A little bit of conjecture about this photo:
...(T)he Cherokees believed that thunder was a horned snake within the rain which connected the sky vault, the human-built houses on earth, and the underground or underwater townhouses (Mooney 1900:481). Mythical stories like this portrayed a tiered cosmos in which similar structures and deities were nested at different levels, but yet interconnected via portals, such as sweat lodge entrances (including front doors, smoke vents, and fire pits), river pools, and caves leading into mountain tops (Figure 2).”
The Socio-Economic and Ritual Contexts of Petroglyph Boulders in the Southeastern United States.
Johannes (Jannie) Loubser, PhD, RPA Stratum Unlimited, LLC
"The serpent effigy is found in many places...The walls of the fort (Fort Ancient) are in the shape of massive serpents, the heads of the serpents forming the gateways..."