A Multistage Geophysical Approach to Detecting ... - Wiley Online Library

Archaeological Prospection Archaeol. Prospect. 18, 231–244 (2011) Published online 16 September 2011 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/arp.418

A Multistage Geophysical Approach to Detecting and Interpreting Archaeological Features at the LeBus Circle, Bourbon County, Kentucky EDWARD R. HENRY* Center for Archaeological Research, University of Mississippi, 38655, USA

ABSTRACT

Magnetic gradiometry and in-phase electromagnetic induction (EM) instruments were employed during a geophysical survey to identify archaeological features at an Adena circular ditch and embankment earthwork site. Three features identified in the geophysical survey were selected for further geophysical examination because of their shape, spatial arrangement, and differences in the gradiometer and EM data. Downhole magnetic susceptibility was used to explore the subsurface shape of these features and to identify the presence of magnetic variations within them. Excavation of these features exposed a buried ground surface below the earthwork’s embankment, a midden-like soil in the ditch, and a large pit situated over a spring/seep inside the earthwork. Magnetic susceptibility data were collected on open wall profiles of excavation units with a KT9 kappameter. Pairing the geophysical results and excavation profiles helped separate a lightly burned ground surface from a buried A horizon below the embankment, map the buried A horizon beneath areas where the embankment is now destroyed, and separate natural from cultural fill episodes inside the pit feature. Correlations between the magnetic susceptibility data and drawn profiles identified specific cultural contexts for radiocarbon dating. The radiocarbon chronology of the site indicates the earthwork was constructed during Middle Adena ritual development (150 BC to AD 1), a time marked by increasing ritual diversity and the appearance of circular structures used in mortuary ritual. This research also demonstrated that the Woodland earthwork was reused by Late Fort Ancient peoples after approximately AD 1450 . Copyright © 2011 John Wiley & Sons, Ltd. Key words: Gradiometry; electromagnetic induction; downhole magnetic susceptibility; KT9 kappameter; Adena; circular earthworks

Introduction A multistage geophysical approach was implemented during archaeological research at the LeBus Circle, Kentucky, USA (Figure 1.) The site is an Adena-age (ca. 500 BC to AD 250) circular earthwork consisting of an exterior embankment encircling a ditch and an internal platform where activities occurred. Prior to this research, only an early nineteenth century sketch map of the site existed (Rafinesque, 1836, Figure 1b). Adena is the name given to a group of complex, semi-mobile societies who lived in the central Ohio River Valley

* Correspondence to: E. R. Henry, Center for Archaeological Research, University of Mississippi, 38655, USA. E-mail: erhenry@ olemiss.edu

Copyright © 2011 John Wiley & Sons, Ltd.

from around 500 BC to AD 250. Adena peoples existed on the border of two distinct archaeologically defined time periods, the Early Woodland (1000–200 BC) and the Middle Woodland (200 BC to AD 400). During this transitional time, they constructed conical burial mounds and geometric earthworks, in addition to expanding and participating in extensive trade routes that stretched from the Appalachian Mountains to the Great Lakes (Railey, 1996, pp.88, 91). One question this research explores is the date of the appearance of geometric earthworks on the Early– Middle Woodland landscape. A chronological model for Adena ritual development has been proposed (Clay, 1991) but it lacks chronometric data. Clay (1991, p.34) suggested that the nearby Mount Horeb circular earthwork was built during the Late Adena ritual phase, yet the chronological evidence comes from

Received 9 February 2011 Accepted 22 June 2011

E. R. Henry

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Figure 1. (a) Location of LeBus Circle, Bourbon County, Kentucky. (b) Sketch map of LeBus Circle from Collins (1882, p.68).

only one fragment of Adena pottery and a few lithic projectile points that were recovered from the excavation of nearly the entire site (Webb, 1941). Archaeological research at LeBus Circle focused on assessing the site’s preservation and determining when the circle was constructed and used. Three stages of geophysical investigation were implemented during archaeological research at LeBus Circle. The first consisted of a 1.52ha gradiometer survey of the earthwork. An electromagnetic induction (EM) meter was used to conduct a magnetic susceptibility survey over a small sample (three 2020m grids) of the area


covered by the gradiometer survey. This was done in an attempt to distinguish between the induced and/ or remnant nature of magnetic features identified in the gradiometer survey. Stage two included the investigation of three features detected during stage one with downhole magnetic susceptibility. The downhole method was utilized to better understand why the gradiometer detected some magnetic features that the EM meter did not. Stage three consisted of mapping magnetic susceptibility variations on the open wall profiles of excavation units with a KT9 kappameter. The data collected during stage three helped separate

