I'm going to go out on a limb & call this stuff ochre. It's dirt, and it's yellowish. Close enough for me right now. It also drys my hands out much like Burning Man Playa dust, so I wonder if it is an alkaline... I might test that out with vinegar to see if it bubbles. If so, maybe I should neutralize it before using it on a painting.
I got tired of waiting for the slurry to dry, so I scooped out the good stuff with a putty knife & spread it on paper towels so it would dry faster. I figured this would be OK, because I was planning on doing two slurrys to get the best of the best.
When it's wet, it gets really dark and brownish. The brown piece between the putt knife & the dried ochre is what a moment prior was dry ochre, but sucked up some moisture from the wet ochre on the left. This makes me think it's not going to mix with walnut oil well. I'm guessing it will work better in egg yolk. I'll have to try them out side by side and see what happens.
On the left is the silt that I scooped out & on the right is all the sand & coarse dirt. I'd guess the proportion is about 1/8 silt.
This much pigment came from about 1/4 pound of dirt. I then took all of this, and mixed it in water again (not pictured). I was surprised to see a fair amount of sand separate out to the bottom! I'll update with final pigment & tests in a few more days. It will have to dry (not get scooped out) this time so I can scratch off the coarse particles more carefully. That takes about a week.