A boulder field that rings like a bell
On Dry Mountain, about five miles of rough road above the Pipestone exit of Interstate 90 between Butte and Whitehall, a heap of rust-coloured boulders sits oddly against the pines, as if dumped from the sky. The blocks are angular, with long flat faces, from about three to twelve feet across, and nothing grows on the pile. Tap one with a hammer and it rings like an old church bell. Each rock has its own note, regardless of size, some ring not at all, and different parts of one boulder can sound different pitches.
The outside world heard of it in 1933. R. T. "Kid" Ogle, a 58-year-old Butte prospector and trapper and a former Yellowstone stagecoach driver, struck a thin metallic note from the pile with his boot, then tested his way across with his miner's pick, hearing tone after tone. For years he led tour groups out along crude signs nailed to fence posts that pointed to "Ogle's Volcano." He was hardly the first to find it: people have lived in the Pipestone area for 8,000 to 9,000 years, and the BLM's archaeologist thinks it logical that tribal people knew the rocks well. Geologists of the day called the stone black gabbro and were more interested in selling it. Samples went to Los Angeles City Hall as a building stone, and army engineers weighed it for riprap on the Fort Peck Dam.
Why they ring is still unsettled. "No one really knows," says Joan Gabelman, a geologist with the Bureau of Land Management's Butte office. The rock, perhaps 76 million years old, sits at the edge of the Boulder Batholith and is rich in pyroxene and olivine, which resist weathering; one idea is that softer rock wore away around it and the mafic core froze, thawed and shattered into blocks. Others think jointing cracked the blocks and gravity toppled them onto one another, leaving open space. Her father, the geologist John Gabelman, believed the pile was a diatreme, a vertical explosion to the surface, and thought the key was suspension: each block rests on a few points, free to vibrate like a bell. Visitors find the loosely piled rocks ring best and the tightly packed ones thud.
Take a rock away and it falls silent, and removing one is illegal anyway. The site is BLM land, the last stretch of road needs high clearance and four-wheel drive, and visitors can leave their cars by the railroad tracks and hike in.
Other places on the map within a couple of hours.