Archaeol. Prospect. 18, 231–244 (2011) DOI: 10.1002/arp

A Multistage Geophysical Approach At LeBus Circle a magnetically enhanced ground surface from a buried A horizon beneath the embankment, in addition to mapping portions of the buried A horizon in areas where the embankment had been destroyed. The KT9 data also acted as an interpretive aid in separating cultural from natural stratigraphy in a large pit feature. These data furthered the understanding of stratigraphic relationships at the site and helped identify specific cultural events where charcoal samples were collected for radiocarbon dating.

Stage one: broad-scale survey A microtopography map of the LeBus Circle was created as a precursor to the prospection survey (Figure 2) Archaeologists have demonstrated that topographic maps can contribute information useful to the study of earthworks and geophysical data (Bowden, 2006, p.80; Kvamme, 2008, p.68). The microtopography map from the LeBus Circle indicates that portions of the ditch and embankment are still intact along the northeastern and southeastern portions of the floodplain. A causeway, or entrance, can be seen on the eastern side of the earthwork in this map. The map also depicts a low flood path beginning along the southern boundary of the landform and bending toward the northwest. This map was used as an aid in interpreting both the gradiometer and EM data, in addition to planning the location of downhole cores.

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Magnetic gradiometry A Geoscan Research FM256 fluxgate gradiometer was used to survey the earthwork in its entirety. Data (presented in Figure 3a) were collected over a total of 38 grids measuring 2020m, with readings taken every 0.125m on transects spaced 0.5m apart. A map of the gradiometer data depicts a well-defined break on the eastern boundary of the earthwork that was interpreted as the entrance to the circle (Figure 3a). A smaller break in the earthwork’s ditch was identified opposite this main entrance. The long-term impact of agricultural cultivation on the site is apparent in the gradiometer image (Figure 3a). Linear anomalies trending in the NNW–SSE directions represent historic chisel or deep plough scars. Less apparent anomalies in the southwest portion of the gradiometer data may represent modern ploughing. A Fourier transform and wedge block filter were applied to the gradiometer data to de-emphasize the linear anomalies associated with the plough scars, which, when viewed at a higher contrast, revealed subtle magnetic variations (Figure 3b). The map of Fourier-processed gradiometer data provides clear evidence that much of the earthwork is still intact. Overlaying the gradiometer image onto the topographic map shows that the circular magnetic high most evident in the data coincides with the earthwork’s ditch, while a faint magnetic high encircling the ditch corresponds to the apex of the embankment (Figure 3c). Burks (2006, 2010) has published similar gradiometer

Figure 2. Microtopographic map of the LeBus Circle and surrounding floodplain. Contours exhibit 8-cm elevation changes.



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Figure 3. (a) Processed FM256 gradiometer data from the LeBus Circle. (b) Gradiometer data processed with a Fourier transform. (c) Topographic contours overlaid onto Fourier-processed gradiometer data.

results from Woodland earthworks in Ohio. However, in his examples the magnetic highs surrounding the ditch correlate to magnetically enhanced eroded topsoil transported to the outside of the embankment, thereby delineating its exterior limits (Burks, 2006, 2010). The comparison of topographic and gradiometer data at LeBus Circle indicates the faint outer circle of enhanced magnetism directly correlates to the modern apex of the embankment (Figure 3c). It should be noted that this interpretation is based only on the current state of the earthwork. The possibility remains that long-term ploughing of the site has caused the apex of the embankment to shift outward from the


ditch. If this is indeed the case, the outer band of high magnetic gradient may represent the original exterior boundary of the embankment. It is likely that portions of the ditch were refilled with magnetically enhanced topsoil as the result of erosion. Gradiometers have been used to identify refilled ditch features on archaeological sites in both Great Britain and North America (Clark, 1996, pp. 125–126; Gaffney and Gater, 2003, pp.123–124; Kvamme, 2003). The break in the ditch on the eastern edge of the earthwork mapped by the gradiometer correlates well with the main entrance identified in the topographic map (Figure 3c). An opening in the ditch


