Colorado Rocks! Flatirons, Red Rocks, Garden of the Gods, and more.
Welcome to Rock Talk with Angel—let’s rock and roll! Today we will be discussing the Fountain Formation, and its most famous outcroppings in Colorado (including the Boulder Flatirons, seen here.)
If you’ve visited Colorado, you’ve likely seen some highly distinctive rock formations. You may have walked around them in a park, seen them looming overhead while visiting a town, or even seen a concert at one of our rock spots. We’re famous for them!
The most famous of our Colorado rock formations include Red Rocks Amphitheater, Garden of the Gods, Flatirons in Boulder, and Roxborough State Park in Douglas County. These geological formations look as if they were built from the same shapes and type of rock, though they stretch across the Front Range over nearly 100 miles. How can this be?
All of these famous Colorado landmarks are part of the same geological formation—the Fountain Formation!
The Fountain Formation is a massive 290-340 million year old arkosic, conglomeratic sandstone formation, which formed from sediments deposited during the erosion of the Ancestral Rocky Mountains. Not to be confused with the modern Rockies, the Ancestral Rockies were, well, ancestral—they existed and eroded long before the birth of the Rocky Mountains.
When the Ancestral Rockies eroded over time, the resulting sediments were deposited as alluvial fans (flat, fan-shaped deposits spreading outward—think of the sediments spreading out from the mouth of a canyon.) These sediments were compressed and lithified over time, forming the sandstone layers we see in Fountain formation structures. But why are these layers tilted upward if they were deposited in a flat fan?
The answer is the rise of the modern Rocky Mountains. Around 65 million years ago, as tectonic activity and mountain building caused the uplift of the modern Rockies, the sandstone “fans” of the Fountain Formation were uplifted as well, and tilted into the iconic structures we know and love today.
So wherever you are in Colorado, make sure to look up—if there are reddish, sharply shaped rocks around, you might be looking at a part of the Fountain Formation, an important part of Colorado’s geological history.
Which of these iconic Colorado rock structures is your favorite? Are there any that are on your bucket list to visit? Comment below!
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