Figure 1. Survey map (Romain 1987) of Serpent Mound superimposed over LiDAR image. Image by William F. Romain.

Located in the rural countryside of Adams County, in southern Ohio, USA, Serpent Mound is probably the best-known and most widely-recognized prehistoric effigy mound in the world (Figures 1 and 2). For many people, Serpent Mound is a sacred site.
Figure 2. Aerial view of Serpent Mound. Photo by William F. Romain.

Given the significance of Serpent Mound, it is of considerable interest to know who built it and when. Recent investigations reveal that, contrary to an earlier report by Fletcher, et al. (1996), Serpent Mound was probably not built by people of the Fort Ancient culture - at about  A.D. 1070. But rather, was built more than 2,000 years ago - during the Early Woodland (Adena) period, around 321 B.C. (OxCal 2-sigma 381-44 B.C.).

The new findings are reported in detail in the peer-reviewed, Journal of Archaeological Science. An on-line, pay-for-view version of the article (for which the authors do not receive compensation) is available at:

http://www.sciencedirect.com/science/article/pii/S0305440314002465

The article is titled:
A New Multistage Construction Chronology for the Great Serpent Mound, USA"  and is co-authored by Edward W. Herrmann, G. William Monaghan, William F. Romain, Timothy M. Schilling, Jarrod Burks, Karen L. Leone, Matthew P. Purtill, and Alan C. Tonetti.

The following discussion is intended only as a brief summary of findings. Interested persons are encouraged to read the full report for a more thorough discussion.

In 2010, The Serpent Mound Project was initiated by William F. Romain. The objectives of The Project were to bring non-invasive and minimally invasive technology to bear on questions of who built the Serpent Mound, when, how, and why. Toward this end, a team of subject matter experts was assembled and pursuant to permits issued by the Ohio Historical Society, investigations were begun. Investigations included: geodetic survey, LiDAR analyses, hand coring, GeoProbe coring, magnetic gradiometer survey, electrical resistivity survey, ground penetrating radar, and limited excavation of a previously undocumented feature at the neck of the effigy. The discussion presented here is focused on the results of our GeoProbe coring work and the new radiocarbon dates. Details concerning our other work will be presented in a forthcoming edited book titled "Serpent Mound Revealed: Context, Archaeology, and Meaning."

GeoProbe Coring

Coring operations were done using a GeoProbe Model 54TR during April and September of 2011. The GeoProbe 54TR is a small John Deere tractor modified to include the DT-21 sampling system. The DT-21 sampling device works by hydraulically driving a core casing and contained sampling tube, into the ground. The core casing prevents the bore hole from collapsing. The soil sample is collected in the soil tube as the casing is driven into the ground. The sample tube is about 2.9 cm (1.1 in) in diameter. Core casings are 1.2 meters (4 ft) in length. Depending on the depth to be sampled, additional core casings are added.

Coring proceeded at 18 locations along the spine of the effigy. Figure 3 shows these locations. The locations were selected with consideration given to maximization of potential information, while avoiding delicate areas such as the oval and tail. Locations for each core were documented by total station measurements tied to a permanent benchmark located near the tail of the effigy.


Figure 3. Coring locations with associated radiocarbon dates. Figure by G. William Monaghan, inset mag image by Jarrod Burks. Reprinted from Journal of Archaeological Science, 50 (2014), Herrmann, et al. "A New Multistage Construction Chronology for the Great Serpent Mound, USA" with permission from Elsevier.

Results

Cores were retrieved as continuous samples in closed tubes. Upon completion of coring, each hole was filled with bentonite to prevent erosion. The cores were analyzed by Bill Monaghan at the Glenn A. Black Laboratory of Archaeology, Indiana University, Bloomington. Cores were examined for color, texture, and other variables, as well as for organic substances suitable for radiocarbon dating and cultural inclusions.

From the 18 cores, a total of 9 soil samples containing possible charcoal were recovered from 5 cores. In all cases, Monaghan was able to clearly distinguish between layers of mound fill, the bottom of the mound, and submound paleosols. Figure 4 shows profiles of the cores that yielded datable samples. In Figure 4, m
ound fill is indicated by light gray; while the bottom of the mound is shown by the thick black line. According to Monaghan (2014:Figure 3 caption), "The submound paleosol was clear in all cores....[and] mound fill units are separated by clear abrupt contacts between units." Importantly, and as Figure 3 shows, the locations for the five cores - from which radiocarbon dates were obtained, come from widely separated areas along the length of the effigy.

