The Earth beneath the rugged mountains and serene plains of Colorado records an ancient saga. Broad tropical seas teemed with life, while reptiles roamed on shore. Continents converged and collided, building massive mountains, only to be torn apart by the movements of colossal tectonic plates. Volcanoes raged, and scalding fluids carrying dissolved metals churned through fissures to make future riches. A huge lake ebbed and flowed, linked to the fortunes of glaciers, and fields of towering sand dunes grew against walls of new mountains. Even today, in the paper-thin history of civilization, rocks and saturated soils rush downhill to remind us that we live on a restless, dynamic Earth.
Tropical Seas and Pangaea
Late in the Paleozoic Era, some 300 million years ago, when the Ancestral Rocky Mountains were being worn by weather to low hills, warm inland seas covered parts of Colorado. Life forms very different from those of today swam and flourished in the waters. Fossil records of those life forms are contained in layers of mudstone and limestone.
At the end of the Paleozoic Era, the restless continental plates collided again to create the supercontinent Pangaea. As the land rose and a Sahara-like desert of enormous proportions covered the continent, the interior seas retreated. But as large as Pangaea was, it too eventually began to be torn apart by the powerful tectonic forces that made it, and the seas returned. Life on land left distinctive marks, most famously the dinosaur footprints and fossil remains throughout the state.
As Pangaea fragmented, a breakaway piece called Laurentia drifted westward and a series of collisions with other, smaller continental pieces gave rise to the present-day Rocky Mountains. This phoenix-like rebirth of the Rockies started between 60 and 70 million years ago, during a mountain-building event called the Laramide Orogeny. As the peaks rose, they were eroded by wind, water, and ice. Wind and water carried the material eroded from the mountains, covering the area we now call the Great Plains. The rise of the Rockies continues today. The sedimentary rocks that formed the floor of the warm inland seas were warped upward against the new mountains, leaving huge triangular cliff facets, locally known as flatirons. From high viewpoints, it is easy to imagine the mountains pushing the flat-lying rocks upward.
Creation of Ore Deposits
As the tectonic plates jostled and collided, some were pushed deeper into the Earth, where they melted. The newly formed liquid rock burned upward to escape back to the surface, where it created volcanoes and lava fields. The mountains of south-central Colorado host the remains of an enormous ancient supervolcano, one that produced the largest single eruption known in the Earth’s entire geologic history. The single eruption rained volcanic material so fast that the thickly falling layers retained enough heat to weld back into solid rock. The molten material was mixed with gases, and during the eruption, the gigantic cavern below the volcano collapsed back on itself, creating a deep, wide crater approximately one mile deep.
This supervolcano is named the La Garita Caldera, after a town on the west side of Colorado’s San Luis Valley. The La Garita Caldera is only one of twenty-plus smaller but similar calderas throughout the San Juan Mountains. The calderas have local names, such as the Silverton, Lake City, and Creede calderas. The calderas are deceptively nestled together in the mountains, with the remnants of their circular outlines hinting at a violent history 25 million years ago.
The destructive eruptions of the volcanoes gave rise to a important facet of Colorado’s history: its vast mineral wealth, which lured a stampede of miners in the mid-nineteenth century. The molten rock beneath the volcanoes often gave rise to superhot and metal-rich waters that pushed for miles outward into cracks and fissures. The invading hot waters dissolved and reacted with the surrounding rocks to make rich ore deposits of gold, silver, copper, lead, zinc, and many other metal-bearing minerals throughout the state.
Great Sand Dunes
About 500,000 years ago—not so long ago in geologic time—the San Luis Valley of south-central Colorado was underwater. Lake Alamosa covered much of the valley, in a cycle of filling and drying as glaciers melted and grew again through many ice ages. Former shorelines, bays, and lagoons are still visible in the southern part of the valley, rimming what was once a body of water nearly 2,000 square miles on the surface and perhaps as deep as 200 feet over the present-day city of Alamosa. As the lake bottom filled with sediments and soils, and again with water, the lake’s surface eventually overflowed a natural dam and cut a deep channel that is now part of the Rio Grande Gorge.
The tallest dune field in the United States lies to the east of ancient Lake Alamosa, protected in the Great Sand Dunes National Park. The dunes are believed to have formed after Lake Alamosa drained and prevailing winds blew much of the sand up and out of the lakebed to rest against the Sangre de Cristo Range. Today, visitors to the park climb, play, and enjoy what glaciers, winds, and water brought to a pocket in the mountains.
Plateaus of Western Colorado
To the west of the Rocky Mountains in Colorado is a region called the Western Slope. Remnants of ancient seas are also present. These ancient-sea sedimentary rocks are warped upwards in some areas with flatiron forms similar to the ones on the Eastern Slope. Where the rocks are still flat-lying, there is often a cap of younger lava, a dark-colored rock called basalt that resists weathering and erosion. The basalt creates a protective cap over the softer underlying rocks, forming distinctive flat-topped hills called mesas (Spanish for “tables”). The largest of these mesas is Grand Mesa just east of Grand Junction. The mesas are often dotted with small lakes and covered with trees, providing important surface and groundwater reservoirs. Hardened sandstones also cap softer rocks in some areas, forming dramatic pillar shapes, explained by some as supernatural. In formations such as the Book Cliffs north of Grand Junction, massive cliffs tower like toppled tomes, with durable mesa covers binding pages of the Earth’s history.
The geologic forces that were active in the past are still active today. In the nineteenth century, tumbling rocks and snow often blocked or destroyed stretches of railroad track and hampered railroad construction. Modern Coloradans were grimly reminded of the state’s geologic hazards in May 2014, when a thick rock-and-debris avalanche tumbled down from a high mesa in western Colorado for nearly three miles at speeds between 45 and 85 miles per hour. Three men died in the avalanche, which occurred after a period of significant rainfall saturated sediments that had been deposited millions of years earlier. The sediments had been exposed by the downward-cutting streams and rivers that carved the modern valley the avalanche rushed into. Another reminder of the precarious interplay between geology and infrastructure occurred on February 15, 2016, when a rock slide in Glenwood Canyon in Garfield County forced the closure of twenty-four miles of Interstate 70 for about a week.
Crossing the plains into Colorado from its eastern neighbors, it is easy to imagine the land once being the bottom of an ancient sea. Looking north and south, where the peaks spike upward from the flatlands, the immense movement of mountains seems impossible, as do the threats of lightning-filled, ash-laden volcanic clouds from millions of years ago. Continuing westward through spectacular canyons, it is hard to fathom that a small shrug of a restless Earth could change the course of a river in a flash. But pause and try to envision the dynamic processes that shaped the land, and that checkered saga will come alive.