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One of the largest fast-moving landslides in generations occurred near the village of Kelly, Wyoming. In just three minutes, huge amounts of rock and debris cascaded down the north slope of Sheep Mountain.
Landslides are natural hazards that cause millions of dollars of damage each year and can also cause many deaths. They are defined as downslope movements of soil and rock under the influence of gravity. They are triggered primarily by water, the soil becomes saturated with high rain fall or melting snow. Earthquakes can also lead to some spectacular landslides.
The stability of a slope can be described as two forces working against one another. Gravity and resistance are the two forces working against each other. A slope becomes unstable when the power of the gravity is greater then the resistance of the materials against each other. Water reduces the resistance and vegetation will help to increase the resistance though its root systems.
As the slope angle increases, the driving forces also increase. Few landslides occur on slopes less than 15 degrees. Certain slope materials also have weak strengths and low resisting forces to landslides. For example, clay, shale, serpentine, and new fill are prone to failure.
If the slope bedrock is inclined and is somewhat parallel to the slope, it is called a dip slope. Landslides are prone along failure planes (clay beds and old soils) on these dip slopes. The Gros Ventre landslide is a good example of this.
To stabilize a slope or prevent landslides, one needs to lower the slope angle, drain the slope of water, build retaining walls, plant vegetation, and avoid building on old landslides. If the slope has moved once, it has a high chance of moving again.
Classifications of different landslides are: Flow, Debris avalanches and Creep. Flows occur when material moves downslope as a thick mass of water and soil. Fastest and largest landslides are called debris avalanches, because they travel at speeds up to 180 MPH and can travel long distances in excess of 18 miles, and are very large. Some movement on slopes can be imperceptible, called creep. Creep is caused by gravity, and is assisted by freeze–thaw or shrink–swell processes operating on the soil sediments. Evidence of movement is found in cracked walls, leaning telephone poles, and leaning trees that adjust (straighten) their direction of growth, creating so-called "trees with knees," also called pistol-butt trees.
The Gros Ventre Landslide.
It is believed that the landslide occurred on June 23, 1925, one of the largest fast-moving landslides in generations occurred near the village of Kelly, Wyoming. In just three minutes, huge amounts of rock and debris cascaded down the north slope of Sheep Mountain. An estimated 50 million cubic yards of rock and soil slide down the face of the mountain, crossed the valley and up the face of the opposing mountainside that you are standing on. The landslide caused a dam in the Gros Ventre River creating the Lower Slide Lake. In May of 1927 the dam failed and flooded the area. It is believed that there were three major factors contributing to this landslide. There was the heavy rains and rapid melting of the heavy snowfall that winter, the river had cut thought the sandstone making a steep slope along the river and the mountain contained many swampy pools on top of the mountain. It is also believed that there were earthquake tremors in this area at this approximate time frame and could have triggered the start of the landslide.
One of the sign at the pull off, A River Dammed
The Gros Ventre slide may have been started by heavy prolonged rainfall. Some 50 million cubic yards of sandstone, limestone, shale and rock, about one mile long, 2000 feet wide and several hundred feet deep in places, plunged down and formed a dam 225 feet high and half a mile wide across the Gros Ventre River. A lake was created above the dam.
For almost two years this earth dam held; then on May 18, 1927, part of the dam gave way and a wall of water, mud and rock flowed down the… (No help here for geocache GCGFE7). Ranch lands were ruined by mud and rubble and the town of Kelly, three and one half miles downstream, was practically wiped out. Six persons drowned.
A search on the Internet will give you a lot to read. Or another interesting way is to visit your local library they have many books on the subject.
------------------------------------------------------ Cache Requirements -------------------------------------------------------------------------------------------------------------
In order to get credit for completion of this cache you can choose between doing item 1-A or 1-B. But you need to do all three requirements; 1, 2 & 3.
1-A. Take a picture of you (prefer of whole group) with the slide in the background or the large wooden sign stating Geological Area. (Post with log entry)
1-B. There are two small signs there also. One is entitled “A River Dammed”. What is the title of the other sign? And please do not include any pictures of either of the smaller two signs in your log. (Email Answer)
2. From the description of the different landslides that I gave you, what type of landslide is this one? Flow, Debris avalanches or Creep. (Email Answer)
3. Email me the information do not include answers in log entry of in photo please..
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