That is a surveyor's mark called an Above Ground Marker. It is used as a point of reference when they survey they land.
What is this thing?
Let us help you identify that mysterious object you’ve found.
Currently in CHALLENGE mode: If you've got something obscure knocking about, post a picture, and let's see how we do. Please prefix such posts with "CHALLENGE:" so we know we've got a fighting chance.
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.
My grandma has a little concrete thing like that that marks where her property ends (or where 1 part of it ends, legally it's 2 rectangles sharing a side). Once after not mowing for like a month and a half I forgot it was there and totally ruined the freshly replaced blades 🥲
They hold the sidewalk down, don't pick it up or the whole block could blow away.