Combining ER and GPR surveys for evidence of prehistoric landscape construction: case study at Mound City, Ohio, USA

Journal of Applied Geophysics 129 (2016) 178–186

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Combining ER and GPR surveys for evidence of prehistoric landscape construction: case study at Mound City, Ohio, USA B.B. Schneider a,⁎, R.D. Mandel b, G.P. Tsoflias a, S.L. De Vore c, M. Lynott c a b c

Department of Geology, University of Kansas, 1475 Jayhawk Blvd, Lindley Hall 120, Lawrence, KS 66045, United States Kansas Geological Survey, University of Kansas, 1930 Constant Avenue, Lawrence, KS 66047, United States Midwest Archeological Center, 100 Centennial Mall N # 474, Lincoln, NE 68508, United States

a r t i c l e

i n f o

Article history: Received 14 September 2015 Received in revised form 28 March 2016 Accepted 1 April 2016 Available online 9 April 2016 Keywords: GPR Electrical resistivity Hopewell Archeology

a b s t r a c t Mound City, located at the Hopewell Culture National Historical Park in south-central Ohio, USA, is a prehistoric earthwork (200 BC–500 AD) that consists of 24 mounds enclosed in a square embankment wall and is surrounded by eight pits. Recent excavation of two of these pits resulted in the discovery of a clay loam liner that appears to have been placed on the floor of the pits by a prehistoric society known as the Hopewell. The aim of this study was to determine the spatial pattern of this liner in one of the pits using non-invasive geophysical techniques, specifically electrical resistivity and ground-penetrating radar. Minimally invasive soil augers and a test trench yielded information that was used to corroborate interpretations of the geophysical data. The geophysical methods proved to be useful in locating and defining the remnants of the prehistoric clay loam liner, and the results of our investigation indicate that almost 50% of the liner still remains in the pit today. This discovery supports a new interpretation that the Hopewell excavated and preserved the pits at the Mound City site because they served as cultural landscape features. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The Mound City Group, located in south-central Ohio, USA (Fig. 1), was originally constructed by the Hopewell in the Middle Woodland Period (100 BC–500 AD). It consists of 24 mounds surrounded by an earthen embankment wall (Fig. 2) that encloses approximately 6 ha (15 acres) and is a characteristic example of Hopewell earthen architecture construction (Brown, 2012). Hopewell sites were used for a variety of social and ceremonial purposes (Lynott, 2015). At the Mound City Group, the earthworks were primarily used for burial ceremonies. The term “Hopewell” implies Middle Woodland societies participating in some expansive form of regional integration that is reflected through the construction of large-scale earthworks and/or earthwork centers and a trade network of exotic artifacts (Carr and Case, 2005; Charles and Buikstra, 2006; Abrams, 2009; Lynott, 2015). Hopewell artifacts were made from materials such as mica, copper, shells, and obsidian, indicating a network of long-distance trading. Hopewell sites occur across the Midwest and eastern United States, but one of the largest clusters of sites is within the Scioto River valley of south-central Ohio. Mound City is located in the center of this cluster and was occupied by the Hopewell between 200 BC and 500 AD. However, intermittent use of the site continued after it was abandoned by the Hopewell (Brown, 2012).

⁎ Corresponding author.

http://dx.doi.org/10.1016/j.jappgeo.2016.04.002 0926-9851/© 2016 Elsevier B.V. All rights reserved.

