Not sure. I know one corner of my parent's land in Montana has a physical marker. Maybe because it's a joint boundary between state, federal and private land. It is also really hard to find, as it's in the bottom of a ravine in heavy brush and it's a ground level bronze (I think) spike like an oversized nail hammered into the ground. You really have to know where the boundary is within a few feet already to have even the slightest hope of finding it.

I found it once as a teen, but 2 decades later, I'd be hard pressed to find it.