Gravel used in road projects in Yellowstone National Park must be free of weeds, a condition that raises project prices considerably.

One five-mile section of the road cost $250,000 more because the gravel had to be "cooked" to kill weed seeds before it could be brought into the park, said George Humphreys, the resident engineer during the summer at Yellowstone National Park.Federal highway officials require gravel to be weed-free to ensure weeds are not imported into the national park, and to get rid of weed seeds, the rocks must be cooked to 300 degrees.

The heating process raises gravel costs from $4 to $4.50 a ton, Humphreys said.

To cook the gravel, contractors put it in an asphalt "hot plant," which is usually heated by diesel fuel, propane or pulverized coal. The process of cooking gravel has been used in Yellowstone for at least a decade, Humphreys said.

A park official could travel to the source of the gravel, inspect it and declare it weed-free as an alternative to cooking the gravel, he said.

The gravel used for work on the Grand Loop came from Bozeman Sand and Gravel in Bozeman, Mont. The gravel must be cooked, Humphreys said.

About 95 percent of gravel used in road work is usually covered by asphalt, but slope material and stockpiles of gravel are left uncovered, he said.