When the quake hit, the earth's crust dropped 19 feet. The land under Hebgen Lake tilted upward; cabins on the north shore were immersed in water and portions of the south shore lay high and dry. Huge waves washed over Hebgen Dam and although it cracked, it held. Several sections of Highway 287 fell into the lake. As a result of the disaster, hundreds of people vacationing in the area were trapped, and 28 people lost their lives.
The massive landslide instantly buried Highway 287 under tons of boulders, rocks and debris. The Madison River, now blocked by the landslide, sweeps through and floods Rock Creek Campground. Muddy water laden with broken trees starts backing up the canyon, beginning to form Earthquake Lake.
The earthquake's impact shocked the world. No one expected something so tragic to happen to them while they lay sleeping. Today, even though the area seems tranquil, geologic tensions are still active underground.
The earth's crust is comprised of 7 major slabs called tectonic plates. As these plates move against each other, energy is stored then rapidly released in the form of earthquakes that most often occur where the plates meet. Here in western Montana, which is part of the Intermountain Seismic Belt, the seismic activity is not occurring where two plates meet. Instead, it is a result of land deformation that is extending eastward from the western edge of the North American Plate. The earth's crust is slowly deforming, stretching and thinning, causing a high level of seismic activity. As massive land movement occurs, the forces produced by a major earthquake can uplift mountains, drop adjacent valleys and form fault scarps - the exposed face of a fault.
There are two huge boulders that were left in place from the earthquake, which have been dedicated to the people who lost their lives in the disaster.
Although there were over 250 people in the Canyon that night, nobody saw the mountain come down. Only the light from the next day did the nature and magnitude of the slide become clear. Over the weeks and months after the disaster, geologists built a story of what had happened. One of the key clues was the scatter of dolomite boulders (such as this one, known as "Sister Boulder," a companion to the Memorial Boulder) that came to rest at the top edge of the slide.
Massive Memorial Boulder and Sister Boulder today rest at approximately the same elevation as the dolomite ledge that they probably came from across the river. They were blasted across the canyon on the leading edge of the sliding mountain, "floating" on debris that was, for those moments, behaving more like a liquid than like rock. In just 20 seconds these six-million-pound boulders traveled half a mile.
|Into the valley|