Colorado’s present-day topography and natural resources have been millions of years in the making. Initially, the land mass known today as “Colorado” was located south of the equator as part of a supercontinent. As the continent drifted, geological processes broke it down and covered it with seawater. During this continental drift, the equatorial heat of Colorado’s greenhouse climate allowed cold-blooded dinosaurs to thrive. The mountains had yet to diversify the low-lying topography, and saltwater covered much of the land.
One hundred million years ago, throughout the Cretaceous period, dinosaurs inhabited the shallow sea’s coastal margins. As they stalked the sandy beaches along dense temperate forests and scattered palm trees, carnivorous dinosaurs dined on eggs, insects, fish and one another. Meanwhile, herbivores such as Colorado’s state dinosaur, the Stegosaurus, grazed on fern and the abundant foliage.
Robert Letscher, assistant professor of chemical oceanography at the University of New Hampshire, explains the Western Interior Seaway as: “a shallow sea stretching all the way from the Arctic Ocean down to the Gulf of Mexico running through present-day Colorado, Kansas, Wyoming, Montana and New Mexico.”
Eighty million years ago, shorelines were present in Western Colorado as the seaway flooded the state. The eastern shoreline became Colorado’s “front range,” eventually resulting in the Dakota Hogback formation visible from Interstate 70, west of Denver.
This seaway contained bony fish, swimming reptiles, and cartilaginous fish like sharks, rays, skates and squid. A plethora of ammonites, an extinct type of mollusk with a snail-like shell, also presided in the sea. Beachy coastlines and sediments formed the sandstone mesas and cliffs that held the dwellings of Pueblo people, now preserved in Mesa Verde National Park.
Letscher told The Sun, “There were corals and lots of coccolithophores, a type of marine algae that secrete a calcium carbonate just like coral do. Calcium carbonate is the mineral that makes up limestone.” Limestone and sandstone settled to the coastal marine seafloor, later to be cut by meandering rivers exposing the layered rock walls of Glenwood Canyon. The continent containing Colorado continued on a northwest trajectory until Colorado reached its current latitude 75 million years ago.
Sixty-eight million years ago, the climate started to cool, and ice began to form around the North and South poles. Geological processes started to uplift and depress the granite continents, impinging on the mantle below; the first mountains rose, tilted and contoured by adjacent faults into the rock layers seen today.
An uplift event called the Laramide Orogeny angled the Maroon Bells into their current configuration. The floor of the sea’s silt deposits remains as sedimentary rock amongst the jagged mountains. As the sea receded, it left an environment of low-oxygen coastal swamps filled with stagnant water. Cyprus trees grew in dense groves in the flooded areas.
In the lush subtropical climate, Letscher said, “organic material built up over millions of years without being consumed by insects, bacteria or fungi … the vegetation that didn’t get eaten just stagnated in low-oxygen coastal swamps, eventually transforming into coal, natural gas and oil.
He continued, “The reason why they are called fossil fuels is that they are fossilized organic materials.” Colorado has an abundance of energy-dense fossil fuels, with over 121,800 oil and gas wells drilled as of 2022. Discovering the prolific coal resources in the Crystal Valley, John Osgood built Redstone in 1902 to house the coal miners. After closing in the early 20th century, Mid-Continent Coal and Coke Company reopened these mines in 1956 and supported Carbondale’s economy until 1991, when the mines were shut down again.
Sixty-six million years ago, an enormous asteroid hit the earth, rendering most of the non-avian dinosaurs extinct. Dinosaur National Monument preserves the oceanic sedimentary rock responsible for fossilizing the bones of over 1,500 species, displayed on the Quarry Wall.
The avian dinosaurs who survived the asteroid collision evolved into modern-day birds. The climate continued to cool; consequently, the warm-blooded mammals, more competitive in cooler temperatures, began to take over the ecosystems. During this time, the Cretaceous period ended, and the Paleogene period began.
Forty to ten million years ago, Colorado’s volcanic igneous activity surged, ejecting basalt rock from local volcanoes, from which the town of Basalt got its name. While continents consist mainly of granite-type rocks, basalt rock originated in the oceanic crust of the ocean basin. The great mudflows, resulting from volcanic flows, would petrify the bases of trees in valleys, petrifying the Redwood tree stumps found at the Florissant Fossil Beds National Monument west of Colorado Springs.
Letscher explained the igneous activity as follows: “The crust starts to pull apart due to forces below it inside the mantle. There is lots of fire and magma. Lava is liquid rock flowing on the earth’s surface … This basaltic type of volcanism is what can break apart a continent and create a new ocean in-between.”
Twenty-five million years ago, basaltic volcanism became the dominant form of igneous activity. Colorado wasn’t split apart by this volcanic exertion, but basalt rock spewed across the region, sizzling at over 2,000 degrees Fahrenheit. The porous black rocks are still prevalent in the area and make excellent garden beds due to their heat retention capabilities.
During the Colorado Gold Rush, gold, silver and copper from the basaltic igneous activity lured many European settlers to the area. The Crystal Mill, near Marble, was built in 1893 to help power a silver mine during this era. Some basalt flows composed the mesas in Western Colorado, including the Flat Tops Wilderness Area and Grand Mesa.
Also, a coveted resource, Yule Marble, Colorado’s state rock, was formed by this igneous activity. During the course of hundreds of millions of years, over 10,000 feet of sediment buried the limestone. Then, as magma flowed into the adjacent rock, 12 million years ago, the extreme heat metamorphosed the limestone into marble.
Eighteen million years ago, uplift and erosion shaped the Rocky Mountains and exposed the marble to the surface. The Rio Grande rift elevated the highlands to altitudes of over 9,000 feet from their base. Glacier and water erosion then carved Colorado’s formidable peaks and valleys.
Throughout hundreds of millions of years, a cooling climate, rifts, faulting, uplift and volcanic activity made our state the mineral-rich, colorful landscape it is today. Understanding Colorado’s vast geologic history informs the stewardship of the state’s plentiful yet finite resources. Looking to the future, Colorado has become a leader in renewable energy, transitioning its economy toward sustainable resources so that humankind can survive and thrive across the Rocky Mountains and beyond.
Much of the information in this article came from a video called “A Brief History of Colorado Through Time” from the University of Colorado Boulder. For more information and visual representation of Colorado’s geologic history, visit: www.bit.ly/COGeology