Note these unstable bluff signs. Of course most
bluffs are unstable; however, we have a higher risk of landslides on bluffs
here in the Pacific Northwest, especially when it rains. Does anyone know why?
(field answers) It is because of our glacial soil layers, and the clay layer
that makes soil slick and slippery. To understand how this came to be, we must understand the geologic history of this area.
About 14,000 years ago, this area was covered by glaciers reaching southwards from Canada. In fact, glaciers created a lobe shape that reached down into the Puget Sound region and helped further carve out the landscape before retreating. Much of the Puget Sound Troth was the result of a massive lake, called Glacial Lake Russell, that was formed when the glacier prevented drainage of water back into the ocean. When the glacier moved over it, it brought in soil and rocks that had been ground up by the movement of the heavy ice. As it progressed, the glacial soil began to fall out of the watery solution first farther away from the oncoming glacier, where the water is more calm. The soils falls out of solution in order of the smallest to largest particles. Accordingly, clay, the smallest particle, falls to the bottom of the lake bed first farthest from the glacier. It is then covered by the next largest particle, sand. As the glacier moves forward, it brings progressively larger pieces of soil with it that continue to stack. The next is silt and then finally, large pieces that are appropriately named glacial till. Thus the soil layers look like this (show picture). Pre-Lawton Sediments (soil here before the glaciers), Lawton Clay, Esperance Sand, Silt, and Glacial Till. When these soil layers sit on top of Pre-Lawton Sediments, the clay layer creates a landslide hazard. Water can permeate down through larger pieces of soil, like till, silt, and sand. However, clay is so small that water can’t really permeate through it, and so the water will actually runoff that soil layer underground. Just imagine your soil sitting on top of a bar of soap. The variety of soils in Washington beg for better knowledge of which soils are good for building on and for specific land uses, and for landowners to heed their predetermined nature.
About 14,000 years ago, this area was covered by glaciers reaching southwards from Canada. In fact, glaciers created a lobe shape that reached down into the Puget Sound region and helped further carve out the landscape before retreating. Much of the Puget Sound Troth was the result of a massive lake, called Glacial Lake Russell, that was formed when the glacier prevented drainage of water back into the ocean. When the glacier moved over it, it brought in soil and rocks that had been ground up by the movement of the heavy ice. As it progressed, the glacial soil began to fall out of the watery solution first farther away from the oncoming glacier, where the water is more calm. The soils falls out of solution in order of the smallest to largest particles. Accordingly, clay, the smallest particle, falls to the bottom of the lake bed first farthest from the glacier. It is then covered by the next largest particle, sand. As the glacier moves forward, it brings progressively larger pieces of soil with it that continue to stack. The next is silt and then finally, large pieces that are appropriately named glacial till. Thus the soil layers look like this (show picture). Pre-Lawton Sediments (soil here before the glaciers), Lawton Clay, Esperance Sand, Silt, and Glacial Till. When these soil layers sit on top of Pre-Lawton Sediments, the clay layer creates a landslide hazard. Water can permeate down through larger pieces of soil, like till, silt, and sand. However, clay is so small that water can’t really permeate through it, and so the water will actually runoff that soil layer underground. Just imagine your soil sitting on top of a bar of soap. The variety of soils in Washington beg for better knowledge of which soils are good for building on and for specific land uses, and for landowners to heed their predetermined nature.
Photo credit for soil diagram: http://www.tubbs.com/ic52/ic52.htm
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