The latest scourge of the desert is the preferred mode of transportation for many athletic environmentalists - the mountain bike.

An obvious appeal of these rugged, fat-tired, 10-speed bicycles is that they're the closest thing to backpacking. You can go onto dirt roads, buzz along with the wind in your hair and the sun on your back.The problem is that mountain bikes are like slow motorcycles. They weigh less, their wheels spin less rapidly, but the knobby tires are capable of damaging the fragile desert crust anyway.

The Southern Utah Wilderness Association, whose members include some of the most hard-line nature advocates in the state, is sponsoring a cross-country mountain bike "wheel around" in June. During this six-day trek participants will bicycle through much of southern Utah.

"We're staying all on paved roads, as a matter of fact," said SUWA's Chris Dangerfield, Cedar City, herself a mountain biker.

Other groups of mountain bikers aren't determined to avoid going off paved roads, and many mountain bikers take their vehicles cross-country.

The "Castle Country Fat Tire Festival," which is going on this weekend throughout the San Rafael Swell and Reef, attracted 235 registered bikers. It is sponsored by the Carbon County Leisure Services, a county agency, as well as several Price businesses.

All four routes are on dirt roads. One will be going near an endangered cactus species, and one route will be on the sandstone Reef itself.

Jim Dryden, the Bureau of Land Management's San Rafael Resource Area manager, said the Reef route is on slickrock - that is, bare sandstone. He said BLM officials will be present to keep riders away from sensitive areas, which have been flagged.

In 1984, no overnight camping permits were requested for mountain bike groups using Canyonlands National Park's White Rim Trail, a 110-mile dirt road that goes around the plateau called Island in the Sky.

"Last year, there was 2,400 nights of use by mountain bicyclists on the White Rim. Quite a jump," said Larry Frederick, an interpreter at Canyonlands.

Karen Whitney, public information officer at Glen Canyon National Recreation Area, said, "People don't realize that off-road use with mountain bikes is illegal (in Park Service areas), and so they're taking them cross-country and over slickrock."

The recreation area posted signs warning motorized vehicles to stay on the roads. But people fail to realize "this applies to bicycles of any kind," she said.

"Bicycle tires, just like footprints and car tracks, will destroy the cryptogamic soil," she said. "When they run over vegetation and disturb it, they're just like any other vehicle. It's difficult to recover in some areas."

Cryptogamic soil is a dark, living crust of mosses, lichens and algae. It stabilizes the ground and traps moisture, helping other plants to survive. It spreads so slowly that it requires years of undisturbed growth to recover from a careless footprint or bike track.

"We're seeing lots of mountain bike tracks. And this is affecting the cryptogamic soil crust," Frederick said.

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Park Service fears

- Mountain bikers are illegally using park trails open only to hikers, not vehicles. Hikers report finding mountain bike tracks and mountain bikers in the backcountry.

- Some park viewpoints near roads are being visited by people using mountain bikes rather than hiking. Cross-country travel damages the delicate soil cover.

- Where four-wheel-drive roads are sandy, the going is tough for bicyclists. So mountain bikers ride on the edge of the roads where there are hard berms. This breaks down the berms, making them sandy, and widening the roads.