As shown by Figure 4, four of the dated samples are from the Ab horizon or ZOD (Zone of Disturbance) that marks the base of the mound. The other three samples are from mound fill situated above the planar mound base.



Figure 4. Diagram showing core profiles and locations for dated samples. All dates are calibrated. Soil horizons as labeled. Figure by G. William Monaghan. Reprinted from Journal of Archaeological Science, 50 (2014), Herrmann, et al. "A New Multistage Construction Chronology for the Great Serpent Mound, USA" with permission from Elsevier.

After retrieval of the samples and analysis by Monaghan, they were provided to paleoethnobotanist, Karen L. Leone for additional assessment. Of the 9 recovered samples, Leone identified 7 samples that contained flecks of wood charcoal. None of the wood charcoal was of sufficient quantity to identify to the species level.

The 7 remaining samples were sent to Beta Analytic, Inc. of Miami, Florida for AMS radiocarbon dating. Due to the relatively small size of the charcoal flecks mixed into the soil matrices, the samples were processed as organic bulk sediments and dating proceeded in accordance with standard protocols. Radiocarbon dating results were forwarded to Tim Schilling - who, along with Bill Monaghan and Ed Herrmann, performed the OxCal analysis shown below in Figure 5.


Results of the OxCal analysis indicate that the Serpent Mound was likely constructed sometime between 381 B.C. - 44 B.C. (with a 95.4% probability). As Table 1 shows, the median calibrated age for Serpent Mound based on our data is 321 B.C.

Table 1 shows that our samples range in age from 639 - 303 B.C. (combined 95.4% probability range of 808 - 116 B.C.). This represents a ~350 year spread. If, however, the two inconsistent ages (Beta 337162 and Beta 337166) are discounted, then the range is even tighter. The Beta 337162 and Beta 337166 samples are 200-300 years older than the median ages from the ZOD; and as Figure 4 shows, they derive from mound fill that is 10-20 cm above the mound base. This age discrepancy is probably the result of charcoal deposited in submound Ab horizons during pre-mound occupations - which was then incorporated into the mound fill when Serpent Mound was built.

In any event, none of the median ages reported from any of our contexts within Serpent Mound are younger than 303 BC. In particular, no Fort Ancient charcoal was found in the sampled mound fill, or at the base of Serpent Mound. If the effigy was built by anyone later than Adena-era people, then one might expect to find some evidence of that in the way of organic materials or charcoal at the base. Such was not the case, however. If someone other than Early Woodland (i.e., Adena) people built the Serpent Mound, they were very fastidious in their work and left no trace at the foundation level. 


Table 1. All radiocarbon dates from Serpent Mound to include Fletcher, et al. 1996. Table by Edward Herrmann and G. William Monaghan. Reprinted from Journal of Archaeological Science, 50 (2014), Herrmann, et al. "A New Multistage Construction Chronology for the Great Serpent Mound, USA" with permission from Elsevier.

Figure 5. Diagram showing all radiocarbon dating results. OxCal refers to the calibration program and model used to plot the date ranges. Beta numbers refer to the laboratory and sample number. Figure by G. William Monaghan. Reprinted from Journal of Archaeological Science, 50 (2014), Herrmann, et al. "A New Multistage Construction Chronology for the Great Serpent Mound, USA" with permission from Elsevier.

What About the Fort Ancient Dates?

If Serpent Mound was built during the Early Woodland period, then the question that remains unanswered is: How do we account for the later - i.e., A.D. 1070, Fort Ancient dates obtained by Fletcher, et al. (1996)?

To answer that, we need to re-visit the modern history of the site.

As readers may know, in the late 1800s, Serpent Mound was saved from destruction through its purchase by
Frederic W. Putnam of Harvard University and the ladies of Boston.  Subsequent to that purchase, Putnam partially restored the effigy and created the Serpent Mound Park. During the course of his work, Putnam cut several exploratory trenches through the effigy. Mostly, however, what Putnam did was restore the effigy to its presumed original height by placing dirt that had washed down from the mound and accumulated along its edges, back up onto the surface of the effigy.