Squier and Davis (1848) originally recorded Mound City in their book Ancient Monuments of the Mississippi Valley. Excavations at the site continued into the 20th and 21st centuries, but the majority of that research focused on the mounds and the embankment walls (see Mills, 1922; Brown and Baby, 1966; Brown, 1994; Lynott, 2015), investigating how they were constructed, what they were used for, and the source of the construction material. Less attention has been directed towards depressions surrounding the embankment walls at the site. The depressions range from 18 m to 54 m in diameter and from 0.5 m to 3 m in depth (Bret Ruby, personal communication, 2013). Squier and Davis originally classified them as borrow pits and the term has been used in subsequent investigations (Brown, 1994, 2012). However, the pits probably served other purposes. Several of the pits are immediately outside the embankment wall. Careful selection and placement of soil in creating both the wall and the pits were required to avoid erosion of the loose, unconsolidated sand and gravel that underlie these features and were exposed by excavation of the pits. Radiocarbon dating and stratigraphic evidence suggest the embankment wall and surrounding pits were among the last landscape features built at Mound City (Brown, 2012; Lynott, 2015). Hence it is likely that the pits post-date the mounds and did not contribute material to mound construction. The use of ditches and pits in association with enclosure walls is well documented among Ohio Hopewell sites, and these features are now considered to be parts of constructed landscapes rather than quarries for construction materials (Cowan et al., 2006; Lynott, 2006, 2015). The

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Fig. 1. Location map of the study area. The study area is directly alongside the Scioto River.

pits at Mound City and perhaps elsewhere may have been designed to hold water. In some Native American belief systems water served to limit the movement of spirits and ghosts (Hall, 1976; Lynott, 2015). Although additional archeological testing is needed to address the function of the pits, the National Park Service (NPC) management policies discourage extensive excavations, especially at prehistoric sites like Mound City. Archeologists must, therefore, investigate sensitive sites without disturbing them. Near-surface geophysical surveying is an efficient, non-invasive means of investigating archeological sites that conforms to the new standards for archeological testing (National Park Service Secretary's Standards, 2016; Whittaker and Storey, 2008). Common geophysical methods used for archeological research include ground-penetrating radar, magnetometry, electrical resistivity, and electromagnetic induction (Dalan and Banerjee, 1998; Kvamme, 2003;

Osten-Woldenburg, 2005; Gaffney, 2008). Our study applied two of these methods, electrical resistance (ER) and ground-penetrating radar (GPR). The NPS has conducted previous geophysical investigations at the Mound City Group. For example, the NPS Midwest Archeological Center completed magnetic gradiometry investigations of the mounds, embankment walls, and ditches, which yielded numerous magnetic anomalies associated with the Hopewell occupation and the World War I training facility of Camp Sherman (De Vore, 2010). Magnetic gradiometry surveys also were completed over the North 40 tract, which is land located just north of the Mound City Group earthwork. Those surveys identified several magnetic anomalies, and based on the results of subsequent archeological excavations, one of these anomalies appears to be a Hopewellian structure with several associated pit features (Brady and Weinberger, 2010).

Fig. 2. Map of the Mound City Group outlining the recorded mounds within the square embankment wall and seven of the pits that surround the wall. The eighth pit in the NE corner was not recorded due to low visibility conditions at the time of mapping. BP1 represents the first pit that was surveyed in 2009 and BP2 represents the second pit that was surveyed in 2010 and 2011 (Courtesy of the Midwest Archeological Center, National Park Service; Lynott and Monk, 1985).

