Oddly enough just for giggles I marked on a piece of paper the spacing of the divots at the lowest point. They are all 5mm apart exactly. And when I run the 'rock' on a piece of paper it leaves a dark line.
I think thewatchers got it with mammoth tooth. I found one similar about 2 years ago in Oklahoma, but cannot take a picture of it currently. Here is one from the web. Soapy water and a toothbrush would probably get you some more detail out of it.