No further exploration into the effigy was done after Putnam's work - until the early 1990s, when Fletcher, et al. (1996) conducted their research at the site.


In 1991, the Fletcher team located and opened one of Putnam's back-filled trenches, expanded the lateral extent of that trench into a non-Putnam back-filled section of the effigy, and from that area, secured charcoal for two radiocarbon samples. A third sample was recovered earlier by hand coring.

Two of the three dates that the Fletcher team recovered are identical – i.e., cal A.D. 1070 ± 70 (Fletcher, et al. 1996:133). The third sample obtained from the hand core came from a depth of 132 cm - well-below the planar surface of the mound. That sample dated to cal 970 B.C. Based on the A.D. 1070 dates and an iconographic interpretation for the head of the effigy, Fletcher, et al. (1996:105) proposed that Serpent Mound was built during the “Late Prehistoric” by people of the Fort Ancient culture.

The Fletcher, et al (1996) conclusion that Serpent Mound was built during the Late Prehistoric differs from the findings of the present investigation. We believe, however, that the 1,400-year difference between an Early Woodland and Late Prehistoric construction date for the effigy can be accounted for.

Specifically, it is suggested that the conflicting sets of dates can be reconciled if Serpent Mound was constructed during the Early Woodland and then later modified. In this scenario, initial mound construction occurred during the Early Woodland (Adena) period about 2,300 years ago, followed by a renovation or repair episode in the area of convolution # 3 about 1,400 years later during the Late Prehistoric (Fort Ancient) period. The posited repair by Fort Ancient people would account for the Late Prehistoric-dated charcoal that was found in convolution # 3 by the Fletcher team.

There are several lines of evidence that support this scenario.

1.
The location of Serpent Mound on a karst-affected promontory makes areas of the effigy susceptible to erosion as well as sinkhole subsidence. Damage to portions of the mound extensive enough to require repairs would not be surprising given the effigy's relatively low, pre-restoration height (generally ≤1 m high immediately preceding Putnam's restoration per early photos), the antiquity of initial construction, and the effigy's ridge-top location amid gullies and karst topography.

More specifically, convolution # 3 - which again, is where the Fletcher-Putnam trench was placed, is located at the head of the largest and deepest gully draining the Serpent area (Figure 6). At this location, however, the drainage of water from the promontory into the gully is blocked by the Serpent. Today, the Serpent's body is built-up - as the result of Putnam's restoration. We suspect, however, that over the course of 1,400 years between the time of Adena construction and Fort Ancient occupation, water, caught inside of convolution # 3 eventually and periodically eroded its way across and through the effigy as it sought to find its way downslope via the gully.


Even today, after a moderate rain, it is clear that the built-up convolution # 3 blocks drainage (Figure 7). Eventual erosion of the Serpent's body at this location seems inevitable.
Indeed, even with the height of the effigy built-up as it is today, one can see from Figure 7 that all it would take is one or two really serious rain events (sometime over the course of more than one thousand years)  to washout convolution # 3 in the area of the Fletcher-Putnam trench.

In short, convolution # 3 has probably long been subject to erosion and likely needed occasional repair due to its location at the head of a major gully.

Figure 6. LiDAR-contour map of Serpent Mound showing location of Fletcher-Putnam trench relative to erosional gully. Figure by William F. Romain.
Figure 7. Water pooling inside convolution # 3 where the Fletcher-Putnam trench was cut. Photo by William F. Romain.

2. The soil stratigraphy of the effigy at convolution # 3 is different from other areas of the Serpent. As geoarchaeologist Ed Herrmann (Herrman, et al. 2015) explains: "The Fort Ancient-age charcoal occurs only in the Fletcher, et al. (1996) trench, whose morphological and depositional contexts are atypical compared to the core samples we obtained. Our cores show that a well-developed, generally continuous paleosol underlies most of Serpent Mound. This paleosol is absent, however, in the Fletcher trench (Fletcher, et al. 1996:121-122, Figure 1). LiDAR and core data also show the mound is typically ~100 cm thick and sometimes directly underlain by a ZOD [Zone of Disturbance], which incorporated the pre-mound Ab-horizon during mound construction. However, the western end of convolution 3, where the Fletcher trench was placed is different. Not only are the submound paleosol and ZOD absent and the mound only ~50 cm thick, but at that location, the mound blocks downslope surface drainage from a depression situated on the inside of convolution 3."