A Multistage Geophysical Approach At LeBus Circle on the western side of the earthwork is also discernable in the gradiometer image. However, this opening is situated in the flood path identified in the topographic map, has a large plough scar running through it, and the magnetic high at the apex of the embankment is visible around this western opening in the ditch (Figure 3c). Given these factors, it is probable that the western opening in the ditch was not part of the initial construction plan and instead represents a destroyed portion of the earthwork. Aside from magnetic features relating to the earthwork itself, a large circular magnetic feature was mapped inside the circle (Figure 3a). This feature exhibits a circular pattern of low magnetic gradient surrounding an area of high magnetic gradient. Because of these characteristics it was interpreted as a possible pit. Data gained from the subsequent excavation of this feature supports this interpretation.

Magnetic susceptibility A small sample of the area surveyed with the gradiometer, including portions of the embankment, ditch, and interior circular feature, was selected for a magnetic susceptibility survey. A Geonics EM38B electromagnetic induction (EM) meter was used for data collection. The EM38B can collect both the quadrature-phase (Q) and in-phase (In) components of the EM signal simultaneously (Geonics Limited, 2003). The Q-phase signal correlates to variations in soil conductivity, while the in-phase corresponds to variations in induced magnetism, also known as magnetic susceptibility (Geonics Limited, 2003, pp.13–14; Clay, 2006, p.93; Dalan, 2006, pp.168–169). The EM survey was conducted in order to differentiate between induced and thermoremnant magnetic features identified in the gradiometer survey. The comparison of corresponding gradiometer and EM datasets have been described as useful complements to each other because magnetic susceptibility surveys can be used to separate features with susceptibility contrasts from those with contrasts of remnant magnetism (Dalan, 2006, pp.162). The ability to measure these two properties, and distinguish between them, can provide data relevant to understanding the origins and characteristics of magnetic features identified using a gradiometer. For instance, a magnetic feature appearing in both gradiometer and susceptibility data could indicate an induced magnetic origin for that feature. Gradiometer and/or EM data from circular ditch and embankment earthwork sites in Great Britain have enabled archaeologists to both detect the earthworks and identify the induced magnetic origins


235 of refilled ditches and pit features (Martin, 2000). These methods have been applied in North America to distinguish between induced and thermoremnant magnetic features on Late Prehistoric, Protohistoric and Historic sites (Haley and Johnson, 2007; Kvamme, 2008). Utilizing the EM meter to measure magnetic susceptibility at the LeBus Circle helped isolate the induced magnetic origins of the feature mapped at the apex of the embankment and the pit feature inside the earthwork. The EM survey consisted of three 2020m grids oriented north–south over an area covered by the gradiometer survey (Figure 4). A segment of the earthwork’s embankment and ditch, as well as the interior circular anomaly were covered in the EM survey. Data were collected at 0.5m intervals along transects spaced 1m apart. Although the Q-phase component of the signal was recorded at LeBus Circle, these data mostly correlated with the topography of the embankment and were not the focus of the EM survey. Maps created from the in-phase data show areas of high magnetic susceptibility at the apex of the embankment but not in the ditch (Figure 4). Based on its circular shape and typical construction procedures of circular earthworks (Webb, 1941, pp.147–148), this feature was interpreted as a collection of magnetically enhanced topsoil that was either displaced during the prehistoric excavation of the ditch or eroded from the embankment as described above. Excavations in this area revealed a well preserved buried ground surface that produced evidence indicating that vegetation had been burned-off prior to the earthwork’s construction, thus providing another tentative interpretation for the outer band of high magnetism. The ditch is not clearly evident in images of the EM data, probably because the magnetic materials are located below the 0.5 or 0.6 m limit of detection of the in-phase signal (Dalan, 2006, pp.167). Tall grasses growing over a portion of the EM survey required the EM38B to be carried higher than normal, resulting in an area of very low magnetic susceptibility over the pit feature. However, this feature is still discernable on maps of the EM data as a circular outline of moderate susceptibility inside the larger area of very low susceptibility (Figure 4). This circular pattern of elevated susceptibility corresponds to the outline of the feature in the gradiometer data. Information obtained from the EM survey supports interpretations made from the gradiometer maps and identified features relevant to answering research questions for the site. Although the ditch was not clearly defined, the EM imagery suggests that portions of the embankment are still intact.


