Geologic Time

Geologic Time

About 300 Million Years Ago...
In the earth’s early history, a mega continent Pangea was formed from the collision of two large land masses. This collision created the Rocky Mountains and the land we know today as Colorado.


Ancestral Rockies

Dinosaurs and Rising Tides


Highlight video

The Flatirons

Ancestral Rockies

Approximately 300 million years ago, two continents, Gondwana & Laurentia collided, changing Colorado’s landscape forever.

Dinosaurs and Rising Tides

Dinosaurs like the stegosaurus roamed Colorado’s low level plains until the Great Interior Seaway puts Colorado underwater, capturing the fossils and footprints in time.


Glacial periods alternate on the Earth between 2.5 million and 11,500 years ago allowing glaciers, rivers and ice to carve and cut through Colorado’s mountains giving them their “rocky” name.

Interested in learning more? Read below!

The Ancestral Rocky Mountains

Approximately 300 million years ago 

During the late Pennsylvanian epoch & early Permian period, two continents, Gondwana & Laurentia collided, starting the formation of the supercontinent, Pangea. Epochs and periods are measures of geologic time which happen over a much larger time-scale than our lives. For reference periods on average last for tens of millions of years, while an epoch is usually several million.

This initial collision caused the ancestral rockies to rise in CO, NM, TX & OK. The ancestral front range spanned across the current Denver basin.

This region was a coastal, tropical coastline where Dimetridons, a proto-mammal,  roamed the riverbeds. Also some of the first known conifer trees started to grow in the mountains.

As water and time eroded the ancestral rockies, rocks and pebbles of many different sizes deposited in alluvial river beds with sand and mud forming the fountain formation. Alluvial beds relate to those that collect alluvium, which is mud, clay, silt, sand or gravel matter. As these materials built up in the riverbeds they cemented over millions of years underground to form the fountain formation. This formation makes up the iconic Red Rocks Amphitheater and the Flatirons in Boulder, CO. 

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Arid Colorado

Approximately 280 million years ago

As Pangea continued to form as the two continents collided, Colorado was lifted up from the coastline & became an arid desert. The ancestral rockies became eroded highlands and massive dune fields covered the lowlands of the Denver basin depositing the Lyon sandstone. This rock is what makes up the Garden of the Gods in Colorado Springs and can also be seen in Lyons, Colorado.

Due to this sandstone having a fine grain and strong cementation, archaeologists believe it has been used for over 6,500 years for building. This is the primary red/orange sandstone used on the University of Colorado Boulder campus.

Dinosaurs in Colorado

Approximately 145 million years ago

It wasn’t long, on a geologic time scale, until Pangea broke up due to the Wilson cycle. Colorado dropped drastically in elevation overtime and became a low plain with slow meandering rivers. Here Stegosauruses roamed the landscape leaving fossils that deposited in the Morrison formation found at Dinosaur ridge. The Stegosaurus was designated as Colorado’s state fossil in 1982.

Colorado Underwater

Approximately 100 million years ago

At this point in time, Laurentia started to look like modern North America, except for a shallow basin in the middle that sank below sea level. The Western Interior Seaway, also known as the Cretaceous Interior Seaway, cut from the arctic to the gulf of Mexico, putting all of modern Denver underwater. 

The Western Interior Seaway was warm, shallow and hosted lots of different marine life such as sharks, mollusks, fish, pterosaurs, early birds, and marine reptiles. The ocean floor would occasionally become anoxic, slowing the decomposition of dead organisms. This is why many of the Cretaceous-aged formations in the Denver Basin have well preserved fossils. 

The Pierre and Dakota shales are two formations in Denver where fossils can be seen. At Dinosaur Ridge, in Morrison, CO, these formations have outcrops where fossilized palm fronds and alligator and dinosaur footprints can be seen. These fossils indicate that the Denver basin must’ve been a tropical environment at this time and it is believed this coastline was a ‘highway’ for migrating dinosaurs. 

The many different alternating sedimentary rocks such as sandstones, mudstones and limestones also indicate that the Western Interior Seaways shorelines were constantly changing as these 3 rock types are deposited at different ocean depths. 

Laramide Orogeny

Approximately 66-47 million years ago 

With time, the oceans fully receded & a mountain building episode, also known as an orogeny, lifted Colorado thousands of feet above sea level. The exact reason for this uplift is still debated today by geologists, as it is believed that many factors contributed. The one factor that most geologists agree on, is the shallow subduction of the Farallon plate under the North American plate which thickened the continental crust. Due to the laws of isostasy, this caused the land to rise. At this time the rockies were a lot less jagged and were believed to look like high rolling plains/prairies. 

Pleistocene epoch

Approximately 2.5 million years ago - 11,500 years ago

Around 2.5 million years ago the Pleistocene epoch began which lasted until around 11,500 years ago. During this time Earth alternated between glacial and interglacial periods, freezing and thawing the Earth many times over. This caused glaciers to form and melt over in the high altitude Colorado landscape many times over. The seasonal freeze and thaw cycles also played a role in erosion along with the alternating glacial periods over tens of thousands of years.

Ice and water are two of the strongest eroding factors and carved the high plains of Colorado minted jagged rocky peaks. Glaciers and rivers form many different formations within the land, many which can be seen in the Colorado Rockies just outside Denver. 

One key difference between glacial and alluvial erosion is the shape of a valley. If a valley has a U-Shape it was most likely formed by a glacier whose heavy flow of ice widens and deepens the valley as it slowly moves down the mountain. These valleys often have flat and wide floors with steep overhanging walls. Rivers on the other hand cut through the mountain at an angle causing a V-shaped valley that follows the rivers path.

At the head of a glacial valley one may find a “cirque” which is a bowl shaped depression where the top of the glacier once laid. As the ice melts away the scars in the mountain are left to tell the mountain’s story.

As the seasons come and go, rivers from the ice and rain form rivers leading out of the mountains. On open U-shaped valley floors, meandering rivers often wind through the landscape. Water always takes the path of least resistance, so it often finds an S-shaped path. Sometimes the water finds it easier to cut off one of the loops, isolating the water creating an ox bow lake. 

The water is also coming from the old mountains bringing minerals and rare metals that broke away due to erosion. Rare earth metals such as gold weigh much more than the other sediments so they get trapped and built up on the turns of the river. These are called placer deposits and many can be found in Colorado, which brought many people during the gold rush.


Did you know that Colorado is the only state to ever turn down hosting the Winter Olympics? Colorado is also home to the tallest sand dunes in North America. There are tons of interesting facts and trivia about Colorado, can you guess them correctly below?


What percentage of Colorado’s population lives in the front range?

Not sure?

Answer: 70%

How many professional teams are there in Colorado?

Not sure?

Answer: There are six professional sports teams in Colorado.

How many NASA astronauts are graduates of the University of Colorado Boulder?

Not sure?

Answer: Eighteen!

Approximately how many days of sunshine does Colorado have per year?

Not sure?

Answer: 300

What Colorado 14er mountain inspired Katharine Lee Bates to write the song “America the Beautiful.”

Not sure?

Answer: Pikes Peak

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