Serpent Mound

Serpent Mound is made up of three parts and extends over 1,348 ft (411 m), varying from 9 in (23 cm) to more than 3 ft (91 cm) in height and between 22 and 24 ft (6.7 and 7.3 m) in width. It conforms to the surrounding land and winds back and forth for more than eight hundred feet, with its tail coiling in seven areas. A triple-coiled tail ends the structure. The mound is made mainly of yellowish clay and ash, with a layer of rocks and soil reinforcing the outer layer.

E. G. Squier and Edwin Hamilton Davis interpreted the head as an open-mouthed serpent nearly engulfing an east-facing hollow oval feature 120 feet (37 m) long, which they believed might represent an egg. The archaeologist J.P. McLean identified the mound instead as the body of a frog. The western side of the effigy features a triangular mound approximately 31.6 ft (9.6 m) at its base and long axis. The mound lies within a meteorite impact structure, the Serpent Mound crater, which is around 8.7 miles (14 km) in diameter and was formed 252-330 million years ago; its topographic expression has been completely erased by erosion.

Throughout the 20th century, anthropologists and archaeologists disputed whether the Adena or Fort Ancient cultures created the mound. In 2014 radiocarbon dating determined that it was built in 300 BCE by the Adena, a date corroborated in 2019, and it was found that the mound was rebuilt or repaired around 1100 CE during the Fort Ancient period.

These dates were challenged by the archaeologist Brad Lepper, who noted that the material dated was organic sediment and not charcoal, and that the samples came from soil cores and not an exposed stratigraphic profile. Peter Steen Henriksen and colleagues have shown that such dates can be as much as 3,000 years older than the actual age of a mound. Lepper argues that the Adena are not known to have built effigy mounds or used serpent symbolism, whereas the Fort Ancient culture built the Alligator Effigy Mound in Granville, Ohio, and frequently depicted serpents. Lepper and colleagues concluded that the best available data indicated construction by the Fort Ancient culture.

In 1996 a team of Robert V. Fletcher and Terry L. Cameron, under the supervision of Bradley T. Lepper, reopened a trench created by Frederic Ward Putnam more than 100 years earlier. Of three pieces of charcoal, two yielded a date of ca. 1070 CE, within the Fort Ancient culture, and the third dated to 2920+/-65 years BP but came from a core sample below the cultural modification level. Charcoal from an ash bed near the conical mound excavated in 2011 was dated between the years of 1041 and 1211 CE, again to the Fort Ancient culture.