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Figure 4. (Left) Location of downhole cores and excavation units on gradiometer data at 50% opacity. (Right) EM38B magnetic susceptibility data. EM survey area denoted by dotted rectangular outline on gradiometer data.

Stage two: downhole magnetic susceptibility After the geophysical surveys were complete, the ditch, embankment, and large pit feature inside the earthwork were subjected to a second stage of geophysical examination. Downhole magnetic susceptibility investigations focused on determining whether differences in detection depths for the FM256 and the EM38B were the reason the ditch was not identified by the EM survey. In addition, downhole investigations provided an opportunity to explore the subsurface extent of magnetic features identified from the stage one surveys. The downhole method also identified stratigraphic variations beneath the embankment, within the ditch, and inside the pit feature. The context of these subsurface variations implied the presence of chronometrically dateable strata relevant to understanding the chronology of construction and use of the earthwork, as well as signifying possible refilling events in the ditch and pit feature.

Downhole investigations of the embankment and ditch Downhole magnetic susceptibility investigations were performed with a Bartington Instruments MS2H sensor. A total of 20 downhole cores spaced 2.5m apart were placed across the embankment and through the ditch. Cores were excavated to depths ranging between


0.6 and 1.8m below surface. Readings were collected in depth increments of 2cm using Bartington’s Multisus 2.44 software and a laptop PC. A profile map generated from the downhole data through the embankment and ditch exhibits elevated susceptibility throughout the embankment and very high susceptibility under the embankment (Figure 5). Excavations at the nearby Mount Horeb earthwork showed that the circular embankment was built using soil removed from the ditch, which was deposited on the original ground surface (Webb, 1941, pp.147–148). Thus, high susceptibility readings in the embankment were interpreted as the anthropogenically altered soil used to build the embankment. The very high readings beneath the embankment were interpreted as a buried A horizon. Downhole investigations through the ditch and embankment also provided further insight into the geophysical survey results. High levels of magnetic susceptibility detected by the downhole sensor in the embankment fill probaly represent the source of the feature at the apex of the embankment in images of the gradiometer and EM data. The downhole image from the ditch depicted high magnetic susceptibility in areas corresponding to high levels of magnetic gradient in the gradiometer image. This could relate to a collection of eroded highly magnetic topsoil transported into the ditch. However, the downhole system detected alternating low and high susceptibility in


Figure 5. North–south magnetic susceptibility profile of the embankment and ditch created from downhole data. The dark band beneath embankment represents the high susceptibility readings relating to the buried A horizon. Darker bands in the ditch reflect susceptibility variations related to pockets of Late Fort Ancient midden-like fill.

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238 the ditch-fill. This suggests the ditch may have refilled, at least partially, through anthropogenic filling events since erosion is generally a slow but continuous process. Therefore, data obtained over the embankment and ditch furthered a more complete understanding of the geophysical surveys conducted during stage one of this research.

Downhole investigations of the interior pit feature Downhole data were recorded in a 1010m grid of cores spaced 2m apart over the pit feature. Cores ranged in depth from 0.8 to 3.3m below surface. Readings were obtained every 2cm down each core hole. High magnetic susceptibility was detected throughout the plough zone, while a moderately high level of susceptibility was identified in the pit that separated its boundaries from the surrounding clay subsoil (Figure 6). A north–south centre profile of the feature generated from the downhole data depicts an asymmetrical shape (Figure 6a). The northern boundary

E. R. Henry of the anomaly angles to the south and becomes level at 3.3m below surface before coming to a nearly vertical southern boundary. Magnetic susceptibility contrasts inside the feature correlate to natural (low susceptibility) and cultural (high susceptibility) layers within the pit (Figure 6a and b). Downhole images of this feature confirmed that it was a large pit based on the shape, depth below surface, location inside the circle and presence of differential filling episodes. Like the downhole investigations conducted on the embankment and ditch, the examination of this feature with the downhole method provided a better understanding of the geophysical surveys over it. Information obtained with the downhole system outlined the shape of the pit below surface and indicated possible areas within the pit where data pertaining to the use of the pit could be accessed. Excavation of the embankment, ditch and interior anomaly were conducted after the downhole examinations. A third stage of geophysical investigation was conducted following excavations.

