Nice photos Ralph. Dave P is on the right track I think. Mortars used for turning acorns into paste which was then soaked in warm water to leach out the tanin the paste was then dried and cakes were formed and cooked on hot rocks. real yummy stuff if you like the taste of chalk.
Many cultures used them(mortars) and the acorn was their primary food as wheat is ours. I’m most familiar with the Kumeyaay of southern California, but the process is the same in most south western cultures. Are there oaks around? That flat spot might have some points, beads and fetishes. It’s tempting even with a cranky neighbor I’ll bet.
