Grand Tetons and the Gros Ventre Slide

The Grand Tetons tower above the valley known as Jackson Hole, Wyoming, provides the epitome of western scenery for many people. A still-active pull-apart fault lies along the front of the range and slopes steeply downward beneath Jackson Hole. From the highest peaks to the fields of the Hole, where elk and moose and bear are common, is well over a mile vertically (roughly 2 km), but the total vertical offset on the fault is almost 6 miles (10 km) (we don’t see this total offset because a lot of rocks have been eroded from the top of the range and deposited in the valley). The uplifted block is primarily old metamorphic rocks that erode only slowly. The faulting is probably related to the Basin and Range extension that also gave us Death Valley, although the complexity of the region makes any interpretation difficult. Dr. Alley recalls huddling next to an overhanging rock, far up on the steep front of the Tetons, watching hailstones rattle off the trail from a black deck of clouds barely over his head. It is a truly awesome place. 

A few miles (few km) east of the park you can visit another interesting feature: the Gros Ventre Slide Geological Site. There, as shown in pictures and the VTrip below, a mountain-sized ridge is made of rock layers that slope steeply, almost parallel to the north slope of the ridge, down to the Gros Ventre River. Those layers include strong, resistant sandstone resting on weak, slippery shale. The river had eroded down through the sandstone and into the shale, leaving the toe of the sandstone unsupported. In June of 1925, after a particularly wet spring, the entire mountainside let loose, sliding along the soft shale down, across the river, and more than 300 feet up the other side; a rancher and his horse who were on the other side barely escaped safely. The slide mass made a dam, and the river then made a lake many miles long and as much as 200 feet (60 m) deep. The entire slide probably required only seconds to occur and moved cubic miles (many cubic kilometers) of rock. 

Such a dam of loose debris is not very strong; water flowing through its porous spaces or over it can remove rocks and weaken it greatly until it collapses catastrophically. Back in Module 2, in the West Yellowstone VTrip, we saw that an earthquake just northwest of Yellowstone in 1959 caused a similar landslide, which dammed a river to form a new lake, and that the Army Corps of Engineers had rushed in to move massive amounts of debris and prevent a collapse of the dam. The Corps knew how likely and how dangerous such a failure would be, in part because the Corps had not been tasked to act at Gros Ventre in 1925.  In 1927, the dam formed by the Gros Ventre slide failed, washing out a small town downriver and killing six people. The loss of life would have been much larger if more people had lived there.  A few of the people living there were saved when a ranger saw the start of the flood, drove downstream faster than the flood and warned the people to flee.  Unfortunately, not everyone listened.   

Take a tour of the Grand Tetons National Park.