Serpent Mound is the world’s largest surviving effigy mound—a mound in the shape of an animal—from the prehistoric era. Located in southern Ohio, the 411-meter-long (1348-feet-long) Native American structure has been excavated a few times since the late 1800s, but the origins of Serpent Mound are still a mystery. Some estimates place the construction of the National Historic Landmark—also called Great Serpent Mound—at around 300 B.C.

What Is Serpent Mound?

As its name suggests, Serpent Mound resembles a giant sinuous snake with a curled tail at the west end, a head at the east end, and seven winding coils in between. In all, the snake stretches a quarter of a mile and ranges from 1.2 to 1.5 meters (3.9 to 4.9 feet) in height and 6.0 to 7.6 meters (19.7 to 24.9 feet) in width.

Serpent Mound is located on a high plateau overlooking Ohio Brush Creek in Adams County, Ohio, about 73 miles east of Cincinnati. It’s on the site of an ancient meteor impact dating to around 300 million years ago; the crater, measuring 8 to 14 km (5.0 miles to 8.7 miles) in diameter, is known as Serpent Mound crater.

Experts disagree about what the head of the mound represents, with some scholars positing the oval shape signifies an enlarged eye while others believe it’s an object—a hollow egg, for instance—being swallowed by open jaws.

Purpose of Serpent Mound

Serpent Mound may have had a spiritual purpose, given that the many native cultures in North and Central America revered snakes, attributing supernatural powers to the slithering reptiles.

Additionally, graves and burial mounds near the site suggest Serpent Mound’s builders may have constructed the structure for some kind of important burial or mortuary function, such as to guide spirits. But the mound itself doesn’t contain any graves or artifacts.

Serpent Mound may have further had temporal significance—the head of the serpent aligns with the summer solstice sunset while the tail points to the winter solstice sunrise. As such, ancient peoples may have used the structure to mark time or seasons.

The design of the mound also matches the shape of the constellation Draco, with the star Thuban (Alpha Draconis, which served as the north pole star from the 4th to 2nd millennium B.C.) lining up with the first curve in the snake’s torso from the head. This alignment suggests another purpose for Serpent Mound: a kind of compass that helps determine true north.

Great Serpent Mound Excavations

In the late 19th century, Frederic Ward Putnam, an archaeologist at Harvard University, conducted the earliest scientific excavations of Serpent Mound.

Since these first excavation efforts, archaeologists have attributed Serpent Mound to one of two Native American cultures: The Early Woodland Adena culture (500 B.C. to 200 A.D.) and Late Prehistoric Fort Ancient culture (1000 to 1650 A.D.).

Though the terms “Adena” and “Fort Ancient” hadn’t yet been coined when Putnam first placed trenches in Serpent Mound and its nearby earthen mounds in 1887 to 1889, the archaeologist recognized that people of two different time periods occupied the Serpent Mound area. He attributed the effigy to the earlier group (the Adena).

Decades later, other archaeologists also attributed Serpent Mound to the Adena, largely based on circumstantial evidence. That is, Serpent Mound contains no artifacts that can be used for identification, but the nearby conical mounds do.

Putman originally excavated a conical mound located 200 meters (656 feet) southeast of Serpent Mound, unearthing multiple burials and associated artifacts, including pottery and projectile points. In the 1940s, archaeologist James Bennett Griffith analyzed these artifacts and identified them as Adena, and thus attributed the effigy to that culture.

Griffith also found both Adena and Fort Ancient materials in nearby cultural features, but he considered it far less likely that the more recent civilization would have built Serpent Mound, especially since the effigy is similar in style to other Adena earthworks in the Ohio Valley, such as Portsmouth Works (a mound complex in Scioto County, Ohio).

Adena Culture or Fort Ancient?

In the mid-1990s, a research team reopened one of Putnam’s trenches and collected charcoal from three locations above and below what they considered the mound base. Using radio carbon dating, they determined that the samples—and Serpent Mound—date back to about 920 A.D., some 1400 years later than originally thought.

This new data, which is based on the first direct aging of the structure, put the effigy in the Late Prehistoric (Fort Ancient) period.

But in 2014, another research team carbon-dated a number of other charcoal samples, placing the construction of Serpent Mound between 381 B.C. and 44 B.C., with a mean date of 321 B.C.

The new evidence suggests, once again, that the Adena were the original builders of Serpent Mound. The research team believes that the Fort Ancient people likely modified and/or renovated it, pointing to the fact that other nearby monuments also show evidence of repair or modification by prehistoric groups.

Serpent Mound Preservation

In addition to leading the first excavation efforts of Serpent Mound, Putnam also led efforts to restore and preserve the effigy. Specifically, his efforts helped raise funds for Harvard University to purchase the site, which the Yale Peabody Museum of Natural History converted into a public park until 1900.

Serpent Mound then became property of the Ohio State Archaeological and Historical Society, now known as the Ohio History Connection, which still manages the site. The organization built an observation tower at the site in 1908, and later built the Serpent Mound museum and other visitor facilities.

Serpent Mound (along with multiple other Ohio American Indian earthworks) is being considered for inclusion on the UNESCO World Heritage List.


Herrmann et al. (2014). “A new multistage construction chronology for the Great Serpent Mound, USA.” Journal of Archaeological Science.
Milam, Keith A. (2010). “A revised diameter for the Serpent Mound impact crater in southern Ohio.” The Ohio Journal of Science.
Serpent Mound; Ohio History Connection.
Fort Ancient Culture: Great Serpent Mound; Khan Academy.
Ohio History Central, Serpent Mound; Ohio History Connection.
Serpent Mound; UNESCO, Tentative List.
Serpent Mound Crater, Ohio; United States Meteorite Impact Craters.