Above: Map of Hopewell Mound Group (Shetrone 1926:196) superimposed over LiDAR image showing location of Great Circle Woodhenge.

Recent discoveries suggest that an Adena or Hopewell woodhenge - perhaps dating to around 2,000 years ago, was designed using sophisticated geometric concepts, built to a special size, and aligned to the summer solstice. 

The newly-discovered woodhenge is located within an earthen walled enclosure known as the Hopewell Mound Group. Situated in Ross County, Ohio, the Hopewell Mound Group is the namesake for the Hopewell Culture and is one of the largest Hopewell sites known. Before it was plowed-down by farming and dug into for its buried grave goods, the site included at least 40 mounds and more than three miles of embankment walls.
Today, the site is protected by the National Park Service and is part of the Hopewell Culture National Historical Park.

The Great Circle embankment shown on the Shetrone map (above) and Squier and Davis map (below) is one of the features that was severely plowed. Today, the embankment is not visible to the naked-eye, or in LiDAR imagery. Originally, however, the more complete Great Circle feature consisted of the earthen embankment shown on the early maps, plus the newly discovered circle of wooden posts (i.e., the woodhenge), as well as an exterior ditch.

Remnants of the Great Circle ditch and woodhenge were first detected during a limited  magnetic gradiometer survey by archaeologist Jennifer Pederson Weinberger (2006). Subsequent to that, a large-scale gradiometer survey showing the entire ditch and woodhenge area was completed by archaeologist Jarrod Burks (2014, 2013) pursuant to contract with the National Park Service. It is the Burks mag imagery that is shown here.

The woodhenge is comprised of a series
of large post holes (estimated by Burks [2013:30] to originally number about 108 for the presumed complete post circle), surrounded by a ditch, 6.5 - 8.2 feet wide (2.0 - 2.5 m). It is the ditch feature that shows-up so prominently in the magnetic gradiometry imagery as a dark circle.
It is not entirely clear at this point if the embankment wall shown on the early maps was exterior to the woodhenge, or if it was built over the post holes after the posts were removed. That is one of the questions currently being investigated by National Park Service archaeologists under the direction of NPS archaeologist Bret Ruby (2014b).
Above: Squier and Davis (1848:Pl. X) map of the Hopewell Mound Group.
Above: magnetic gradiometer image with alignment, geometry, and mensuration data added.
Above: although at map scale the Great Circle looks small; it is actually quite enormous. This photo was taken from the approximate center of the Great Circle. The archaeology crew is seen in the distance, excavating at the exterior ditch. Photo by Romain.
Above: NPS excavation crew, clockwise from lower left: Patrick Zingerella, Jocelyn Connolly, Bret Ruby, Andrew Weiland, Nissa Salvan, Jarrod Trombley. Cailey Mullins is in the center. Not pictured: Timothy Everhart. Photo credit: Tim Black and Dean Alexander, provided courtesy of Bret Ruby.
Above: Photo showing profile of exterior ditch.
Below: Photo showing profile of excavated post hole. Photos by Romain.


William F. Romain, Ph.D.

The Adena and Hopewell aligned many of their earthworks to celestial events such as solstices. They also built many earthworks in geometric shapes and used standard units of measurement (Hively and Horn 1982; Romain 2000). One of these units of measurement is equal to 1,054 feet. For convenience, we can call this the Hopewell Measurement Unit, or HMU. Thus 1 HMU = 1,054 feet. This unit of length is based on the diameter of the Newark Observatory Circle (Thomas 1894:16). This unit of length or its sub-multiples (e.g., 527 ft., 263.5 ft., 131.7 ft.) are found in the dimensions of every Hopewell earthwork assessed thus far, as well as most Adena earthworks.

1/4 HMU = 263.5 feet

If a square having sides that are each 1/4 HMU (or 263.5 ft.) in length is constructed, then the resulting diagonal will be 372.6 feet.

According to Burks (2013:30) the diameter of the inside edge of the Great Circle ditch is 114 meters, or 374 feet.
Accordingly, the inside diameter of the Great Circle is equal to the ideal 1/4 HMU square hypotenuse length to within 1.4 feet (negligible at ground level and at map scale). It is not uncommon to find Adena and Hopewell earthworks sized using similar iterations of the HMU based on hypotenuse lengths.

The interesting thing about the Great Circle design is that the summer solstice alignment is orthogonal to the diagonal of the 1/4 HMU square; and further, the diagonal of the square and its solstice orthogonal establish the locations for at least two of the gateways into the circle. In this design, astronomic observations, geometry, and a specific unit of length are integrated in a sophisticated and unique way.

The interior post holes that comprise the woodhenge are of special interest. As mentioned, it is estimated that there may have been about 108 post holes. A likely reason why all of the posited post holes are not visible in the mag imagery is explained by Burks (2013:30):
"Not all of the posts are visible in the magnetic data and this could be because those postholes that were detected contain burned earth, while others do not."