Figure 6. (a) North–south profile of downhole data through centre of pit. Arrows denote the pit boundary and susceptibility variations related to differential fill. (b) East–west profile of downhole data through centre of pit. Arrows denote pit boundary and susceptibility variations related to differential fill.




Stage three: magnetic susceptibility mapping of excavation profiles Excavations at the LeBus Circle consisted of a 14m test unit through the pit feature and a 126m mechanically excavated trench through the embankment and ditch (Figure 4). The pit feature yielded few artefacts but displayed a complex soil stratigraphy including a dark silty-loam cultural fill with intermittent natural clay laminations and wash-fill. Mechanical excavations through the ditch and embankment revealed 30cm of mottled clay embankment fill below a plough zone roughly 30–40cm thick. Underlying the embankment fill was a dark silty-loam layer 5cm thick that exhibited a large amount of charcoal flecking. A soft loamy textural difference was noted 10cm below the dark siltyloam stratum. The ditch contained a very dark and soft silty loam with an extremely high density of charcoal in the lower 50cm of the ditch-fill. An unusual gap was noted between the mottled embankment-fill and the ditch. Normally the construction of these earthworks includes soil excavated from the ditch being placed directly outside of the ditch to create the embankment (Webb, 1941, pp.147–148). Magnetic susceptibility was measured on the wall profiles of excavation units using an Exploranium KT9 Kappameter. This single-coil electromagnetic induction meter records volume magnetic susceptibility at the same sensitivity (110 5 SI units) as the Bartington MS2H downhole sensor (Bartington Instruments, 2008, p.11; Exploranium Radiation Detection Systems, undated, pp.23). Exploring features with geophysical methods after excavations provided an opportunity to further examine the magnetic nature of these features. The data gathered during this stage of research were used to better understand stratigraphy documented after excavations and aided general interpretations of the site.

Mapping magnetic susceptibility inside the pit feature KT9 data recorded on wall profiles inside the pit feature were collected in 10-cm increments on the north wall and in 20-cm increments on the 4-m-long east wall. Data were processed using a gridding method and low-pass filter, and compared with excavation profiles in Voxler. Susceptibility variations mapped with the KT9 coincide well with the variations delineated by the downhole data but exhibit slightly higher SI values (Figure 7). Nevertheless, both instruments accurately outlined the boundaries of the feature and detected interior contrasts that relate to natural and cultural stratigraphy. When images of the KT9 data are compared with wall profile drawings


239 (Figure 7a), the outline of the pit is defined by high susceptibility values. Areas of feature-fill exhibiting high susceptibility contained the highest amounts of artefacts and burned limestone. Thus, strata inside the pit exhibiting high susceptibility signify anthropogenic filling episodes (Figure 7b). Low susceptibility values in the pit correlate with clay laminates and mottled laminate/ dark silty-loam fill denoting natural refilling events related to rain and floods. However, a collection of very thin clay laminates were also observed below the pit feature. Lamination can be caused by both the slow separation of clay and silt particles in low-energy water contexts and particle separation occurring by the pressure of overlying strata that push lighter clay particles upward into denser sediment (Rapp and Hill, 1998, pp.43–44; Waters, 1992, pp.216). The presence of these sedimentary deposits below the pit demonstrates that the feature was situated over a small spring or seep. Mapping magnetic susceptibility on wall profiles in the pit provided a means to analyse stratigraphy within the feature and separate the natural and cultural refilling episodes. Separating these events by comparing geophysical data, artefact densities and stratigraphy was important in building a holistic interpretation of this feature’s depositional history. The specific function of this feature is unknown but the structure and history of refilling is indicative of a small spring or seep that was refilled like a large pit. The shape of this pit, outlined by the downhole data, observed in profile and verified with the KT9 data, implies that prehistoric people possibly shaped its boundaries to maintain access to water at the base, before abandoning it and allowing it to refill (Henry, 2009, pp.166).

