Friday, September 23, 2011

Clay Slip

I noticed that the shale in this section of stream bank was decomposing much more rapidly than any of the other exposed sections. Cracks have actually formed near the top of the exposed face. This indicates a major shift of material downward toward the creek. I haven’t seen any of the other shale outcrops behave in this manner.

Instead of standard stream bank erosion, there is a slip in progress here. A slip is a condition where a mass of soil moves downhill as a single unit, as opposed to erosion where soil is stripped away only from the exposed surface. No amount of vegetative cover or root mass is going to stop a slip. As seen here, the soil just slides out from under the plants.

Plants that began life high up the bank, end up at the bottom. This Ironwood will eventually be washed on downstream. There’s a chance it could lodge somewhere and reestablish in a new location. This is another example of how plants colonize new areas.

As the toe of the slip moves into the creek channel, normal stream bank erosion carries it away. This cycle will continue until the slipping hillside stabilizes.

I was still puzzled by the rapid formation of the slip. There are larger areas of exposed shale that stand almost vertically against the creek and show no signs of slipping. The appearance of this shale bed is no different than any other.

A more physical examination shed some light on the occurrence. My thumbnail could easily carve a depression in the sample. The entire bank is composed of compressed clay in layers identical to the shale formations. Given enough geologic time, this clay could have become shale. As it is now, the clay has the look of shale, but not the strength. There’s no telling how large this pocket of clay could be. The slip could eventually reach far up the hillside. I know it won’t go to the top of the hill, because limestone takes over somewhere between here and there. I’ll just have to wait and see what happens.

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