Colorado by Nature: Eldorado Canyon State Park

For many people, mountains are a metaphor for permanence. But when we expand our outlook from the human lifespan to that of geologic time, which encompasses millions—and even billions—of years, it becomes apparent that hills, mountains, and other seemingly enduring parts of the landscape are far from everlasting.

Nowhere is geologic transience more evident than at Eldorado Canyon State Park south of Boulder, where no fewer than three mountain ranges have risen—and eroded—in roughly the same spot. Beginning about 65 million years ago, tremendous earth forces initially raised the Rocky Mountains, creating the raw ingredients through which South Boulder Creek eventually carved the dramatic canyon where Front Rangers enjoy hiking and picnicking today.

The rocks comprising “Eldo’s” steep, golden cliffs are the evidence that an earlier mountain range stood in this area about 300 million years ago. After being uplifted, the so-called “Ancestral Rockies” wore away very, very slowly, lowering the range’s craggy peaks grain by grain and depositing them in a thick cone of sediment at the toe of these ancient mountains. Nearly 200 million years later, the uplift of the Rockies dramatically tilted this thick sediment pile to create Eldorado Canyon’s steep walls, a favorite destination for many technical rock climbers.

Incredibly, there is evidence in Eldorado Canyon that an older range once existed in the area. On the south side of the dirt road through the state park, just before it crosses South Boulder Creek, a few scraps of a rare, bullet-hard rock are exposed. Known as the Coal Creek Quartzite, this is one of Colorado’s oldest rocks, checking in at a whopping 1.7 billion years old.

Found only in a narrow, 10-mile band of foothills between Eldorado and Coal Creek canyons, this slick rock was once a pile of sand and gravel shed off an even earlier range of mountains before being buried so deeply that its grains were fused together. Because the quartzite’s parent material is sand rather than something more coarse, geologists speculate these ancient mountains lay a few tens of miles to the west of present-day Eldorado Springs. The ancient range was probably raised at Colorado’s very birth, when a massive collision between tectonic plates added our state onto the early North American continent.

Although the processes tearing down the modern Rockies seem imperceptibly slow to human eyes, the inexorable forces of nature will eventually level this range as well. Given enough time, however, the forces of plate tectonics will likely once again build another range nearby—as baseball legend Yogi Berra once famously said, “It’s like déjà vu all over again.”