I found this Indian grinding stone (mortar or metate) yesterday afternoon while on a hike in the Santa Barbara backcountry. I had, funnily enough, after hours of hiking, just arrived at a place of interest to me for camping when it caught my eye. Apparently the location had served well for other campers of times long ago past. This is a remote, rugged place with no trail access.
The photo above makes it hard to see the concavity of the dish, but the second photo below, which shows the stone on edge, gives a slightly better view of its curved basin form.