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Geophysical investigations have also been successful in identifying earthen structures and anomalies at nearby Hopewell earthwork sites, including the Hopewell and Hopeton sites. At Hopewell magnetic, electrical resistivity, and electrical conductivity methods identified possible remnants of the enclosure walls that are no longer visible at the surface (McKee, 2005). Magnetic and resistance surveys were also conducted in areas of the Hopewell site that lack earthworks. Excavations guided by the geophysical observations found very little evidence of intense or long-term activities during the Middle Woodland period, contrary to the long-term or large-scale Hopewell settlements suggested by Morehead in 1922 and Griffin in 1997 (Lynott, 2009). In 2001 and 2002, Weymouth (2003) used a cesium gradiometer, fluxgate gradiometer, and a fixed-offset resistance meter to conduct surveys at the Hopeton Earthworks. His work showed that the walls of the rectangular enclosure are quite visible in the magnetic data, even though they are only very subtle topographic features at the surface due to years of agricultural activities (Lynott, 2009). He was also able to identify a weak magnetic anomaly, relative to background values, that was believed to represent the remains of a small circular earthwork, known as the North Small Circle, which is not visible at the surface today. The anomaly was excavated and magnetic susceptibility methods (see Dalan, 2008) proved successful in delineating the boundaries of the feature and surrounding soils. Several Hopewell sites in the Scioto River valley of Ohio have been altered or destroyed by plowing and development. The archeological integrity of the Mound City site has diminished over time as a result of land leveling for cultivation, construction of Camp Sherman, a World War I military training camp (Ohio Historical Society, 2013), and reconstruction of the earthworks in the 1920’s and 1960’s. All of the pits at the site were leveled during the construction of Camp Sherman and were later excavated in the 1920’s by the Ohio Historical Society using heavy equipment, with the exception of the southeast borrow pit that was identified and excavated by Dr. James Brown (see Brown, 2012) in the 1960’s, as described below. However, investigations have still continued at the site in order to collect any remaining information about the original construction and purpose of the site. In recent investigations at the Mound City site, a distinct clay loam liner was discovered in two of the pits. This discovery, combined with the fact that the Hopewell people excavated the pits after most of the earthworks were constructed, challenges previous interpretations about the function of the pits (Lynott, 2015). The first step in addressing this question was to assess the spatial pattern of the clay loam liner. The primary objectives of our investigation were to determine the effectiveness of geophysical methods, in particular GPR and ER, for imaging the clay loam layer and then incorporate the geophysical results with information gathered from limited augers and a single test excavation unit to address the question about the function of the pits. 2. Site setting Mound City was constructed on a high, Pleistocene terrace of the Scioto River, a tributary of the Ohio River, USA. The eastern edge of the site is about 45 m from the modern channel of the Scioto. The terrace fill typically consists of sand and gravel capped by a thin veneer of loamy alluvium. Thick till deposits that accumulated during the Wisconsin Glacial Episode (ca. 110,000–10,000 years ago) mantle the uplands surrounding Mound City (Thornberry-Ehrlich, 2013). 3. History of research on the pits at Mound City Archeological investigations have been conducted at Mound City since the mid 1800’s. Squier and Davis (1848) recorded and mapped the site and excavated the interiors of some of the mounds. They concluded that the site had over 20 mounds surrounded by embankment walls, and seven “borrow pits” were recorded outside the embankment walls. William C. Mills excavated some of the mounds in the 1920’s

