grind stone

Found this on the Santa Ynez trail, still getting familiar with natural erosion patterns. The depression gets wider on one end, which is where my doubt lies. Otherwise it’s pretty symmetrical. Any thoughts? Thanks!

I found this grindstone while metal detecting in Eastern NC. I also found a few archaic period artifacts nearby. On closer inspection I noticed intersecting lines / channels and notching along some of the channels. I would appreciate any feedback on whether the marks represent run of the mill...