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Steamboat Springs — At the end of a twisting road in southern Routt County, a bulldozer noses cascades of black rock down the side of a hill.

The fine material, left behind by a volcano, probably will end up spread across snow-covered roads. Bratton Enterprises, the company that runs the operation, provides nearly all of the scoria for the Routt County Road and Bridge Department.

The out-of-the-way pit sits a few miles from McCoy.

"People say, 'Why don't you put a sign up?'" owner Jim Bratton said. "We can't keep up as it is."

Bratton said he ships truckloads and railcars of road sand and decorative rock to Vail, Carbondale, the Front Range and beyond.

"I went back to Wisconsin one year to look at a front-end loader," he said. "The guy selling the front-end loader - I didn't know this guy from Adam - had some of my decorative rock."

The volcanic area, called a cinder cone or a scoria cone, is easy to mine, Bratton said. The rock is porous and can be pushed off the hill, sifted by size and then sold. It comes in different sizes - the boulders aren't as popular as Bratton expected - so the company rarely has to crush it down to road sand size.

"We use a lot of that," said Paul Draper, director of Routt County Road and Bridge. The department typically buys more than 3,000 tons of the rock each winter, he said.

The volcanic rock is good because it's porous and doesn't freeze into a pile, Draper said. The scoria also provides more traction than beach-type sand, he said. That's because the county uses three-quarter-inch pieces. They sit on top of the snow, but sand would get lost in it, he said.

The material also has been mined for railroad ballast, the rock that forms the base for tracks, Draper said.

"There's a lot of history down there," he said.

The age of rocks

Jerry Magloughlin, a professor in the Geosciences Department at Colorado State University, said it was common to mine cinder cones for such uses. But the volcano near McCoy is unusual for the region, he said.

"To the best of my knowledge, there's not a lot of scoria or cinder mining going on in Colorado," Magloughlin said. "There's a lot more in New Mexico."

Bratton said he thought deposits existed in Dotsero and Alamosa. Magloughlin confirmed the Dotsero site.

"That is probably several thousand years old," the geologist said of the Dotsero cinder cone. "That's the youngest volcanism in Colorado that I'm aware of."

Magloughlin said the McCoy cone probably was about the same age.

"If you find nice, pristine, bubble-filled cinders, it's got to be young - tens of thousands, maybe hundreds of thousands" of years old, he said.

That's clear to scientists partly because the porous rock doesn't last long, Magloughlin said.

"It tends to weather pretty easily into clays and goo," he said. "Basically, if you can find good deposits of this stuff, it's a pretty young volcano."

The rift

The Dotsero cone is a result of the Rio Grande Rift, Magloughlin said, adding that the McCoy cone probably is, too. A rift is a spot where the Earth's tectonic plates are pulling apart and thinning. Volcanic activity is common along such a spot, he said.

The Rio Grande Rift runs from the San Luis Basin in Colorado through New Mexico to Mexico. Some geologists think it runs farther north, possibly to Wyoming, Magloughlin said.

The plates still are shifting, he said, which means new volcanoes could appear. They are more likely to pop up in New Mexico than Colorado, however. And the mounds don't pose a huge danger, he said.

"Cinder cones are small volcanoes," Magloughlin said. "They are just commonly not more than 1,000 feet high or maybe 500 feet."

Bratton's volcano

Bratton has worked at Crater, the McCoy site, since he started running a bulldozer there in 1963. Much of the land is government-owned.

He took over the operation with his wife, Donna, in 1991. His son Roger helps with hauling on Fridays, and his daughter Stephanie does the accounting. The family lives in Yampa.

Bratton expects the mine to last at least until he retires, though he's not sure when that will be.

Right now, he's busy stockpiling sand scoria for winter. He had a big pile set up, but last winter's huge snowfall ate it all up, and Bratton Enterprises had to scrape out more during the season.

Demand, he said, always is high.

"It's really interesting," he said of the business. "But the challenging part sometimes is that a lot of customers are so demanding. If we don't have it, they get upset. But there's times we can't keep up."