The Madison Slide

For some 200 vacationers camping in a deep canyon on the Madison River just downstream from Hebgen Lake, not far west of Yellowstone National Park, the night of August 17, 1959, began quietly, with almost everyone safely bedded down in their tents or camping trailers. Up to a certain point, it was everything a great vacation should be—that point in time was 11:37 P.M., Mountain Standard Time. At that moment, not one but four terrifying forms of disaster were set loose on the sleeping vacationers— earthquake, landslide, hurricane-force wind, and raging flood. The earthquake, which measured 7.1 on the Richter scale, was the cause of it all. The first shocks, lasting several minutes, rocked the campers violently in their trailers and tents. Those who struggled to go outside could scarcely stand up, let alone run for safety.

Then came the landslide. A dentist and his wife watched through the window of their trailer as a mountain seemed to move across the canyon in front of them, trees flying from its surface like toothpicks in a gale. Then, as rocks began to bang against the sides and top of their trailer, they got out and raced for safer ground. Later, they found that the slide had stopped only a few car lengths from the trailer. Pushed by the moving mountain was a vicious blast of wind. It swept upriver, tumbling trailers end over end.

Then came the flood. Two women schoolteachers, sleeping in their car only a few feet from the riverbank, awoke to the violent shaking of the earthquake. Puzzled and frightened, they started the engine and headed for higher ground. As they did so, they were greeted by a great roar coming from the mountainside above and behind them. An instant later, their car was completely engulfed by a wall of water that surged up the riverbank, then quickly drained back. Although the two women managed to drive the car to safety, others were not so lucky.

After the first surge of water, generated as the landslide mass hit and blocked the river channel, the river began a rapid rise. Great surges of water were overtopping the Hebgen Dam, located upstream, as earthquake aftershocks rocked the water of Hebgen Lake back and forth along its length. In the darkness of night, the terrified survivors of the flood had no idea what was happening or why. The water had risen 10 m (about 30 ft) above ground level in just minutes.

The Madison Slide, as the huge earth movement was later named, had a bulk of 28 million m3 (37 million yd3) of rock. It consisted of a chunk of the south wall of the canyon, measuring over 600 m (about 2000 ft) in length and 300 m (about 1000 ft) in thickness. The mass descended more than half a kilometer (about a third of a mile) to the Madison River, its speed estimated at 160 km (100 mi) per hour. Pulverized into rock debris, the slide crossed the canyon floor, its momentum carrying it over 120 m (about 400 ft) in vertical distance up the opposite canyon wall. Acting as a huge dam, the slide caused the Madison River to back up, forming a new lake. In three weeks’ time, the lake was nearly 100 m (330 ft) deep. Today it is a permanent feature, named Earthquake Lake.

The Madison Slide is an example of mass wasting—the downhill motion of rock and soil under the influence of gravity. Although a rock avalanche is very rapid, other types of mass wasting are slower and less dramatic. However, they all act to carve and shape the landscape into distinctive landforms.