smooth with scratches (by Ale*)
Pothole Dome is a “roche moutonée”, a block of rock over eroded and polished by a glacier in motion over it. Among the various indicators of such motion, one is the very smooth surface of the rock surface, caused by the grinding of very fine (“rock flour”) sedimentary particles. Another is the presence of parallel scratches on the rock surface, caused by bigger sedimentary particles trapped at the bottom of the ice mass in motion. In this image we can see the smooth surface of the granite, which shows crystals of quartz, biotite, potassium feldspar and very big white crystals of sodium plagioclase, and the scratches, which are particularly visible on said plagioclase crystals. An idea of the size of these features is provided by the lens cap edge on the right.
Pothole Dome, Yosemite National Park,
Tuiolumne County, California
Fuente: Flickr / greenriver