Mortar holes in a California suburban backyard

If you are momentarily startled by the title of this post, that may be because you are associating the word "mortar" with artillery.  Reconfigure your thought process toward "mortar and pestle."

I found the embedded image (which I've cropped for size and emphasis) in the archaeology subreddit.  The accompanying text is limited to a caption, which indicates that the homeowner's yard is in Mariposa County, California.  The current owners correctly note that these holes were created by generations of Sierra Miwok, who used them for grinding acorns into meal for food.

If you zoom the photo, it's evident that this outcropping of rock had over several centuries become covered with dirt and eventually turf, which has been scraped away toward the front and back. It is a careful restoratioin of an historic site, evidently destined to be protected for the future.   Nicely done.

For completeness, I'll add that the word "mortar" is almost as ancient as the practice, and is virtually unchanged from the Middle English: 

The English word mortar derives from Middle English morter, from old French mortier, from classical Latin mortarium, meaning, among several other usages, "receptacle for pounding" and "product of grinding or pounding"; perhaps related to Sanskrit "mrnati" - to crush, to bruise.