This time I found the pestle too (I think)

[SurfaceHunter]

You should post the mortar in a separate post it’s very nice looks almost like a stone bowl.

[SurfaceHunter]

Maybe even a anchor. That’s one for CMD to see he has a lot of knowledge. Won’t get much people looking at it here do another post by itself

[tomf]

I was trying to figure out how this could have been worn quite so far (all the way through and then some), then someone suggested it was used for leeching acorns, as opposed to grinding.

I've read that acorns were leeched in baskets, but a bowl like this lined with reeds or grass would work well too.

I think it's an interesting idea.

Otherwise it's a mystery why the hole is worn through to the extent it is.

[Sage hen jack]

That pestle is nice find. Any Artifact find in California these days is rare. I once saw an old turn of century photo taken on a ranch property-foothill region of Sacramento Valley. Hundreds of huge Mortar & pestles all lined up along a rock fence! I have a pestle very similar with portion of the wide end missing like yours but worn smooth too. My guess -they were still continued to be used as long as they were functional.