On June 23, 1925, one of the largest fast-moving landslides in generations occurred near the town of Kelly, Wyoming. According to the written records published by the USDA Forest Service, in just three minutes, huge amounts of rock and debris cascaded down the north slope of Sheep Mountain and changed the area forever.
Quoting from the Forest Service document, the rockslide hurled down the slope at 50 mph, the mile-wide slide carried 50,000,000 cubic yards of debris. The mass rode 300 feet up the opposite slope, blocked the Gros Ventre River, and formed a five- mile long body of water known today as Lower Slide Lake. The piles of debris seen today contain large chunks of Tensleep Sandstone, along with remnants of the original forest.
Throughout the years, many people have wondered what caused this tremendous slide. Three primary factors are thought to have contributed to the unusual event:
- Heavy rains and rapidly melting snow saturated the Tensleep Sandstone, causing the Amsden Shale rock layer on Sheep Mountain to become exceptionally slippery;
- The Gros Ventre river, cutting through the sandstone, produced a “free side” with no extra support holding it in course;
- Swampy pools with no outlets, on top of the mountain, indicating water-saturated soil.
Earthquake tremors (which were occurring) added to these already unstable factors and could have precipitated a landslide.
However, relatives of the Arapaho, the Gros Ventre Indians, have their own version of this slide.
Shown pictures of the sliding hillside by Ella E. Clark in the 1950’s, a Gros Ventre man on the Fort Hall Reservation told the following legend to which his people had handed down for generations.
“Long ago, there was a large cave which was entered from the side of this mountain in the picture. A stream flowed through the valley, and there was plenty of wood for fires. Few people ever went into the cave because it was so very large and because its far interior was dark. Only during times of danger would the Indians use it as their retreat.
Once when a large party of hunters were killing buffalo, the sun became covered with something. Darkness came in the middle of the day.
People were very much frightened. They thought that the great ball of fire (Coona) had gone out. So they went into the cave, driving before them a herd of buffalo. Almost immediately, the entrance fell in and completely closed the cave. As the day was very dark anyway when they entered, the people did not know they were shut in. They felt safe in their retreat.
As there were no other opening to the cave, they are still there. They are now a large tribe. The buffalo also have increased and are now an immense herd. When the Indians within the cave gather to hunt and kill buffalo, they have a great roundup. Then the ground shakes and trembles. The mountain is made to quiver and great landslides occur.
When the mountain is quiet, the Indians are sleeping.”
Page 271, Indian Legends from the Northern Rockies by Ella E. Clark, 1966
For more eyewitness accounts of this slide, you can follow these links. Unfortunately, six lives were lost as a result of this show of nature. You can also visit the site itself by Kelly and still see the damage caused by the slide.