Something I Found

On my Tuesday walk, I spotted this perfectly round rock laying at the edge of the ditch next to a stubblefield. Thought I'd better carry it home, rather than leaving it there to be turned under when the cultivator goes through, or to disappear when the grass and weeds grow tall again.

Wonder what it could be. 

If it could be an artifact, I need to report it somewhere. 

If it's an artillery ball, wouldn't it be darker? 

It's quite heavy. 

I'll post it on FB and Twitter, see if anyone has ideas.