Hercules Slump and Earthflow EarthCache
Hercules Slump and Earthflow
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This is a drive-up earthcache located within the city of Hercules and may be observed and logged from your car or by standing on the sidewalk. It is not necessary to venture off the public sidewalk.
The soils of the California landscape are notoriously unstable. With its seasonal rain, no winter goes by without media reports of mudflows which imperil property and human life. More common and less spectacular are slumps with subsequent earthflows. This is a form of mass wasting in which a mass of rock or, as in the case of this earthcache, a thick accumulation of cohesive clay moves down slope as a unit. Usually the slumped material does not travel spectacularly fast nor very far. The surface of rupture beneath the slump block is characteristically spoon-shaped and concave upward or outward. As the movement occurs, a crescent-shaped scarp (cliff) is created at the head and the block’s upper surface is tilted backwards. Sometimes water is impounded between the base of the scarp and the top of the tilted block. As anyone living in the Bay Area knows, these clays become slick when wet. So as the water percolates downward, the upper layers lose their strength and slide easily past each other resulting in slope failure. The displaced soil then flows down hill as an earthflow.
The slump at the coordinates occurred several years ago and continues to move a few inches or feet every winter. The last couple winters have been somewhat dry, so little movement has occurred. Just above the sidewalk you can see the remnants of a concrete retaining wall that has partly collapsed or has been topped by the moving clay. During some future wet winter the soil will again become saturated and the mud will flow onto the sidewalk and street. That undoubtedly will send a city crew scurrying to clean up the mess.
From the sidewalk, you can see the scarp on the hill above. You can move to either side of the slide on the sidewalk to observe the upper part of the slump. A broken concrete drainage ditch, a black plastic drain pipe and a failed attempt to prevent water from entering the slump using black plastic sheeting are also visible.
TO LOG THIS CACHE you must do the following:
1. The earthflow is irregular in shape and height. But you can still estimate of the length (L), width (W) and height (H) of this feature. Then calculate the total volume of displaced earth in cubic feet and send me your answer by e-mail. Do not post your answer.
Additional Hints
(Decrypt)
Ibyhzr = Y k J k U
Treasures
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