Excavation into two post hole features by Bret Ruby (2014a) and colleagues revealed that the post holes are about 4 feet deep. The posts that were set into these holes were roughly 2 1/2 feet in diameter. Ruby (2014a) calculates that four-foot deep holes
could have supported poles as tall as 18 feet. Note too the four pit-like features at the center of the circle. These may be additional post holes or other kinds of features.

For more details concerning excavation of the post holes, see the NPS video:


There are several things worth noting about the discoveries just documented.

First, this is the only Adena-Hopewell woodhenge I know of that is not only solstice-aligned, but also uses the orthogonal of that alignment to establish a second opening into the circle. The significance of this finding is that it demonstrates in a ground-truthed manner, the importance of orthogonal relationships relative to astronomic sightlines - a design variable that was used at other sites to include those of the later Mississippian period.

Second, this work demonstrates that much remains to be discovered about the ancient Adena and Hopewell. All is not lost. Although seemingly plowed into oblivion, new technologies can - in some cases, retrieve useful data. We must continue to support and fund the kind of research demonstrated by this project.

Lastly, studies such as this demonstrate the value of using multiple approaches in our efforts to understand the past. In this case, geophysics combined with excavation, combined with archaeoastronomic assessment yielded informative results - thereby furthering our understanding....And that after all, is the point of what we do as archaeologists.


I wish to thank Jarrod Burks, Ph.D., and Bret Ruby, Ph.D. for sharing their discoveries of the Great Circle woodhenge with me and for various permissions. Many thanks also to the National Park Service archaeology crew for showing me their excavations.


Burks, Jarrod
2014  Recent Large-Area Magnetic Gradient Surveys at Ohio Hopewell Earthwork Sites. International Society for Archaeological Prospection ISAP News 39:11-13.

2013 Large Area Magnetic Survey at the Hopewell Mound Group Unit, Hopewell Culture
National Historical Park, Ross County, Ohio, OVAI Contract Report #2012-52-1. Contract
P12PX15855. Prepared for Midwest Archeological Center, National Park Service, Lincoln, NE.
Prepared by Ohio Valley Archaeology, Inc., Columbus, Ohio.

Hively, Ray, and Robert Horn
1982 Geometry and Astronomy in Prehistoric Ohio. Archaeoastronomy
(Supplement to Vol. 13, Journal for the History of Astronomy) 4:S1-S20.

Pederson Weinberger, Jennifer
2006 Ohio Hopewell Earthworks: An Examination of Site Use from Non-Mound Space at the Hopewell Site. Unpublished Ph.D. dissertation, Department of Anthropology, The Ohio State University, Columbus, Ohio.

Romain, William F.
2000 Mysteries of the Hopewell: Astronomers, Geometers, and Magicians of the Eastern Woodlands. University of Akron Press, Akron, Ohio.

, Bret J.
2014a Current Research in the Park: The Great Circle Project at Hopewell Mound Group. Presentation for the National Park Service Harness Lecture Series, June 26, 2014, Hopewell Culture National Park, Chillicothe, Ohio.

2014b Research Design to Evaluate the “Great Circle” at Hopewell Mound Group. Report on file, National Park Service, Hopewell Cultural National Historical Park, Chillicothe, Ohio.

Shetrone, Henry C.
1926 Explorations of the Hopewell Group of Prehistoric Earthworks. Ohio Archaeological and Historical Quarterly 35:5-227.

Squier, Ephraim G., and Edwin H. Davis
1848 Ancient Monuments of the Mississippi Valley; Comprising the Results of Extensive Original Surveys and Explorations. Smithsonian Contributions to Knowledge, Vol. 1. Smithsonian Institution, Washington, D.C.

Thomas, Cyrus
1894 Report on the Mound Explorations of the Bureau of Ethnology for the Years 1890-1891. In Twelfth Annual Report of the Bureau of American Ethnology. Smithsonian Institution, Washington, D.C.



Al Tonetti
07/09/2014 8:16am


Although the earthworks at many of these sites are important to understanding, and certainly for us today, recognizing and appreciating the sites, what's more important to understanding them is what we don't see until we do the digging, i.e., the evidence of the wooden architecture and related features associated with the activities for which these wooden structures were built. The earthworks marking many of these wooden structures are impressive to be sure, but to really understand and appreciate the complexity of these sites and the people whose built them we need a robust effort of geophysical surveying and excavation. Much remains to be learned at and from these sites, especially those less well preserved and truly in danger of being lost or destroyed because their earthworks have been plowed away and forgotten.

Al Tonetti

07/28/2014 7:17pm

Great trans-disciplinary archeology. Thank you for the discovery.
You have determined the solar alignments. Are there any possibilities that there might be lunar alignments? What other archeo-astronomical alignments might there be in the Chillicothe Woodhenge Circle beyond the seasons and rhythms of the sun?


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