Its called a benchmark, used by surveyors as a known point from which to take readings. We had a benchmark on the edge of of our property in the town where In grew up. It was a square, white marble column, like a truncated obelisk with the point cut off to make a 4" square flat top, buried at the roadside and standing a couple inches proud of the grass. It had a cross with a dot in the middle, and a geodetic ID number engraved on the top.

Nailed it with the lawnmower once or twice. That'd really put the Fear in you.