The lichens have grown over the paint, so it's at least a couple of decades old. Wikipedia says it grows at around 1mm per year.
Hessians sometimes used that type of cross, and fought in the American Revolution - including battles in South Carolina. Would be impressed if their paint would last 240 years in the SC humidity, though.
Could have just been some bored kids. Iron crosses were popular with heavy metal bands. Or it could have been some neo-nazis or something. Too bad the other shapes are so deteriorated. Can't really tell what they are supposed to be.
I want it to be some kind of treasure map, but its likely just graffiti. You could take a sample of the paint and pay to have it analyzed. The composition of the paint would tell you its age. If it turns out to be very old, then you could take the pictures to an archaeologist or historian at USC or Clemson and see if they can make any sense of it.