In most respects, the town of Calhan, located on U.S. 24 in northeastern El Paso County, is typical of the small towns dotting the state’s vast High Plains. But Calhan hides a colorful secret: vibrant purple, red, pink, and yellow badlands that are the only source of high-grade clay anywhere on Colorado’s Great Plains. Indigenous tribes have mined the clay for pottery and ceremonial paint for the last 10,000 years, and left behind an archaeological record so extensive that the site is listed on the National Register of Historic Places.

The badlands, which are preserved in El Paso County’s 750-acre Paint Mines Interpretive Park, also hold a scientific secret that is the reason for their vivid color. These ancient soils formed during an intense heat wave that gripped the globe 55 million years ago and lasted for 10,000 years. The clay’s colors were dictated by ancient soil conditions: the yellow and brown splotches are characteristic of well-drained soils, whereas the purple color formed in low spots where the soils were waterlogged year-round.

When the heat wave struck, the Earth was already much warmer than it is today. The temperature then spiked 18 degrees Fahrenheit in just a few thousand years—a lightning-fast change on a geologic timescale. Although scientists studying this rapid warming event have concluded the heat wave was triggered by the injection of 2,000 trillion tons of carbon into Earth’s atmosphere, they still debate the carbon’s source. Some argue it was released during massive volcanic eruptions in the North Sea and eastern Canada; others attribute it to the burning of huge peat bogs (wetlands) or the melting of frozen methane buried beneath the seafloor.

Regardless of its source, the amount of carbon released is comparable to what humans will have released by the year 2200, if not before, so scientists have studied this ancient heat wave for insight into how ecosystems respond to the kind of warming expected within the next two centuries. The results are sobering; during that heat wave, North America experienced its largest-ever extinction of mammals and a huge influx of invasive plants and insects. About 40 percent of oceanic microorganisms also went extinct.

Fortunately, since 2004, when Colorado became the first state whose voters passed a renewable energy standard, we have been a leader in renewable energy technology and energy efficiency. Thanks to our ample wind and sunshine, rooftop solar installations are appearing across the state, and wind farms are sprouting up across the Colorado’s plains, including Xcel’s proposed $1 billion Rush Creek Wind Project that the utility plans to build not far from Calhan. If we continue to promote a clean energy economy, there is hope that our descendants will not experience a heat wave like the one preserved at the Paint Mines.

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