BEDROCK CHANNEL EROSION

There are four main mechanisms of bedrock channel erosion—abrasion, dissolution, cavitation, and weathering-and-plucking. The latter occurs when weathering along joints and bedding planes of the bedrock loosens slabs or clasts, which are then entrained (plucked) during high flows. Cavitation is difficult to observe or prove in the field, but likely occurs in the stream I visited this week, Raven Run (near Lexington, KY). The other mechanisms all clearly exist.

Weathering and plucking is the dominant erosion mechanism of the bedrock streams hereabouts—the photo shows the flat surfaces and angular features that result from weathering along the horizontal bedding planes of the limestone and the frequent vertical joints, and subsequent removal of the resulting slabs.

Raven Run, Kentucky.

 

In this reach of Raven Run, however, there are numerous potholes and cavities that result from a combination of dissolution (this is a fluviokarst area, after all) and abrasion. Abrasion is not dominant, because it requires “tools” (gravel to abraid the bedrock), and there isn’t much here, and few are rounded, as generally occurs with abrasional “grinders.”  However, the near-circular shape of some of the potholes and presence of grinders in some of them indicates that some abrasion is going on. In addition to the potholes, there are lower-relief sculpted forms on the channel bed indicating dissolution.

In the photo below, check out the difference between the moss coverage on the left side of the photo on the shaded north slope, compared to the slightly less shaded south slope. What is the relationship between the moss cover and channel processes? The moss probably facilitates weathering during lower flows by holding moisture, adding CO2 and organic acids to it, and facilitating microbial activity. During higher flows, does it provide significant protection against abrasion? What kind of hydraulic conditions or abrasive bedload transport does it take to remove the moss cover?

Danged if I know, but there’s a thesis there for somebody.

Raven Run, Kentucky.

 

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