(Mills, 1922), and James Brown directed investigations in 1963 to further examine several submound features, relocate and reconstruct the southeast embankment wall, and relocate and reconstruct several of the mounds (Brown, 1994). During Brown's investigations, an eighth pit was discovered in the southeast corner of the site. The excavations revealed prehistoric artifacts such as pottery, mica, bladelets, and projectile points within the pit. Also, burials were uncovered in the southeast and northwest pits (Brown and Baby, 1966). Based on the presence of artifacts in the fill of the southeast embankment wall and a radiocarbon age from charcoal at the base of the wall, Brown (1994) concluded that the pits and embankment wall were likely built simultaneously after the construction of the majority of the mounds. In the summers of 2009 and 2010, the National Park Service's Midwest Archeological Center conducted geophysical investigations including GPR, ER, magnetometry, and electromagnetic induction over the southeast pit (BP1) and the eastern pit north of the gate (BP2) to determine their archeological integrity (Fig. 3). The geophysical observations were then tested using minimal excavations, including a 1 × 12m trench that was excavated to a depth of 1 m in BP1. Also, four soil augers were placed in BP1 that were excavated to a depth of 20–30 cm. Testing in BP2 included a 1 m × 2 m trench that was excavated to a depth of 60 cm, plus nine soil augers that were excavated to a depth of 40 cm–80 cm. The soil augers were collected using a 3-in. diameter bucket auger. The geophysical results revealed that there is little to no archeological integrity in BP1 due to a buried pipe that was emplaced in the pit during the construction of Camp Sherman, and that most of the remaining clay loam liner had been removed when the pit was excavated during the 1920’s and 1960’s. The only remnants of the clay loam liner occur along the edges of the pit (Benson, 2012). The geophysical results of BP2, which are presented in this paper, revealed that almost 50% of the clay loam liner is still intact, with minimal impact from Camp Sherman and the 1920’s reconstruction. In 2009, a clay loam liner was recorded on the floor of the trench in BP1 and in 2010 was subsequently discovered in the trench of BP2. In BP1, the liner is 15 cm–50 cm thick and overlies unconsolidated sand and gravel. The liner in BP2 is 20 cm–35 cm thick and also overlies unconsolidated sand and gravel. Along the southwestern edge of BP1, the liner occurs on steps cut into the side of the pit; it is in a near-vertical position on the risers of the steps, but flattens out on the treads. A small burned feature consisting of burned soil and pine charcoal was found on one of the steps and is comparable to other small burned features found in association with landscape construction features at the nearby Hopeton Earthworks (Lynott, 2015). Remnants of clay loam liners have subsequently been recorded in other borrow pits at Mound City (Brown, 2012; Lynott, 2015). In summary, these liners are not natural features. It appears instead that the clay-rich sediment was emplaced by the Hopewell soon after the construction of the borrow pits. The clay loam liners do not fine upward and gravel is scattered throughout the fine-grained matrix, which is not indicative of a flood deposit. Also the geometry of these liners is not natural. As noted above, in BP1 the clay layer is in near-vertical positions where it mantles the risers of steps cut into the wall of the pit. Natural processes cannot account for such plastering of sediment. Our paper presents the results of the application of two non-invasive near-surface geophysical investigations, GPR and ER, to locate and trace the remnants of the clay loam liner recorded in BP2 at Mound City. Information gained from the geophysical investigations change the interpretation about the function of the pits. The results of this study may also help guide future investigations that require non-invasive techniques to search for subtle cultural features in earthworks and other archeological sites. 4. Geophysical methods GPR and ER data were collected over the whole survey grid shown in Fig. 3. The pit is 3 m deep and extends 35 m in a north–south direction

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Fig. 3. a) Survey grid of BP2 showing surface elevation contours (m) and the locations of the trench and auger holes. Auger numbers were assigned in the order they were excavated. b) Contemporary photograph of the pit, facing NE. c) GPR data collection within BP2. d) Electrical resistance data collection within the pit.

and 15 m in an east–west direction. The surveys covered a total area of approximately 640 m2, and a line spacing of 0.5 m was used for both methods. The geophysical data were compared to in-situ observations made in the trench and auger holes in BP2. ER is an active source method that applies current at the ground surface through metal probes to measure the potential difference between them (Burger et al., 2006). It has often been used to detect subtle archeological features on site surveys (Martínez et al., 2015; Schmidt, 2013; Bongiovanni et al., 2011). The survey at Mound City was conducted using a Geoscan Research RM15 Resistance Meter twin array with a 40-V output. The electrodes had a constant spacing of 0.5 m for all Nseparations and a 0.5 m spatial sampling was used along the survey lines. Distance to the stationary electrodes was not recorded because there was a large distance between the remote and mobile electrodes (N15 m), and so the changes in earth resistance due to varying electrode geometry is negligible (Schmidt, 2013). The twin array was chosen for this research because it has good horizontal resolutions, with a lateral resolution of approximately half the electrode spacing (Zonge et al., 2005). The depth of investigation using this array and electrode spacing is approximately 0.5 m, which was deemed reasonable for detecting a layer that ranges between 20 and 50 cm in thickness and lies roughly 20 cm below the ground surface. When evaluating the shallow subsurface with ER, it is important to recognize that the movement of electrical current in soils is influenced by several factors, including the amount of water present in the soil, the salinity of the water, and soil texture, especially the clay content. Increased clay content will generally decrease

