Adventurers who bicycle the 10 miles along the C&O Canal towpath from Williamsport, Md., northwest to the Clear Spring, Md., area usually know what to expect along the trail--muddy ditches, tree roots and fallen branches.

But along with those hazards is another: heaps of shale used by the National Park Service to repair the 150-year-old path. Bikers and hikers have made a few complaints to the service through the years about popped inner tubes and jarred limbs, but this summer, one group indicated that the rock piles have created an unusually severe hazard.

The 184-mile towpath along the canal from Georgetown to Cumberland, Md., is a national historical park heavily used by bikers and hikers. Of the estimated 721,400 visitors to the northwestern stretch of the canal last year, about 11 percent were bikers, park officials said.

Vince Berg, a civil engineer and bike leader who guided a group of 15 bikers for the Audubon Naturalist Society on a trip last month, wrote to Park Superintendent Richard L. Stanton that the mounds of shale dumped directly on the towpath posed "a glaring maintenance problem."

Shale is used because local materials traditionally are employed for towpath repairs, park spokesmen said. Construction fill dirt that has a good mixture of clay, silt, and gravel often is used in the Washington area, for instance. Near Hancock, which is in a more mountainous region about 125 miles to the northwest, shale from the Martinsburg, W.Va., area is used.

"This material may be more historically accurate for repairs ," Berg wrote, "but it is dangerous and possibly life-threatening to bicyclists and dangerous even for hikers."

In the three years that Berg has guided groups down the path, "there were never spots . . . that bad," he said. In the course of the trip, Berg repaired 11 flat tires for cyclists, as well as bent rims and broken spokes.

Stanton said the Hancock stretch was smoothed over several days after Berg's group rode by.

"What worked for the original canal company works for us," Stanton said, referring to the 1830s, when shale was laid for the mules who pulled the barges and who were the primary users of the path.

Park workers have been shoveling and hammering shale into potholes and over worn out sections after heavy traffic, storms and floods for 27 years, said Dale Sipes, maintenance chief of the C&O Canal. Some sections are smoothed by rolling machines, and others are raked by hand. Larger pieces take about a year to disintegrate into the path and firm up, he added.

Along the Georgetown-to-Great Falls stretch in the Washington area, traffic is heavy enough to keep the towpath packed down, Stanton said. In the lesser-used areas to the northwest, trail conditions depend on the weather. Impressions left by horses' hoofs and bikers' rims during a wet spring will often hollow out into potholes, Stanton said.

In the 1960s, Sipes said, crews laid crushed bluestone in the more remote area, but reverted to shale after receiving complaints from preservationists who opposed the use of modern grading techniques. When not crushed, shale is also less expensive to use than bluestone, he said.

Stanton said conditions on the towpath often cannot be predicted because the trail keeps changing. "If you hit it at a time when it's rough, it's bad. It's very fragile . . . It is not like a road. When people hike, they are expecting an adventure; they don't expect everything to be perfect. We are constantly out there dealing with it . . . "

Cyclists who take to the path should realize it is a slow kind of riding, said Carl Modig, a safety expert for the Washington Area Bicyclists Association. "If they the National Park Service put down a lot of shale, it's trading one kind of trail hazard for another. They have a multiple group of users--joggers, bicyclists, hikers, personnel who ride it with a jeep and naturalists who'd object to having it paved. The best they can do is strive to achieve an all-weather dirt surface," he said.