Further, Herrmann (Herrman, et al. 2015) explain,
"Evidence of erosion and re-deposition within in this part of the mound is provided by the age of charcoal buried 70 cm below the Fort Ancient-age mound fill in the Fletcher trench. The age and depth of the charcoal (2920 BP, 95.4% probability 1368-925 BC, BETA 47212; Table 9.1) led Fletcher, et al. (1996:132-133) to conclude that it was related to a previous Late Archaic occupation and was 'carried to the lower depth by bioturbation or some other mode of transport.' We agree with this; but further suggest that the 'other mode of transport' was probably long-term erosion and re-deposition processes associated with gully formation. Based on the age of the buried charcoal, gully formation and erosion were ongoing processes. Erosion extensive enough to completely remove the original (Adena) mound fill and the upper part of the paleosol is consistent with the lack of a submound Ab/Eb horizons in the Fletcher/Putnam trench. That substantial erosion occurred at or near the Fletcher/Putman trench, at the apex of the convolution is further indicated by the absence of a submound paleosol [color added for emphasis]. As might be expected, the long convolution sections that extend along the same trajectory as the erosional gully were apparently not significantly altered by erosion."
3. Yet another line of evidence in support of the hypothesis that the effigy was repaired at convolution # 3 is the discovery of at least one other episode of prehistoric mound alteration. As Figure 3 shows, magnetic gradiometer survey by Jarrod Burks found evidence for what might be called a 'stealth' or 'abandoned' convolution at the neck of the Serpent. This feature is not visible on the surface by naked-eye, or in LiDAR imagery. What is left of the feature is situated below the present ground surface - but still detectable by magnetometer survey. Based on the superimposition of the present neck embankment over the 'stealth' coil, as well as the presence of Late Archaic artifacts recovered from our limited excavation into and across the stealth coil, it appears that this feature was probably built very early on, and subsequently erased from the surface, with the new neck configuration then built.

Given that at least one episode of prehistoric modification was made to the effigy, there is no reason to think that another episode - this one involving repair at convolution # 3 could not also have been made.

Discussion
  
It is possible that some people will argue that what we dated was mound fill and that mound fill could come from anywhere and date to anytime. Such a representation, however, would not be accurate. As the data show, in no less than four instances, we did not date mound fill - but rather, charcoal situated in the Ab horizon, or Zone of Disturbance, at the very base of the effigy.

The question then becomes, 'what do the dates obtained from the Ab horizon or Zone of Disturbance represent relative to time of construction?' The matter is complicated; but basically can be summarized thusly:
"Our approach to resolving when Serpent Mound was first built is similar to that commonly used to date paleosols, which assumes that the time of paleosol burial is approximated by the youngest ages found (Herrmann, et al. 2014:121)." Following upon that with reference to Serpent Mound, "The distribution of ages...strongly suggest that the submound paleosol was more likely buried during an Adena construction" (Herrmann, et al. 2014:122). For further details and source documentation on this approach the reader is encouraged to review pages 121-122 of the JAS article.

In any case, it is again worth emphasizing
that no charcoal or other organic materials dating to the Late Prehistoric period were found at the base of the effigy. One might expect that if someone other than Adena people built the effigy, some evidence of their presence at the foundation level of the effigy would have been found in no less than 18 core samples. The lack of Fort Ancient-dated charcoal or organic materials at the base of the effigy is strong evidence that that culture was not responsible for construction of Serpent Mound. Moreover, the radiocarbon dates we obtained are internally consistent within a fairly narrow range of several hundred years.

The idea that Serpent Mound was built by people of the Adena culture is not new. Early-on, and based on his excavations, Putnam (1890) recognized that two different people occupied the Serpent Mound area during two different time periods. The terms Adena and Fort Ancient had not yet been applied to Moundbuilder peoples. However, Putnam - probably the most authoritative of all Serpent Mound investigators, attributed the effigy to the earlier of the two peoples he recognized. Likewise, Emerson Greenman (1934) (former Curator of Archaeology for the Ohio Historical Society), and William S. Webb and Charles E. Snow (1945:341), and Webb and Baby (1957:106) (archaeologists who literally wrote the books on Adena) also attributed Serpent Mound to the Adena.