the electrical resistance of a soil (Burger et al., 2006). This makes ER particularly useful for differentiating deposits of coarse-grained sands from deposits consisting of clay; sands and gravel will have higher resistance whereas clay will have relatively lower resistance. GPR is an active source method that transmits into the ground a short pulse of high frequency electromagnetic energy, usually in the range of 10–1000 MHz, and responds to changes in the electrical properties of the ground (i.e. the dielectric constant and electrical conductivity) (Jol, 2009; Davis and Annan, 1989). At Mound City, a Geophysical Survey Systems Inc. (GSSI) SIR 3000 cart system with 400 MHz antennae was used. A grid of parallel 2-D lines were recorded at a trace spacing of 0.02 m along the survey lines and a sampling interval of 0.17 ns. When evaluating the shallow subsurface with GPR, higher frequencies achieve shallower depth of penetration but higher resolution (Annan, 2005). At a frequency of 400 MHz and a velocity of 0.09 m/ns, features or depositional units as thin as approximately 6 cm can be resolved vertically, which is sufficient for identifying the boundaries of the targeted clay loam liner. Attenuation of the signal is greater in fine-grained materials like clay as these minerals absorb the EM signal, so areas where the clay loam liner is absent or thin are expected to exhibit higher GPR signal amplitudes compared to areas with a thicker clay layer present (Annan, 2005). Processing of the geophysical datasets is necessary in order to improve subsurface image quality (see Table 1). For the ER dataset, processing was completed using Geoplot v. 3.0 (Geoscan Research, 2014). First, a de-spiking filter was applied to remove random, spurious

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Table 1 Geophysical processing steps for the ER and GPR datasets. ER processing steps

GPR processing steps

1. De-spiking filter 2. Edge matching 3. Interpolation 4. High-pass filter

1. Time drift corrections 2. Time zero corrections 3. Low-pass frequency filter 4. Background removal 5. F–K migration 6. Topographic corrections

readings followed by edge matching to align the survey lines. The data were then interpolated and a high-pass filter was applied using a Gaussian weighted 3 × 3 window that removed low-frequency noise and the geological background response that is common in resistivity data. Topography is another important factor than can affect ER data and cause anomalous changes in the values. However, smooth topography, which is typical of soil-covered archeological monuments like the Mound City Group, has been shown to have negligible effects on ER measurements (Schmidt, 2013). Hence topographic corrections were not applied to the ER data. For the GPR dataset, processing was completed using MATLAB and MatGPR (Tzanis, 2010). Time-drift and time-zero trace corrections were applied, followed by a low-pass frequency filter that removed frequencies higher than 550 MHz. A background removal filter was applied to eliminate the horizontal banding created by antenna ringing, revealing the geology of the site. The data were F–K migrated with a velocity of 0.09 m/ns, which was estimated using a hyperbola fit to subsurface diffractions. Automatic Gain Compensation (AGC) gain was applied during acquisition of the data. The last processing step corrected the data for elevation changes across the survey. The 2D GPR lines were then concatenated into a pseudo-3D grid and imported into IHS Kingdom® for interpretation. Although not a full resolution 3D grid (Grasmueck et al., 2005), the 0.5 m GPR line spacing is adequate for creating horizontal time slices and thus being able to image a laterally extensive layer across the survey area. 5. Results 5.1. Soil and stratigraphy of BP2 The stratigraphy of BP2 was gleaned from a 1 m × 2 m trench excavated on the southwest side of the pit (Fig. 4 upper). Three stratigraphic units, numbered I to III from top to bottom respectively, were identified in this trench (Table 2). Unit II is the clay loam liner that occurs at approximately 25 cm below the ground. The liner is 20–35 cm thick and has abrupt, wavy upper and lower boundaries. The material used to construct the clay loam liner probably was derived from the argillic (Bt) horizon of a native soil. This interpretation is based on the presence of clay films (argillans) in the liner. An interesting observation was that Unit II only occurs in the western part of the trench; it pinches out to the east. Nine soil augers were collected in BP2 (Fig. 3), five of which (augers 3, 4, 5, 7, and 9) encountered the clay loam liner. The boundaries separating the individual units were apparent in both the trench and auger holes. In the augers, the thickness of the clay loam liner ranged from 20 cm–55 cm, with the thickest liner in auger 5 and the thinnest in auger 7 (Table 3). The clay loam liner was encountered at a depth of approximately 20 cm in four of the five augers, and was intercepted at a depth of approximately 35 cm in auger four. The sand and gravel layer occurred at depths of 40 cm–76 cm when the clay loam liner was present. The depth to the sand and gravel layer in the four augers without the clay loam liner ranged from 25 cm–35 cm, except for auger 6 where the sand and gravel layer was not encountered until 45 cm below surface. In auger 6, several historic artifacts including coal, fistsized rocks, brick, glass, bone, nails, and slag were recovered in Unit 1,

