Familiar Facies (Are Geology Puns Rock Bottom?)

Familiar Facies (Are Geology Puns Rock Bottom?)

Natural resource professionals in Muir Woods National Monument are gearing up for a Salmon Habitat Enhancement Project scheduled for next summer. This project will restore sections of Redwood Creek by removing large boulders cemented into place over 80 years ago to create rip rap. While rip rap was installed for erosion control, it has created poor habitat conditions for juvenile Coho Salmon. Juvenile Coho need pools to feed and grow, but rip rap interrupts natural geomorphic processes like pool formation.

Rip Rap

In general, pools are deeper and calmer areas in the creek where the bottom is characterized by finer materials like sand. Often times, pools occur before and after riffle features where shallow water moves quickly over gravel. To help understand the impacts of rip rap and how it affects the geomorphic processes that create features like pools and riffles, scientists can map the distribution of different rock sizes with a facies map.

To make a facies map, a patch that appears to have a similar rock size composition  (sand, gravel, cobble, etc.) is designated and drawn onto a base map of the creek. A randomized selection of rocks within that patch are measured to the nearest millimeter. Depending on the individual measurements, each rock is placed within a certain category (very fine gravel, large cobble, medium boulder) and then, using a sample size of 100 or more rocks, the relative abundance of each size class can be determined and the patch can be classified accordingly.

Measuring a Cobble Patch

This week in the creek, we used a tool called a gravelometer to help us quickly and efficiently measure a selection of rocks for our facies patches. The gravelometer has a series of cutouts corresponding to the various established rock size classes used in facies mapping.


Pushing a rock through the smallest allowable cut out identifies the size class… and will keep you entertained as you work through the rocky creek bottom. Although some people may take this kind of work for granite, I think it’s the schist (metamorphic-ally speaking). 


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