Mapping magnetic susceptibility in the backhoe trench Magnetic susceptibility mapping on three wall sections of the mechanical trench helped interpret the downhole data and the trench profile. Section 1 was 2m long and located under the apex of the embankment. Measurements were collected on a 20cm grid. Data were processed using a Kriging gridding method in Surfer 8. All KT9 profiles from the backhoe trench were overlaid on to georeferenced drawings of the trench profile in ArcGIS 9.3. Results from section 1 exhibit moderate susceptibility through the embankment, elevated susceptibility at the dark stratum beneath the embankment and the highest susceptibility in the soft loamy area just below the dark zone (Figure 8a). These results indicate that the dark charcoal-flecked stratum below the embankment is an original ground surface that was covered by the embankment construction. The


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Figure 7. (a) Drawings of wall profiles in pit feature showing natural and cultural strata. (b) KT9 data collected on wall profiles depicted in (a).

charcoal associated with this layer demonstrates that the ground was burned to remove vegetation before being covered by embankment fill. The highest magnetic susceptibility recorded just beneath the charcoalflecked stratum is associated with the 10-cm-thick soft area of loam in profile. The zone below the buried ground surface represents the true buried A horizon. The second KT9 section was located north of the embankment where the mottled embankment fill and the buried ground surface are no longer visible in the wall profile. Section 2 was 2m long and gridded for 20-cm


measurements. A map of the data collected from section 2 exhibits high magnetic susceptibility values below where the embankment would have once been (Figure 8b). High susceptibility values in section 2 correspond to the location of the buried A horizon identified in section 1 and high susceptibility values recorded in this area during the downhole investigations. Images of the susceptibility data from the KT9 and downhole surveys confirm there was not originally a gap between the ditch and embankment. Rather, they suggest that ploughing has destroyed the embankment



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Figure 8. (a) Map of data from KT9 section 1, (b) section 2 and (c) section 3. Maps are overlaid on to corresponding portions of the western wall profile in the backhoe trench. Greyscales represent volume susceptibility (K) in SI units.

in this section of the earthwork. The absence of the embankment would leave the ground surface and buried A horizon vulnerable to leeching effects, probably making them invisible in profile. Section 3 covered a 4-m area that began in the middle of the ditch and extended south past the visible boundary of the ditch. Results from this section reveal susceptibility highs clustered inside and along the bottom of the ditch, as well as south of the ditch below the plough zone (Figure 8c). This image indicates that the non-visible buried A horizon continues to the edge of the ditch. The presence of another non-visible portion of the buried A horizon similar to that detected in section 2, implies that the embankment was originally a low and wide feature. Similar to maps of downhole and KT9 data from section 2, the image of section 3 supports an interpretation that the visible gap between the ditch and embankment in profile is an issue of


preservation rather than a representation of the earthwork’s original construction. Susceptibility highs clustered in the bottom of the ditch relate to concentrations of midden-like debris. The density of plant and wood charcoal suggests that the midden-like soil is not the product of redistributed trash debris. It is possible that these deposits represent the clearing of first-generation plants growing in the ditch through burning. If this is the case, radiocarbon samples from the ditch would determine when it began to refill.

Discussion The multistage approach to geophysical investigations described here provided data used to make highly informed decisions during each phase of


