It is one of America’s most iconic and mysterious prehistoric earthen monuments, and at times, also one of its most hotly debated: Ohio’s famous Serpent Mound.
Located in Adams County, Ohio, the ancient site features a massive, undulating serpent whose coiled tail and gaping jaws have stood as an impressive monument to the Buckeye State’s ancient past, prompting serious investigations by archaeologists that have spanned nearly two centuries.
First documented in the landmark work Ancient Monuments of the Mississippi Valley in 1848 by Ephraim Squier and Edwin Davis, the famous site features many peculiarities, including the large oval-shaped feature positioned within the serpent’s open mouth. “This oval is formed by an embankment of earth, without a perceptible opening,” Squier and Davis wrote, noting that the feature “is perfectly regular in outline” and “slightly elevated” while also containing an area of “large stones, much burned once, [that] existed in its center.”
However, research involving the Serpent Mound also suggests that Ohio’s most famous animal effigy earthwork looks quite different today from how it once did—a curious fact that raises haunting questions about its original appearance, and what that may have meant to its mysterious ancient builders.
“The Serpent Mound is one of the most astounding earthworks you can see,” says Jason Pentrail, an environmental scientist, avocational archaeology researcher, and author of Adena: Mound Builders of the Ohio Valley, a two-volume set of books that looks extensively at America’s enigmatic Adena culture.
A United States Coast Guard veteran, Pentrail spends much of his retirement conducting independent research, volunteering at excavation sites in the Southeastern United States, and podcasting about archaeology. Speaking with The Debrief in a recent interview, he addressed some of the mysteries that have lingered about America’s largest serpent effigy site.
“Of course, there are other serpent effigies in the world; this is not the only one,” Pentrail said. “However, it is—especially for North America—probably the most complicated, and the most detailed without a doubt.”
Pentrail addressed several of the famous earthwork site’s intriguing aspects, including its known astronomical alignments, its unique location within a massive ancient meteorite impact crater, and perhaps most controversial of all, the debate that has continued for years over when it was first constructed, as well as which ancient culture was responsible.
“Some intense debate over the years has involved the dating,” Pentrail told The Debrief. “Who built it has always been a controversy.”
During some of the earliest investigations at the site, its very close proximity to a well-established Adena-period earthen mound led to the initial interpretation that the Serpent Mound was also built by the same culture.
However, in 1991, when radiocarbon dating was finally undertaken at the site based on samples collected by avocational archaeologist Robert Fletcher, the findings turned those expectations on their head.
“They came back with radiocarbon dates that placed it as a Fort Ancient structure,” Pentrail explained, “which would have placed it over a thousand years later than Adena.”
Dating back to between 1000 and 1750 CE, the Fort Ancient culture, like the Adena and Hopewell before them, as well as the contemporaneous Mississippian culture, lived along the Ohio River valley mainly between Ohio and western West Virginia, as well as parts of northern Kentucky and even southeastern Indiana. This new interpretation of the Serpent Mound’s age and the ancient culture that built it prevailed for many years, but the controversy was far from being over.
“Another team came in more recently, and they did some pretty detailed coring of the mound,” Pentrail explained, referring to work in 2011 undertaken as part of a multidisciplinary reevaluation of the site that incorporated magnetometer surveys, LiDAR imaging, and additional core samples from the mound structure.
What the new analysis determined effectively reopened the debate over Serpent Mound’s ancient roots.
“Those cores came back with much earlier dates that would place it perfectly in the Adena time period,” Pentrail told The Debrief.
Writing of the discoveries, Ohio archaeologists Jarrod Burks, William Romain, and colleagues noted at the time in a paper detailing their findings that “Our data suggest that the mound was first built ~1,400 years earlier and contemporary with an Adena occupation, as presumed throughout most of the 20th century.”
“What looks to be the case today is that it most likely is an Adena construction that was repaired over the years,” Pentrail told The Debrief, adding that “different groups probably used it” over the millennia that it has existed.
“It’s been eroded, and of course, it’s had trees growing out of it. All these things had to be repaired or replaced, and of course, that’s mixing up the soils, mixing up the carbon dates. But these core dates came from the very bottom of the mound, and more than one location, giving it a firm Adena affiliation.”
Despite the confirmed Adena-era dates, Pentrail says there are a few holdouts in the ongoing debate.
“People today will still argue with that point,” Pentrail told The Debrief, “but I think the science is in, and as of now, I can say [Serpent Mound] is of Adena origin, and that it was probably used again and repaired by the Fort Ancients.”
However, the famous mound’s suspected provenance as an Adena construction isn’t all that studies from over the years have revealed about America’s largest ancient serpent effigy. At some point in the distant past, the structure very likely looked quite different from how it appears today, including additional coils that were, for various reasons, lost over time.
Of equal intrigue, other ancient portions that the mound once featured, but which are missing today, may have even included a pair of “horns” or other protrusions near the serpent’s head—features that were recognized as far back as Squier and Davis’s original documentation of the site, appearing as a pair of small, dark appendages just behind the serpent’s open jaws in drawings featured in their landmark survey of Ohio Valley earthworks.
“This particular part is fascinating, because again we see the interaction of various cultures over time,” Pentrail told The Debrief. “This seems to be leading back to a very old story that, really, is an international phenomenon. The great horned serpent is well established in parts of South America, in Asia, and throughout Europe. This is a known theme.”
“But in 1887, the restoration of the Serpent Mound was completed under the supervision of Frederick Ward Putnam, who was really the first person to recognize the importance and do some really extensive work out there,” Pentrail said. “Charles Willoughby was someone who was also interested in the mound at the time—this was 1919—when he describes a portion of the restoration of the mound.”
As Pentrail explained, “Willoughby went on to state that Putnam only recognized the head with the open jaws and the oval,” the latter referring to the large ellipsoid portion of the mound which rests within the jaws of the serpent, a feature for which there are also a range of different interpretations.
“If you visualize the head, or you pull up a picture of [the mound], you’ll see what looks to be sort of a mouth or jaws wrapping around this oval shape,” Pentrail explained. However, according to Willoughby, a front portion of the effigy also featured “projections at the base of the head had also been restored and would be relatively higher and more conspicuous than they appear at present.”
“So he was stating that at the back of the jaws, there were these two distinct portions that came off that you don’t necessarily see there today,” Pentrail explained. “If you go with a trained expert—somebody who knows exactly where these would have been, there are subtle land features that you can see that show that they may have actually been modified.”
Pentrail also told The Debrief that ground penetrating radar surveys of the site from over the years also showed that there was “potentially another loop of the body that essentially was erased over time.
“We don’t know who would have done that,” he adds, “it wasn’t done by Putnam or anyone at that time. It would have been done in antiquity, prior to these excavations.”
“So it does appear that, over the years, that horned serpent effigy portion may have been changed or altered, and there may be a loop or undulation of the body that may have been ‘edited’ for whatever reason—we don’t know if that was due to an [astronomical] alignment, or it had something else to do with the culture that was using it at the time.”
For Pentrail, while a range of interpretations for this exist, one likely meaning behind the changes that have taken place at Serpent Mound over the ages could have to do with the various groups that “inherited” the site over time, and with the changing of hands, the changing of use and meaning that was attributed to the ancient effigy site.
“Again, it lends to that idea that it could have meant more than one thing, to more than one culture,” Pentrail says.
The Serpent Mound is one of many archaeological sites featured in Pentrail’s exhaustive new two-volume book series, Adena: Mound Builders of the Ohio Valley, now available on Amazon.com.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.