How the Boulder Flatirons Were Formed

The Flatirons, tilted rock slabs pasted on Green Mountain and South Boulder Peak, form Boulder’s iconic mountain skyline. The huge red Flatirons were named for their resemblance to flat irons, metal pieces that were heated and used to press clothes by pioneer women in the 19th century. Five main formations—called the First, Second, Third, Fourth, and Fifth Flatirons—are on Green Mountain, while a hundred more scatter on the slopes with imaginative names like Satan’s Slab, The Willie B, The Seal, The Fatiron, The Maiden, Devil’s Thumb, and The Maiden.

What Kind of Rock Forms the Flatirons?

The Flatirons are composed of sandstone and conglomerate in the Fountain Formation, a 1,000-foot-thick rock unit along Colorado’s Front Range between Cañon City and Fort Collins. The stone was originally deposited by rivers and creeks on the eastern edge of a mountain range called the Ancestral Rockies about 300 million years ago. The Fountain sandstone found at Garden of the Gods and Roxborough State Park is soft and easily eroded, but at the Flatirons the rock grains are cemented together by adularia, forming the erosion-resistant formations.

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How Were the Flatirons Formed?

After the initial rock deposition, the Flatirons formed much later during the uplift of today’s Front Range Mountains between 70 and 64 million years ago. Uplift was along several faults which tilted the Fountain Formation rock layers downward and eastward, leaving today’s 50-degree-angled Flatirons. Water erosion later attacked the slowly rising sandstone, chiseling out today’s iconic cliffs over the last 30 million years.

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The Best Flatirons Trail

Over 100 miles of trails lace the Flatirons on Green and South Boulder peaks in Boulder Mountain Park, offering plenty of easy to strenuous hiking adventures. The best Flatiron hike is the Mesa Trail, which winds along the edge of the Flatirons for seven miles from Chautauqua Trailhead to South Boulder Trailhead. The trail passes all the big rock formations, crosses a variety of terrain, passes through grasslands and forests, and offers postcard views. Arrange a car shuttle or do a 14-mile out-and-back trek.

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The Best Flatirons Climbing Route

The best climbing route in the Flatirons is the Standard East Face route up the 1,300-foot-high East Face of the Third Flatiron, one of the most prominent formations on Green Mountain. The route, one of Colorado’s most popular climbs, is ascended almost every day. The route’s been climbed in roller skates, naked, by moonlight, without the use of hands, and speed climbed. The current speed record from base to summit is 5 minutes 59 seconds by climber Stefan Griebel. The route, rated 5.4, is usually climbed in eight pitches, with the last pitch being the hardest. Consult the Rock Climbing Colorado guidebook for a route description.

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Third Flatiron East Face: Colorado’s First Climb

The Standard East Face route on the Third Flatiron, first climbed in 1906 by Floyd and Earl Millard, is Colorado’s earliest recorded technical rock climb. The brothers scrambled up and down the cliff without safety gear. The first roped ascent was made in 1919. Climbing guides Ev Long and Hull Cook installed six beefy eyebolts on the East Face route in the early 1930s to use as belay anchors when they took clients up the rock. The bolts are still used today as belay anchors.

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3 Best Flatirons Photo Spots

The Flatirons are visible from across Boulder, but here are three of the best vantage points to shoot the best photographs of the formations (Remember that morning is always the best time for Flatiron photos!): 1) Chautauqua Park on Baseline Road offers a classic Flatirons scene (Find great images on winter mornings with fresh snow and in mid-summer when wildflowers fill meadows below the rocks), 2) Drive up to the National Center for Atmospheric Research or NCAR and take a short hike west to an overlook that yields a perfect view north to the main Flatirons, and 3) The Davidson Mesa Overlook on the west-bound lanes of U.S. 36 east of Boulder offers a postcard view of the city with the angled Flatirons on the mountains above.

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Freddy Flatiron and Groundhog Day

The big question on Groundhog Day is: Will it be an early spring in Colorado or six more weeks of winter? To get the answer, you need to ask Flatiron Freddy at Chautauqua Park below Boulder’s Flatiron formations. Colorado doesn’t have groundhogs, so you need the next best thing—a stuffed marmot, since all marmots except Freddy are hibernating in February. In 2017, Flatiron Freddy skied into the park, clad in a top hat and cape, with a park ranger to make his prediction. Freddy couldn’t see his shadow since it was overcast with light snow. His pronouncement? An early spring.