Saturday, January 24, 2015

Geo 1095: January 23, Day 753: Rock Fall

A week to two weeks ago, when I was discussing the large waterfall upstream from this spot, I emphasized the importance of the sedimentary beds between flows of Columbia River Basalt. Those beds weather and erode out faster than the basalt, undercutting it. This leaves suspended slabs of rock hanging, unsupported from below. From time to time, large blocks break off, which helps to maintain a vertical drop- that can be seen particularly well in this post. The same process works along the cliffs surrounding this portion of the canyon, though without the influence of a major stream, at a slower pace. Nevertheless, all along this reach of the stream, hikers often see large boulders on the valley floor. These have broken off the precipices surrounding the gorge and rolled to the bottom. I doubt even the largest trees would slow down blocks of this size. However, that vegetative cover couldn't have developed very quickly. I suspect these required on the order of decades to develop that thick moss and fern pelt. So it's not very frequent, and not something to worry about. But one definitely wouldn't want to build any kind of permanent structure down here.

Photo unmodified. August 30, 2012. FlashEarth Location (approximate).

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