My guess is that it would be a Native American bedrock metate where they were grinding nuts and things. But I am not an expert others may see something else in it.
Yep grind in the flat area leach in the hole...I bet that sang a song that pretty much had the same lyrics...with many verses added over the 1000 years it took to wear in that hole..
The picture I put up was from Indian Grinding Rock State Park but it could be any one of many places in the Sierra foothills with native culture history.
Metate of all shapes and forms are found all over the Americas. Here in the southwest deserts they were kept in several locations and just turned over until the next use. I've found several Metate and Mano in the open still waiting for their owners return. They were a basic kitchen appliance for many thousands of years and are still used in many areas of the Americas.
They are indeed a treasure to those who depended on them.