Most of us have a difficult time wrapping our brains around the shift from one year to the next, so it’s hard to imagine what our world was like centuries ago. Leave it to geologists to consistently think in terms of millions—if not billions—of years when studying Earth’s long history.

Over such vast spans of time, the landscape and the creatures that inhabit our world can change dramatically, thanks to variations in climate and the forces of plate tectonics. The excellent exposures of rock—and their fossil treasure troves—found across the Front Range have provided clues that have enabled geologists to reconstruct what the region looked like in the past.

The Denver Museum of Nature and Science brings the Mile High City’s history alive at its Ancient Denvers exhibit, a fascinating series of paintings by local artists (and a companion science book) that vividly and accurately illustrates “snapshots” of the Denver area and its inhabitants through geologic time. The series begins 300 million years ago, when an ancient mountain range, known as the Ancestral Rockies, stretched from the center of the state to present-day Arkansas. Streams flowing out of this range wound through thick forests with 100-foot conifers and, near Vail, enormous horsetail-like plants, while giant millepedes—up to several feet long—crawled farther east, near the coastline of a large and salty sea.

Fast-forward 20 million years, and a totally different scene appears. All the world’s continents had amalgamated into a single landmass known as Pangaea, and in its dry climate, enormous, Sahara-sized sand dunes covered most of today’s American West, including Colorado. Strange animals, including dog-sized “proto mammals” (meaning they exhibited mammalian traits such as fur and warm blood), left their tracks on the steep dunes as they wandered across this desolate landscape.

By 250 million years ago, Denver’s environment had evolved to a scorching, tropical seashore covered by slimy bacterial mats. Winged pterosaurs, the largest creatures ever to take to the skies, circled a vast inland sea that stretched from Denver to Illinois 70 million years ago. What Denver looked like just before, and just after, Earth’s collision 65.5 million years ago with the famous asteroid that killed the dinosaurs is also captured on canvas. The “after” painting shows life’s amazing resilience, as it depicts the monsoon-drenched rainforest that thrived in Castle Rock a mere 1.4 million years after three-quarters of all the planet’s plant and animal species died in the mass extinction. The Castle Rock rainforest was among the first ever to grow on Earth.

As the exhibit approaches the present, museum visitors see Highlands Ranch 16,000 years ago, in the depths of the Pleistocene Ice Age. Camels and tusked mammoths meander through tall grasses and around pine trees beneath the snow-capped Rampart Range. The exhibit closes with a photograph of the same area today, covered with a seemingly endless sea of houses—a striking transition that happened, as the exhibit shows, in the geologic blink of an eye.

(Read more from 5280‘s Colorado by Nature series)

Terri Cook
Terri Cook
Terri Cook is an award-winning freelance writer based in Boulder. More of her work can be found at