242 archaeological research at LeBus Circle. In this case study, gradiometer and EM surveys identified multiple features and demonstrated that much of the earthwork is still intact. Furthermore, they highlight the careful planning that went into the construction of these landscape features. The gradiometer survey shows that LeBus Circle was originally a nearly perfect circle. However, one error in construction can be seen along the western edge of the earthwork’s ditch. Just south from the western break in the ditch, the shape of the ditch is slightly angular (Figure 3). Apart from this small portion of the ditch, the earthwork creates a perfect circle. This is a testament to the great skills Adena people had in earthwork construction. The continued geophysical evaluation of features with the downhole system provided information that facilitated a better understanding of the gradiometer and EM surveys. This was done by depicting the magnetic strength, extent and shape of the features recorded below the surface. Maps of the data collected with the downhole sensor confirmed that differential filling episodes were present in the ditch. The downhole system was also used to detect stratigraphic variations underneath the embankment. Downhole investigations in the pit feature identified 3.3m of deposition that exhibited variations in magnetic susceptibility relating to natural and cultural refilling episodes. This dataset also depicted the shape of the pit below surface. The specific use of the downhole system to explore the shape and dimensions of archaeological features detected in geophysical surveys is significant. This method can and should be used to explore multiple features found in magnetic geophysical surveys when time and/or funding only allows for the excavation of a small sample. The post-excavation geophysical examination of features with the KT9 Kappameter furthered the understanding of exposed excavation profiles. The KT9 profile under the embankment identified a separate burned ground surface directly above the true buried A horizon. Without consideration of the KT9 data, the charcoal-flecked surface would have been interpreted as the buried A horizon. This information was crucial in understanding what cultural context was being dated when the charcoal-flecked buried ground surface was submitted for accelerated mass spectrometry (AMS) radiocarbon dating. The first AMS sample was comprised of charcoal-flecked soil from this stratum. This sample spanned 1006–904 BC at the 2-sigma range when converted to calendar years using the Calib (version 5.0.2) radiocarbon conversion program (Stuvier and Reimer, 1993; Reimer et al., 2004). This date suggests that the earthwork was constructed about 1000 years earlier than archaeologists (Clay, 1991, p.33;


E. R. Henry Applegate, 2008, p.534) previously thought. Because of this very early date, a second AMS sample comprised exclusively of charcoal from the burned ground surface was submitted. The second sample dates between 152 BC and AD 3 at the 2-sigma level. The second date suggests soil accompanying the first sample affected the outcome. Bulk soil samples from buried A horizons have been considered less accurate in dating events similar in nature to earthwork construction because of foreign carbon intrusions (Wang et al. 1996). Such intrusions are most likely the cause for the early date from the first sample. The KT9 map from the ditch revealed that a zone of high magnetic susceptibility extended south of the ditch’s visible boundary. This area of high susceptibility was considered a non-visible extension of the buried A horizon, which suggests the embankment probably originally extended to the edge of the ditch. High susceptibility readings in the downhole and KT9 data in the bottom of the ditch related to pockets of midden-like deposits containing large quantities of charcoal in addition to shell-tempered ceramic artefacts. An AMS radiocarbon sample from the bottom of the ditch returned with two intercepts at the 2-sigma range that span AD 1451–1632. This date correlates well with comparative dates for shell-tempered ceramic artefacts recovered from the ditch, in addition to those recovered from the pit feature (Henry, 2009, p.154). Magnetic susceptibility profiles made from the KT9 data in the pit feature demonstrate that the shape of the pit is consistent with the downhole images. The sloping northern half of the pit may represent a humanmodified route to the spring or seep, since the near vertical southern half probably represents a portion of the pit that was cut back prehistorically to maintain access to the water source. However, the ability to separate the natural and cultural refilling events with help from the KT9 data confirmed that the pit had a long, complex, history of refilling. A charcoal sample from the first cultural refilling event in the pit feature (260–270cm below surface) was submitted for AMS radiocarbon dating. The 2-sigma calibration of the date exhibits seven intercepts spanning AD 1667–1952. This date indicates that the cultural refilling of this feature began during the Late Madisonville Horizon (post-AD 1550 ) of the Late Fort Ancient period. Fort Ancient peoples were sedentary farmers who lived in the Middle and Upper Ohio River Valley from around AD 1000 to 1750 (Henderson, 2008, p.741; Sharp, 1996, p.181-182). They are contemporaneous with Mississippian populations in the southeastern USA, but were not ruled by elite kinship lineages and did not construct large platform mounds (Griffin, 1992; Pollack and Henderson, 1992, p.289,


