[Gutenberg 56850] • The Night the Mountain Fell · The Story of the Montana-Yellowstone Earthquake

At 11:37 P. M. on Monday, August 17, 1959, one of the severest earthquakes recorded on the North American continent shook this area. It sent gigantic tidal waves surging down the 7-mile length of Hebgen Lake, throwing an enormous quantity of water over the top of Hebgen Dam, the way you can slosh water out of a dishpan, still keeping it upright. This water—described as a wall 20 ft. high—swept down the narrow Madison Canyon, full of campers and vacationers who were staying in dude ranches and at three Forest Service campgrounds along the seven-mile stretch from the dam to the point where the canyon opened up into rolling wheat and grazing land. Just about the time this surge of water reached the mouth of the canyon, half of a 7,600-ft.-high mountain came crashing down into the valley and cascaded, like water, up the opposite canyon wall, hurtling house-size quartzite and dolomite boulders onto the lower portion of Rock Creek Campground.