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Rockfall, Bookslam!
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N 37° 43.761' W 119° 33.549' |
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Fish Camp, California 93623 United States Near By Caches
Hidden On: 30 Sept 2006
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ROCKFALL, BOOKSLAM! July 10, 1996
A VERY UNUSUAL ROCKFALL: Looking up from these coordinates, you are viewing the results of a very unusual rockfall that occurred on July 10, 1996 at 6:52 pm. Something different happened here that is rare on the planet Earth. This rockfall was different because the rockage fell away from the cliff it was attached to, flew downward through the air, and collided with the lower part of the mountain. Upon impact, the rocks hit with such force they created a tornado strength wind that snapped the tops of trees and blew large trees over. This wind was deadly. It caused destruction far beyond the reach of the avalanching rocks. BOOKSLAM: When you slam a book on a table, you create a wind that blows dust and small objects away. The wind caused by slamming the book, is the same effect caused by a rockfall slamming thousands of tons of rock into the side of a mountain. This was two rockfalls to start: First one mass of rocks fell and then 14 seconds later, another rock mass fell from the 2,182 foot high cliffs. The two impacts were recorded by seismographs as much as 124 miles away in central California and western Nevada. The impact area of the rockfalls was approximately 10 acres. The airblast propelled in front of and beyond the avalanche with velocities exceeding 246 miles per hour. It created a huge gray sandy dust cloud that was abrasive and dense. It devastated the area below the impact sites, snapping the tops of trees, and toppling full sized trees, pulling them up by their roots. This was in addition to the rockslide avalanche that clobbered everything in its path. GPS machines were used: National Park Service observers used GPS readings from a helicopter to estimate the volume of the rock involved in this rockfall. They estimated that the combined volume of this rock fall was of 78,000 cubic yards weighing 80,000 tons. EXFOLIATION: The source of the rockfall was an arch of exfoliating rock, the majority of which detached in two large blocks. These two blocks slid down a steeply inclined ramp or shelf and then fell through the air on a ballistic trajectory about 1,800 feet at 270 miles per hour before hitting the northern part of a talus slope at the base of the cliff. The dust kicked up from the pulverized rock billowed hundreds of feet into the air and covered everything nearby with up to 2 inches of dust. It blocked out the sun and coated tents and recreational vehicles, not unlike the ashfall from a volcano. DESTRUCTION: The impacts of the blocks generated atmospheric pressure waves resulting in a wind comparable in velocity to that of a tornado or hurricane. At 174 mph the airblast uprooted and snapped about a thousand trees. Falling trees damaged a nature center, crushed a snack bar, destroyed a bridge, killed a hiker, and seriously injured 14 people. One young woman who had to be lifted out by helicopter, was listed in critical condition with fractures in her neck, arms, and legs. DUSTCLOUD: Just after the airblast, the billowy dense sandy cloud of pulverized rock descended from the impact site toward the nature center, abrading trees and depositing gravelly coarse sand. Dust from the cloud rose rapidly into the air and plunged the area near Happy Isles into darkness for many minutes. If you were there: If you were standing at the coordinates when this rockfall happened, the tornado like wind would have torn off your clothes and thrown you towards the Vernal Falls trail on the other side of Happy Isles. AFTERAFFECTS: Additional, smaller rockfalls occurred later that night and the next morning. TO LOG THIS CACHE: Go ahead and log your find. Then email to me the answers to the following questions: Standing at the coordinates, 1. What barriers limit where you can walk? 2. What damage do you see that was caused by the rockfall? 3. Does it appear that this damage was caused by the avalanching rocks, the tornado like wind, or both? 4. What is the first and last word from any one of the informational signs. ADDITIONAL INFORMATION: Walk on the island paths to enjoy the beauty and to see more damage from the rockfall. Be sure to visit the nearby nature center and the explanatory signs next to the nature center at coordinates N37 43.807 W119 33.572. |
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