Fig. 4. (upper) Photograph of the south and west walls of the trench in BP2 along with interpreted contacts between units I, II and III shown as black lines. The trench is 2 m long by 1 m wide and approximately 0.6 m deep. As indicated by the black outline, Unit II (clay loam liner) pinches out from west to east along the south wall. This corresponds with an increase in radar amplitude from west to east in the corresponding cross section of the 2D GPR profile data (lower). The interpreted tops of unit II (U2) and III (U3) are also marked on the GPR line.

suggesting that portions of BP2 may have been used as a trash pit during the Camp Sherman occupation. This would account for the greater thickness of Unit I in the area of auger 6. 5.2. Electrical resistance results The electrical resistance data along with the soil auger observations are shown in Fig. 5. The ER data exhibits alternating regions of high and low resistance values with respect to the background values. Clay should have relatively low resistance values and gravel should have Table 2 Description of the south wall of the trench in BP2. Unit

Depth (cm)

Description

I

0–20

II

20–40 shallowest 20–55 deepest

III

40–60

Brown (Munsell color chart (MCC 10YR 4/3, dry) silt loam, weak, fine granular structure; friable, many biogenic features, including open insect burrows and worm casts; many fine and very fine roots; 30–40% gravel; abrupt wavy boundary. Dark reddish brown (MCC 5YR 3/4) clay loam, reddish brown (MCC 5YR 4/4) dry; moderate medium prismatic parting to moderate fine subangular blocky; hard, firm; common distinct continuous reddish brown (MCC 5YR 4/3, dry) clay films on ped faces and dark reddish brown (MCC 5YR 3/2, dry) clay flows in macro pores; common fine and very fine and few medium roots; common worm casts; abrupt wavy boundary. Yellowish brown (MCC 10YR 5/4, dry) sand and fine gravel; loose, single-grain.

B.B. Schneider et al. / Journal of Applied Geophysics 129 (2016) 178–186 Table 3 Thicknesses and depths of the units encountered in the augers. The topsoil across the survey was 3 cm thick. Auger number

Unit 1 thickness (cm)

1 2 3 4 5 6 7 8 9

23 33 17 33 19 42 18 27 18

Unit 2 thickness (cm)

Depth to top of unit 2 (cm)

35 35 43

20 35 22

19

21

34

21

Depth to top of unit 3 (cm) 25 35 55 70 76 45 40 31 55

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oriented GPR profile at position East 888 with intersecting augers 8 and 9 and the corresponding resistance profile (see Fig. 5 for location). GPR reflections vary in continuity and amplitude along the profile. Auger 8 did not contain any clay loam lining, whereas auger 9 did, which is supported by the resistance data exhibiting relatively high resistance values below auger 8 compared to low resistance values at auger 9. When compared to the GPR data, there is a loss of radar signal amplitude below the position of auger 9 where the clay loam liner is present, as compared to radar amplitudes below the position of auger 8. This GPR signal amplitude attenuation can be seen across the lines where resistance data values are low. GPR reflections could not be consistently correlated to boundaries between soil units. 5.4. Comparison of ER and GPR results