Contributing to assessment of the Serpent Mound as Adena was the cultural affiliation of the large conical mound situated about 650 feet southeast of the Serpent effigy. The conical mound had been excavated by Putnam and was found to contain multiple burials and associated grave goods. Analysis of these materials by world-renowned expert James B. Griffin (1943:56-64) identified diagnostic Adena projectile points and pottery. Additional Adena materials were further identified by Griffin from nearby features. As Putnam anticipated, Griffin also found evidence for a much later Fort Ancient occupation of the area. The totality of evidence, however, led Griffin (1943:57) to conclude: "Although artifacts taken by Putnam from the conical mound south of the serpent and from the lower level of the near-by village site cannot be positively assigned to the builders of the effigy, it is considerably less likely that the later Fort Ancient occupants built the serpent." Both the consensus of opinion and radiocarbon evidence suggest an Adena construction.

On a final note, it is useful to consider that although Serpent Mound is an exceedingly sophisticated earthwork in terms of its design, there are are other examples of similarly complicated earthwork designs found in the Eastern Woodlands that predate Serpent Mound - in some cases by thousands of years. Watson Brake and Poverty Point are examples. Watson Brake is solstice-aligned (Davis 2012; Romain 2013) and designed using geometric shapes (Sassaman and Heckenberger 2004). So too, Poverty Point is one of the most impressive earthworks ever built and like Watson Brake, uses geometric shapes in its design (Clark 2004; Romain and Davis 2013). Watson Brake dates to about 3400 B.C. (Saunders, et al. 2005:1); Poverty Point dates to about 1600 B.C. (Gibson 2001:94). Given this, there is no reason to think that intricate earthwork designs such as found in the Serpent Mound were beyond the capabilities of Adena people at ca. 321 B.C. The imaginations of Adena people were most certainly not limited to building circle earthworks and conical mounds. Indeed, if archaeologists have learned anything over the past decades it is to not underestimate the capabilities and accomplishments of the ancient cultures and people we endeavor to understand.

Acknowledgements

I wish to express my sincere thanks to the principal investigators who made-up the core of the Serpent Mound team: G. William Monaghan, Edward W. Herrmann, Timothy M. Schilling, Jarrod Burks, Karen Leone, Alan Tonetti, Matthew Purtill, and Michael J. Zaleha. All volunteered their time and expertise. Each played a crucial role; each was a delight to work with.

Many other persons contributed
time, effort, and material support to The Project including: Jeff Wilson, Delsey Wilson, Albert Pecora, Jim McKenzie, Beverly McKenzie, Ross Hamilton,
Lesley LaBoda, Kris Phipps, Glen Horton, Jeff Dilyard, Harry Campbell, Jamie Davis, Dave Kuehner, Crystal Marvin, Meggan McCane, and Horton Hobbs. Thank you.

Permission to conduct research at Serpent Mound was granted by the Ohio Historical Society. For various OHS assistance and support we thank Burt Logan, George Kane, Brad Lepper, Bill Pickard, and Karen Hassel.

GeoProbe equipment was graciously provided by the Glenn A. Black Laboratory of Archaeology. Funding for radiocarbon dating was very generously provided by Friends of Serpent Mound, Gray & Pape, Inc., Ohio Valley Archaeology, Inc., and ASC Group, Inc.

Thank you to all.

William F. Romain, Ph.D.
Director, The Serpent Mound Project



References

Bronk Ramsey, C.
2013  OxCal 4.1 Manual. Electronic document, http://c14.arch.ox.ac.uk/oxcalhelp/hlp_contents. html, accessed November 2013.

Clark, John E.
2004  Surrounding the Sacred: Geometry and Design of Early Mound Groups as Meaning and Function. In Signs of Power: The Rise of Cultural Complexity in the Southeast, edited by Jon L. Gibson and Phillip J. Carr, pp. 214-233. University of Alabama Press, Tuscaloosa.

Davis, Norman L.
2012  Solar Alignments at the Watson Brake Site.
Louisiana Archaeology 34:97-115.

