Wow... I can't believe it's taken me so long to process through my Billy Goat Trail photos from a month ago! Here's the final batch: a collection of images of potholes. Here's a typical pothole in metagraywacke of the Mather Gorge Formation:
Differential etching of the mica-rich and quartz-rich layers suggests that sand or silt is responsible for much of the carving of potholes: picture a liquid tornado with suspended grit, focusing abrasion on a specific area of the sub-river bedrock. Later, the brittle fins of quartz may be snapped off by shearing stresses when larger clasts smash into them, such as the pebbles and cobbles seen above.
Here's a trio of itty-bitty potholes in migmatite:
And, lastly, a nice waterfall-carved (now dry) series of chutes and plunge-pools, again carved into migmatitic metagraywacke:
...And a more zoomed-in shot to give a better sense of the complicated topography here:
I'm heading out on the Billy Goat Trail again today, and also Thursday and also Friday... a busy week of field-tripping. Hope you can make it outside too!
Labels: geomorphology, hydrodynamics, rivers, sediment