A Multistage Geophysical Approach At LeBus Circle 2000, p.212; Drooker and Cowan, 2001, p.93). Shelltempered ceramic artefacts recovered from this feature during excavation support this date. However, the AMS date does not indicate when the spring or seep was active or when it was used before the pit began to refill. Mapping magnetic susceptibility variations in the ditch and pit feature led to the separation of two distinct cultural contexts (the ditch and the bottom of the pit feature) where carbon samples could be obtained for radiocarbon dating. Charred wood and plant remains in the bottom of the ditch may indicate the earthwork was maintained by burning vegetation growing there. The date from the bottom of the ditch correlates to a Fort Ancient use of the site, which implies that late prehistoric inhabitants in the area were revisiting ancestral earthworks and refurbishing them. The date from the bottom of the ditch reveals that the earthwork was reused post-AD 1450 during the Early Madisonville Horizon of the Late Fort Ancient time period. The first refilling episode in the pit dates to the Late Madisonville Horizon (post-AD 1550 ) of the Fort Ancient period in Kentucky. These dates demonstrate that the reuse of this ritual earthwork was not a singular event. Unfortunately, the exact function of the pit cannot be determined on the basis of these data. However, given that the spring or seep pre-dates the refilling of the pit, the possibility remains that the placement of the earthwork is associated with the small water feature.

Conclusions This research demonstrates that the application of geophysical methods in a multistage research design can help archaeologists better understand and interpret geophysical data collected during general surface surveys. The continued evaluation of geophysical features in pre- and post-excavation contexts allows archaeologists to develop more informed and accurate interpretations at the feature and site level. Although it is important that this methodology can guide archaeological investigations and provide geophysical information that is rarely sought after or attained, it is more important that this methodology can initiate comprehensive archaeological interpretations. The correlation between more data and better interpretations will not always be the case. However, gaining the best possible understanding of features detected during broad-scale geophysical surveys through continued evaluation of their geophysical characteristics makes the success of these methods in archaeology more likely.


243 The buried ground surface detected beneath the embankment provides a way to date the construction of the earthwork and gain an understanding of the construction sequence. The burning of vegetation from the ground surface signifies either a functional and/ or ritualistic landscape-clearing event aimed to remove vegetation prior to construction. This research presents the first dateable evidence of a construction event from a circular earthwork in this region. The more acceptable of the two dates submitted from the burned surface indicates that circular earthwork construction begins during Middle Adena (150 BC to AD 1) ritual development. This period of Adena ritual development includes the appearance of circular paired-post structures associated with mortuary ritual (Clay, 1991, p. 33). This overlap in chronology may support one interpretation made by Clay (1998, p.10) that circular earthworks and paired-post structures are two types of ritual space that serve similar functions in Adena society. However, it presents evidence contrary to Clay’s (1991) statement that circular earthworks begin to appear on the Adena landscape during the Late Adena (AD 1 to AD 250) phase of ritual development. Thus, it can be stated that two separate forms of circular space appear during Middle Adena ritual organization. It is unclear exactly why people living 1200 years after the construction of the earthwork decided to reuse it. However, one explanation for the reuse of this Woodland earthwork by Late Fort Ancient people lies in a possible want and/or need to reconnect with ancestral landscape features. This research is significant in this regard because the reuse of Woodland ceremonial earthworks by Late Prehistoric peoples in the region is not commonly documented.

Acknowledgements The 2008 Summer Research Grant from the Graduate School at the University of Mississippi, a Research Grant from the Kentucky Organization of Professional Archaeologists (KyOPA), the 2008 Fall Research Grant from the Graduate Student Council at the University of Mississippi, and private donations from Buck and Lane LeBus supported this research. The FM256 was used on loan from the W. S. Webb Museum of Anthropology at the University of Kentucky, while the remainder of geophysical equipment was provided by the Center for Archaeological Research at the University of Mississippi. Jay K. Johnson was the chair of my thesis committee at the University of Mississippi. He greatly supported my research and provided excellent feedback on this paper. Bryan S. Haley sat through more conversations about this project than he will probably care to mention but his guidance and help throughout my studies at the University of Mississippi was


244 invaluable and I greatly appreciate his help. Drs Larry Conyers, Rinita Dalan and Berle Clay, in addition to one anonymous reviewer, provided very helpful comments and discussion on this manuscript. Lastly, I thank my wife Andrea Schuhmann who helped with GIS maps, copy-edited drafts of this manuscript, and whose unending devotion has undoubtedly benefited this project and me.

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