Assuming these Markings are real(I am not there so I could not tell for sure). What I would do is try 2 different things.
Get yourself a strong battery operated Black Light(ultraviolet)(I would recommend a short wave) and go out there at night and shine the black light on the stone. The markings will likely be clearer and you might be able to see more of what looks like markings that have worn away.
The second option would be to learn about inferred photography. There is plenty of information available. Experiment and learn. Then go out there and shoot inferred photos of it. More details might be revealed.
ericwt