Fletcher, Robert V., Terry L. Cameron, Bradley T. Lepper, Dee Anne Wymer, and William Pickard.
1996  Serpent Mound: A Fort Ancient Icon? Midcontinental Journal of Archaeology 21 (1):105-43.

Greenman, Emerson F.
1934  Guide to Serpent Mound (booklet). Ohio Historical Society, Columbus.

Gibson, Jon L.
2001  The Ancient Mounds of Poverty Point: Place of Rings.
University Press of Florida, Gainesville.

Griffin, James B.
1943  The Fort Ancient Aspect: Its Cultural and Chronological Position in Mississippi Valley Archaeology, Anthropological Papers No. 28, Museum of Anthropology, University of Michigan, Ann Arbor.

Herrmann, Edward W., G. William Monaghan, William F. Romain, Timothy M. Schilling, Jarrod Burks, Karen L. Leone, Matthew P. Purtill, and Alan C. Tonettti
2015  Radiocarbon Dating of Serpent Mound. In Serpent Mound Revealed: Context, Archaeology, and Meaning, edited by William F. Romain. Book manuscript in preparation.

Herrmann, Edward W., G. William Monaghan, William F. Romain, Timothy M. Schilling, Jarrod Burks, Karen L. Leone, Matthew P. Purtill, and Alan C. Tonetti
2014  A New Multistage Construction Chronology for the Great Serpent Mound, USA. Journal of Archaeological Science 50:117-125. Electronic document,
http://www.sciencedirect.com/science/article/pii/S0305440314002465, accessed July 2014.

Putnam, Frederic W.
1890. The Serpent Mound of Ohio. Century Illustrated Magazine 39:871-888.

Reimer, P. J., E. Bard, A. Bayliss, J. W. Beck, P. G. Blackwell, C. Bronk Ramsey, P. M. Grootes, T. P. Guilderson, H. Haflidason, I. Hajdas, C. Hatte, T. J. Heaton, D. L. Hoffmann, A. G. Hogg, K. A. Hughen, K. F. Kaiser, B. Kromer, S. W. Manning, M. Niu, R. W. Reimer,  D. A. Richards, E. M. Scott, J. R. Southon, R. A. Staff, C. S. M. Turney, and J. van der Plicht
2013  IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0-50,000 years cal BP. Radiocarbon 55(4):1869-1887.

Romain, William F.
2013  Letter to the Editor Regarding Watson Brake. Louisiana Archaeology 36 (2009, printed February 2013):3-4.

------------------------
1987  The Serpent Mound Map. Ohio Archaeologist 37(4):38-42.

Romain, William F. and Norman L. Davis
2013  Astronomy and Geometry at Poverty Point. Louisiana Archaeology. Electronic document, http://www.laarchaeology.org/articles.html, accessed July 2014.

Sassaman, Kenneth E., and Michael J. Heckenberger
2004  Crossing the Symbolic Rubicon in the Southeast. In Signs of Power: The Rise of Cultural Complexity in the Southeast, edited by J. Gibson and P. Carr, pp. 214-233. University of Alabama Press, Tuscaloosa.

Saunders, Joe W., Rolfe D. Mandel, C. Garth Sampson, Charles M. Allen, E. Thurman Allen, Daniel A. Bush, James K. Feathers, Kristen J. Germillion, C. T. Hallmark, H. Edwin Jackson, Jay K. Johnson, Reca Jones, Roger T. Saucier, Gary L. Stringer, and Malcolm F. Vidrine
2005  Watson Brake, A Middle Archaic Mound Complex in Northeast Louisiana. American Antiquity 70(4):631-668.

Webb, William S., and Raymond A. Baby
1957  The Adena People No. 2. Ohio Historical Society, Columbus.

Webb, William S., and Charles E. Snow
1945  The Adena People. Reports in Anthropology and Archaeology, Vol. VI. University of Kentucky, Lexington.




 
 
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.


ASTRONOMY AND GEOMETRY OF THE HOPEWELL MOUND GROUP GREAT CIRCLE

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:


https://www.facebook.com/photo.php?v=734509149921663&set=vb.192277604144823&&theater


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.

Acknowledgements

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.

References

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.

Ruby
, 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.