At the site of the slide, a relatively strong and nearly vertical layer of dolomite rock supported a huge bank, or mountain, of comparatively unstable schist and kept it from sliding into the valley in the same way that a retaining wall keeps a hillside terrace from slipping downhill. The tremendous shock waves of the earthquake fractured this dolomite buttress, and some 43 million cubic yards, or 80 million tons of rock, timber, and other mountainside debris cascaded off the slope, hurtled into the canyon, and surged up the opposite side, carrying huge trees and house-sized boulders as if they were weightless, hollow toys.
When this huge mass whomped down onto the river bed, it forced out the water and air trapped underneath at hurricane velocity. The huge slide spurted mud, air, and water with such force as to send two-ton cars sailing through the air, and to grind others to suitcase thickness against the rocks.
All this happened in seconds.
It would take eight seconds for the mass at the top of the mountain to fall to the valley floor 1,200 ft. below. At the time it reached this point, the mass would be travelling 174 miles per hour. The time it took to zoom half-a-mile across the valley, up the opposite canyon wall, then split and flow three-quarters-of-a-mile up and down the valley (the slide lies one-and-a-half-miles-long in the valley), was less than thirty seconds!
The fact that timber from the face of the mountain is spread in relatively uniform fashion over the entire surface of the slide is interpreted to mean that there was little tumbling action—that the slide moved as a single, if shattered, mass.
One important scientific controversy has emerged from the earthquake. It relates to the time relationship, or sequence between the initial shock, the tidal waves, or seiches, how fast the huge quantities of water which overtopped the dam moved down the valley, and whether these slugs of water had rushed through the canyon in time to reach the site of the slide before the mountain fell.
The stretch of the Madison running through the canyon is fresh, fast water, but normally it takes up to two hours for an object to run the sparkling, seven-mile, trout-rich stretch from Hebgen Dam to the mouth of the canyon. The big surges of water—the seiches overtopping the dam—would make it a lot faster.
There are two big, related questions.
Could the big surges of water reach the point of the slide soon enough?
And just how soon after the first shock did the mountain fall?