writing?

I found this large rock in an abandoned garden, just north of Crescent City, California. Can anyone tell me what it is and what made those marks? The rock is approximately as big as a platter, and the marks go all the way around the rock in by a complete circle. Thank you!

While fossil hunting with my kids in South Jersey (Sewell area) in late Cretaceous green marl, my 4 year old son found this triangle shaped metal?/stone? object in a stream. It didn't look like a fossil, so I ignored it but let him put it in the pail of fossils since he's 4. Later he wanted to...