Looks like marble which means it can’t be that old.
I’m more likely to think it’s in the range of 500 years old. Probably a left-over from some Spanish expedition into the region.
Re “Looks like marble which means it can’t be that old”.
Sorry, it would exactly the opposite, esp. depending on whether it is very old marble or another form of limestone (softer, due to precipitation in an ocean, shallow sea, lake, etc). Much of the very ancient US, up to the end of the dinosaurs about 65 million years ago, was under water (i.e. the Great Salt Lake/Bonneville Flats, Dinosaur National Park, the Badlands, Green River formation (full of fish fossils), etc).
Marble is actually pressured limestone, often due to it being broken up and reformed under tremendous deeps of material and heat.
Limestone can come from the erosion of earlier limestone formations or marble, which is often precipitated by micro-fossils including foraminifera, globogerinua (sp. is off), life forms, etc.
Give it the old “acid tests” and find out what its chemical composition is. That will be a good clue as to how old the “rock” is and maybe where it came from.
Otherwise, I suspect a bored Spaniard with a couple of good chisels and time on his hands.
Wouldn’t that depend on how much rain the area gets? Marble gravestones can wear smooth in 100-150 years in a climate that gets 40-50 inches of rain per year. I also wonder how dust and sand storms would factor into amount of wear. Either way these look modern to me, either forgeries or Spanish markers.