u/Imaginary-Point-7721

Found this odd rock on the banks of the Snoqualmie River in Washington. The dark, oddly round spot caught my eye. Upon further inspection the spot is a bit of a depression, is smoother than the surrounding rock, and the darkness doesn’t appear to be from a change in the rock grain, like it could be stained. Wondering if this might be a very old tool of some sort?

Should have given something for scale, but you can see the tip of my boot in the first photo. Spot is maybe 2” diameter.