They’re drill marks from cutting the stone.

That’s a good point Phil. We did see posts still in use with pins in them to hold the wire though.

Yeah, they'd use that way of attaching the wire. Those grooves left by the drills were handy for keeping wire in place too! Those posts can weight several hundred pounds; that's probably one reason you often find them where they were put originally :). I have just a piece of one that often finds its way into some seasonal decor here at the house...

https://i.imgur.com/dafltK9l.jpg

I find it interesting to see how different regions solve the same problems. Fence posts are a good example. Depending where you go they’re made out of different materials, using what was available regionally. Osage orange is pretty common where I’m from, Kansas definitely takes the award for heaviest fence post and probably most long lasting!

The Midwest prairie can grow Osage Orange also. It is some of the hardest — and heaviest — of all woods. You can dull a chain saw or burn out a stove with it.