Serpent Mound
United States · not visited
Four hundred metres of earthen serpent, with almost nothing inside it to date. Its age has been argued back and forth for thirty years, and the newest answer is that both camps may be right.
- Latitude
- 39.0256° N
- Longitude
- 83.4300° W
- Length
- c. 411 m
- Dating
- c. 300 BCE and/or c. 1070 CE, disputed
- Open questions
- 4 logged
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A serpent with nothing inside it
When Frederic Ward Putnam of Harvard dug into Serpent Mound in the 1880s, he did what archaeologists of his day did with a mound: he looked for the burial. There was none. The earthwork, roughly 411 metres of coiled tail, curving body and open jaws around an oval, held no grave and nothing that could be dated.
An effigy that large, with no grave goods, has to be dated with whatever small scraps of charcoal can be found in it.
It lies on a ridge in Adams County, southern Ohio, and it is the largest serpent effigy known. The ridge also sits on a disturbed patch of bedrock usually described as a possible ancient impact structure, hundreds of millions of years old. Whether anyone who built the serpent knew or cared about that geology is a separate question, and I return to it below.
The dating argument, in four rounds
It is one of the cleanest examples in North American archaeology of a date that will not settle, and the pattern matters more than any single number.
Round one: an assumption. For most of the twentieth century the mound was attributed to the Adena culture, roughly 1000 to 100 BCE, mainly because Adena burial mounds stand nearby. That was proximity, not evidence.
Round two: 1070 CE. Robert Fletcher and Terry Cameron remapped the mound, and Bradley Lepper and Dee Anne Wymer took cores and made limited excavations there in 1991. Two charcoal samples from undisturbed parts of the mound both came out at about 1070 CE, which points to the Fort Ancient culture (about 900 to 1600 CE) rather than Adena. The result was reported in 1996.
Round three: about 300 BCE. In 2014 Edward Herrmann and colleagues published seven new radiocarbon ages, taken from cores that reached the base of the mound, and analysed them statistically. Their conclusion in the Journal of Archaeological Science was that the serpent was first built about 2,300 years ago, in the Early Woodland (Adena) period, and renovated about 1,400 years later in the Fort Ancient period, probably to repair eroded sections. The 1070 CE charcoal, on this reading, dates the repair. The same paper notes a possible abandoned coil near the head, visible only in a magnetometer survey.
Round four: the argument from images. In 2018 Lepper, Duncan, Diaz-Granados and Frolking argued in the Cambridge Archaeological Journal for an Early Fort Ancient origin. They do not offer new dates. They say three earlier attempts at radiometric dating were inconclusive, and they turn to iconography instead: serpent imagery is common in Fort Ancient and wider Mississippian contexts, and the pictographs of Picture Cave, Missouri, dated to about 950 to 1025 CE, show a serpent, a female figure and a vulva-like shape together, which they read through Siouan traditions of First Woman and the Great Serpent. They see the same three elements in the mound's original design.
So the live dispute is no longer simply Adena against Fort Ancient. It is whether the first build is the older or the younger one, and whether a comparison with pictures can outweigh a radiocarbon sequence. Those are different kinds of evidence, and neither side's evidence is invented.
The sky here
The 1996 dating team also reported alignments. As Archaeology magazine summarised it, features of the mound line up with the summer solstice sunset and, less clearly, the winter solstice sunrise. A pile of burned stones in the oval of the head sat several feet north-west of its centre, and the team suggested it may have been placed to line up better with the point of the V in the serpent's neck and the summer solstice sunset.
For scale: at this latitude, on a flat horizon, the summer solstice sun sets at an azimuth of about 301 degrees, a little north of north-west. That is my own arithmetic, not a measurement from the mound, and I have not been able to read the original survey, only the magazine summary. Two cautions apply. The mound was restored after Putnam's work, so its present shape may differ from the original. And a serpent with many bends will always have some stretch that lines up with some sunrise or sunset. An alignment that was meant has to be picked out in advance, not found afterwards.
The Fort Ancient date sits close to the year 1066, when Halley's Comet blazed across the northern hemisphere and was recorded in Europe and China. What if somebody in Ohio looked up, saw a serpent of light, and put it into the ground?
Why it is worth asking: because the 1070 CE charcoal is the only direct date from the mound before 2014, and because comets and serpents are linked in many traditions. I have not found this idea argued in the literature, so treat it as my own question, not a finding. What speaks against it: Halley returns about every 76 years, so any date with a margin of a century contains a comet. And on Herrmann's chronology 1070 CE is a repair, not the moment of design. What would tell the readings apart: a modelled date for the original construction from the head or the possible abandoned coil, independent of the repair. What stays unknown is whether the builders needed any sky event at all.
What is still open
Built once and rebuilt, or built once?
The 2014 chronology says first built about 2,300 years ago and renovated about 900 years ago. The 2018 argument says Early Fort Ancient from the start. A dated sample from the oldest, sealed fill under the head or the abandoned coil would help, and so would a test of whether the radiocarbon charcoal is from the construction itself or from later activity on the surface.
What is the head aligned to, if anything?
Solstice alignments have been reported, with the summer solstice sunset better supported than the rest. What would settle it is a modern survey of the mound's present axes, set against the 1880s restoration record, and a test of whether the pattern is stronger than chance for a winding earthwork.
What does the serpent mean?
Great Serpent figures are widespread in the traditions of the eastern woodlands, tied to water, the underworld and the journey of the dead. Those readings use later ethnography and comparison with other images, which is legitimate but is not evidence from the mound. What would help is any datable image or object from the same landscape that shows the same pairing of serpent and oval.
Why this ridge?
The mound sits at the edge of a strange geological feature. That may be chance, or it may be that the place was known as odd. Nothing in the evidence so far tells the two apart. A site with a long, earlier use of the same ridge would shift the odds, and none has been reported.
Sources
- Herrmann, E.W. et al., “A new multistage construction chronology for the Great Serpent Mound, USA”, Journal of Archaeological Science 50, 2014, pp. 117 to 125.
- Lepper, B.T., Duncan, J.R., Diaz-Granados, C. and Frolking, T.A., “Arguments for the Age of Serpent Mound”, Cambridge Archaeological Journal 28(3), 2018, pp. 433 to 450. Abstract.
- Archaeology magazine, November 1996, news brief on the radiocarbon dating of Serpent Mound by Fletcher, Cameron, Lepper and Wymer, and on the solstice alignment. Link. I read the summary, not the original paper.
- Lepper, B.T., “The Mississippian Iconography of Serpent Mound”, Journal of Ohio Archaeology 7, 2020, pp. 38 to 55. Listed in another paper's references; I have not read it.
- Putnam, F.W., excavation of Serpent Mound, Peabody Museum, 1880s.
Not visited in person. Written from published sources. Last reviewed 12 October 2026.