The variability of grain size in this boulder is pretty awesome. A nice thing about coarse material like this is that you can really get a sense of the parent material. Sometimes, with sands, you can get sense under a microscope, but often, even then, it's frustrating. There's a granitic/dioritic clast near the center of the photo, which is what I'd suspect if we are in Tyee Formation. Tyee was formed when the drainage feeding this basin apparently captured a stream coming across the northern portion of the Rockies- in particular, its sediment was derived from erosion of the Idaho Batholith, or something similar and close-by, which might now be obscured by younger cover. Prior to Tyee deposition, this was a much more limited sedimentation area, the Lookingglass Formation, dominated by the much smaller Klamath Mountain area, and with a correspondingly higher proportion of low- to moderate-grade metamorphic rocks. Given the limited number of granitoid clasts in this boulder, I'm wondering if this might in fact be Lookingglass rather than Tyee.
Photo unmodified. March 9, 2012.
FlashEarth location. (This is probably accurate to within 10-20 miles. Sorry, no clue.)
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