Looking at a cut into the Cape Blanco terrace deposits, one can see a generally fining upwards trend, meaning the environment was becoming less energetic over time. There appears to be a very specific horizon that appeals to some sort of burrowing creature, likely some sort of shore bird. But we didn't see any inhabitants, and I don't know birds well enough to hazard a guess as to what kind they might be.
I have presumed these were of marine origin, but if you look carefully at the gravel in the lower portion of the cut in the full-size image, it looks imbricated (clasts are consistently tilted in a particular direction), in this case up to the right and down to the left. This would indicate a consistent flow, as in a river, to the right, and weigh against a shore environment. So perhaps this was deposited by an ancestral Sixes River? As always, I reach a point where I have to say, "I don't know."
Followup: When I went to find the FlashEarth location, I realized this was taken looking almost directly south, and the face we're looking at is nearly east-west, with west on the right. This means flow was to the west, and would be entirely consistent with a river draining to the ocean.