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The Cut in I-70 at Morrison: The geology of Colorado’s Dakota Hogback

May 31, 2021

I don’t know how many times we’ve passed it entering the mountains on I-70 heading into Colorado’s high country over the past 52 years. 1000 times? Probably more. I did stop about thirty years ago, when the geological markers had not yet been trashed with graffitti by whomever trying to deny the reality of evolution – human or otherwise. Now the paths are in a state of disrepair, those information signs helping a visitor navigate through some 80 million years of geological history have either been removed or seriously and systematically written over by graffiti, the remaining signs themselves bludgeoned with either hammers or axes, as if violence could erase the earth’s 4.5 billion year history.

But still, that cut in the road remains – a breathtaking manifestation of Mesozoic sedimentology and Cenozoic tectonic plate activity, evidence by its very existence that the age of the earth is a gazillion times older than the 5200 years right-wing religious wackos of all faiths would have their followers and the rest of us believe. If the geology of the area is intensiviely studied it is in part because of its dramatic geological history but also because it is believed that some 10,000 feet below the surface are petroleum bearing strata containing potentially huge volumes of oil and gas.

Thinking that it was, once again, time to take a stroll down geological memory lane, I headed back up to the 400 high, half-mile long road cut by the Morrison exist of I-70 to poke around that slice of some 40 million years of geological formations that are visible today as clearly delineated strata ranging in hue from bright yellow and orange to red, green, brown, tan, gray and black. The oldest strata on the western side of the cut date from around 145 million years ago, the youngest at the eastern end some 95 million years old when Colorado was “a broad flat plain populated by dinosaurs.”

“The cut” revealed to as the Dakota Hogback, a steeply sloped ridge that extends for southern Wyoming across the Front Range of Colorado at the eastern edge of the Rocky Mountains into Northern New Mexico. It includes such geological wonders as Dinosaur Ridge in Morrison, Colorado, the Red Rocks Amphitheater, the Flat Irons of Boulder and Colorado Sprongs’ Garden of the Gods.. The Dakota Hogback is a remnant from a period of mountain-building that occurred approximately 70 million years ago, when the modern Rocky Mountains were uplifted in a process that tilted and exposed older, underlying rock strata — some fossil-bearing.

During the 1971 construction of I-70, an estimated 25 million tons of rock were removed from the road cut, revealing some 40 million years of geological formations that are visible today as clearly delineated strata ranging in hue from bright yellow and orange to red, green, brown, tan, gray and black.

As Pueblo Chieftain reporter Lynda La Rocca noted in a 2017 article:

Walkways along both sides of the road cut are lined with interpretive signs (many, however, have unfortunately been defaced and vandalized) explaining the origins of this sedimentary sandstone, mudstone, limestone, siltstone and shale, and illustrating how different strata represent different paleoenvironments, including freshwater marshes, beaches, tidal flats, tidal plains and sea bottoms.

Indeed, at a certain point one could smell the odors of past organic materials, still seeping their way to the surface.

A Wikipedia article notes that prior to the emergence of the Rocky Mountains, some 67 million years ago, the region was a shallow sea. As sediments fell to the bottom of the water, they were compressed into soft sedimentary rock. Thus, oyster and clam shells, sand, and mud built slowly into layers of sandstone, shale, limestone, and “mudstone.” As the Rocky Mountains rose over the last 67 million years, up to nearly 30,000 feet above sea level, the soft sedimentary rocks were quickly weathered and washed away from the high mountains. That soft matter, deposited in the layers visible in the cut was kept from eroding by a top layer of hard sandstone, the Dakota Sandstone, from which the ridge gets its name. This sandstone “cap” protects the softer shales and limestones beneath it from weathering and erosion.

“I hardly ever see anyone here.” These were the words of Roy Johnston, a local artist I mt there, exploring the formations on the north side of I-70 where I was also probing. We stopped and talked, shared a few observations about evolution – human and more general, both enjoying the wonder of it all. It was obvious from his red MAGA cap and my KGNU hat that had the circumstances of our meeting taken place in a different milieu that we might not have had such a friendly connection, but there we were before the enormity of geological history where those more mundane political differences didn’t amount to much, if anything. Had me thinking about how context “is everything.”

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