June 23, 1925. In under three minutes, fifty million cubic yards of landmass broke free from the north slope of Sheep Mountain and crashed down the slope, damming the Gros Ventre River behind the debris. To this day, the Gros Ventre Slide is the second largest landslide in human history, and the largest the United States has ever seen.
The Gros Ventre Slide can be thought of as a perfect storm. Conditions were just right for incredible landscape change. You can think of the conditions leading to the slide as ingredients for a deliciously destructive cookie. The first two ingredients are rocks. Sheep Mountain is composed of Tensleep Sandstone, which is permeable, and Amsden Shale, which is impermeable. The third ingredient is the Gros Ventre River, which cut into the mountain side and removed support from the slope. The fourth ingredient was precipitation: leading up to June 23, 1925, Jackson Hole saw unusually high levels of precipitation. This precipitation saturated the permeable sandstone upper layer, and the shale layer below provided a perfect sliding surface for the sandstone. Add in the final ingredient of an earthquake and you have the Perfect Slide Cookie.
To William Bierer, the slide was not a surprise. Bierer saw the ingredients for the Perfect Slide Cookie adding up, so he sold his ranch on Sheep Mountian to the unsuspecting Guil Huff. On the day of the slide, Guil heard loud noises on Sheep Mountain and decided to ride his horse to investigate. When Guil saw 50 million cubic yards of mountain barreling down the slope, he raced ahead of the landslide and escaped by a mere 20 feet. When he made it back to his ranch, Guil’s wife didn’t believe the story. After he finally managed to convince her, they evacuated the ranch with all of their possessions. In just 24 hours the ranch stood in 18 inches of water, and soon Slide Lake stretched for five miles behind the dam, with Guil’s ranch and a ranger station floating on the lake.
Two years after the Gros Ventre Slide, the dam created by the slide broke, and the resulting flood destroyed the town of Kelly. Hundreds of livestock were killed, and some residents that refused to leave or were not able to escape in time died in the flood. A bank safe was washed away in the rush of water and could not be found. Just ten years ago, the safe was discovered, buried under debris many miles downstream from Kelly.
At the time of the flood, Jackson and Kelly were competing for the county seat. With the destruction of Kelly, Jackson was awarded the county seat. Today, Kelly has a population of 138.