeast trending fracture, igneous rocks rose from their abyss, uplifting or emplacing through the surrounding host rocks to form plutons (intrusive igneous rock crystallized from magma), dikes (a subvertical sheetlike intrusion of magma) and sills (a nearly horizontal intrusion of magma) in the upper crust.

We would be witnessing the beginning of the present day Rocky Mountains in a mountain building event known as the Laramide Orogeny. In Colorado, one the earliest of these intrusions is located in the southwest portion of the state, forming the 72 MY Ute Mountain plutons. Between 70-65 MY ago, igneous activity would be found extending northwest from the present location of Salida to near Eagle and trending northeast from Aspen toward Breckenridge. On the beaches, and emerging land we would find numerous species of dinosaurs dwelling among the vegetation of palms and broadleaf ferns, including T. Rex, Triceratops and Stegosaurus (Colorado’s state dinosaur). Living among these ferns, we see early, rodent-like mammals, burrowing amid the ancient forest floors. Perhaps we would even see Pteradactyles flying, with wing spans over thirty feet, above the ancient waterway. This was truly like a scene portrayed in Jurassic Park.

By 65 MY ago, nearly all of Colorado was above sea level, and it was at this time that a major asteroid impact in the Gulf of Mexico brought severe climatic distress, wiping out the dynasty of the dinosaurs. During the last 65 MY and with continued plate tectonic activity on the west coast, there have been episodes of renewed faulting, folding and igneous intrusions in Colorado. All the while, the Colorado Rockies have been in a see-saw battle between this episodic uplift and constant erosion. The finishing touches on our topography was provided by glaciation events; with the last major glacial epoch ending just 12,000 years ago.

Alexander (Sandy) Gunow received his doctorate in geology from the University of Colorado. He also he worked as a miner at the Henderson Mine and eventually worked as an exploration geologist for the Climax Molybdenum Company. He is currently enjoying his retirement in the mountains near Silverthorne.


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