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.
That area probably averages about 1-2” per month of moisture (rain/snow).
Not a lot of sandstorms in that area, dust sure.
I was leaning toward bored sheepherders. They left lots of interesting things in that area.