Al-Naslaa rock in Saudi Arabia is one of the most photogenic petroglyphs in the area. The perfect slit between the two standing stones and the flat faces are completely natural.
It is said, it is most likely the ground shifted slightly underneath one of the two supports and the rock split. Could be from a volcanic dike of some weaker mineral that solidified there before everything was exhumed. Or, it could be an old pressure crack (you see a parallel crack just to the right of it) that has been pushed/pulled apart some. Or, it could be an old (minor) fault line, since fault motion often creates a zone of weakened rock that erodes relatively easier than the surrounding rock.