I agree with Bodrk43: it looks like a benchmark or surveyor's mark of some sort. If it's a US Geological Survey benchmark, then destroying it is a crime. If it was set by some other agency or a private party, I don't know what the law says but I would be hesitant about destroying it.

I'm a member of the Geocaching hobby, and some of us also engage in the even geekier sub-hobby of searching for old benchmarks like this. If you could post the GPS coordinates or even the approximate location of this marker, I could look it up on some government databases and other listings of benchmarks to see if there's a record that would tell us what exactly it is.