relatively higher resistance values (Burger et al., 2006). Spatially varying ER values are most likely associated with the remaining areas of emplaced clay loam versus the areas that have gravel directly beneath the topsoil. The ER data correlate very well with the auger observations and the results of the trench that was excavated into the pit (Fig. 5). Auger locations that encountered the clay loam liner are shown as open diamonds and lie in the low resistance regions of BP2. Auger locations where the clay loam liner was absent are shown as open circles and lie in regions of high electrical resistance, which is consistent with the presence of gravel. The linear feature running diagonally in the western portion of the ER grid corresponds to a modern-day walking trail next to the borrow pit. The amplitude scale range of the dataset was set to ±10 Ω in order to enhance prehistoric features that may be present. 5.3. GPR results When compared to a GPR cross-section at the location of the trench, a noticeable increase in amplitude can be seen occurring from west to east as the clay loam liner pinches out (Fig. 4 lower). Distinct GPR reflections defining the top and base of Unit II are not observed. This trend is seen in the radar lines across the grid. Fig. 6 shows a south–north

A cross plot of the resistance data with the GPR RMS amplitudes at the auger locations is shown in Fig. 7. RMS amplitude is a measure of the overall GPR signal strength in the interval under consideration. In this study we computed GPR RMS amplitudes in the upper 40 ns. Regions of the grid containing the clay loam should exhibit overall lower RMS values because GPR signal amplitude attenuates in clay-rich soils (Annan, 2005). Overall, a good correlation between resistance data and GPR RMS amplitudes is shown in Fig. 7. Auger locations that intersected the clay liner exhibit overall relatively lower resistance and lower GPR RMS compared to augers with no clay liner. These trends are examined over the entire survey area by overlaying the GPR RMS amplitude and resistance data (Fig. 8). GPR RMS amplitude values below 50 are interpreted as the clay loam based on the trends observed in Fig. 7 and correlate well with the low resistance data across the survey. Amplitudes above 50 were left transparent and assumed to be unknown or gravel. The two data sets presented in Fig. 8 show good agreement with each other and with the auger and trench in BP2. The predicted distribution of the remaining clay loam liner in BP2 was mapped in the areas where both the GPR and ER datasets agreed, or where auger and trench data provided additional information (Fig. 8). Based on this interpretation, it appears that almost half of the liner is still present within the pit. The thickness varies across the pit, ranging between 10 cm and 55 cm; the majority of the thickest remnants are located in the south-central portion of the pit, and the thinner remnants generally surround the edges of the pit. 6. Discussion and conclusions

Fig. 5. Resistance data from BP2 with trench and auger locations (see key). There is clear agreement between ER data and auger results identifying the spatial pattern of the clay liner (see key). The dashed line marks the location of resistance and GPR lines shown in Fig. 6.

The objective of this research was to evaluate the effectiveness of non-invasive geophysical methods for imaging a recently discovered clay loam liner in the pits at Mound City, Ohio, USA. GPR and ER methods were successfully used to locate and map the remnants of the liner in one of the pits at the site. These methods are advantageous because they are non-invasive and provide greater spatial coverage in a shorter amount of time than auger testing. In particular, the successful integration of the two supporting datasets is an important technique that can be used at similar sites, but with the understanding that these methods detect changes between contrasting units (i.e., grain-size distribution, moisture content, mineral composition, etc.). In addition, calibrating geophysical measurements with in-situ observations from auger holes and trenches strengthens the interpretation of the data. The geophysical results suggest that the clay loam liner in BP2 originally extended across the entire area of the pit. The depth to Unit 3 is greatest in the center and southern portion of the pit where the liner is still present, and appears to thin along the edges of the pit. This suggests that the liner was originally thickest in the center portion of the pit and thinned gradually towards the edges. Variability in the thickness of the liner, as well as its absence in some portions of the pit, are believed to be products of disturbance associated with both the Camp Sherman occupation and the reconstruction of the pit in the 1920’s. Based on the likely presence of a trash pit in the area of

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Fig. 6. (upper) BP2 GPR line at position 888 m east moving north and the corresponding resistance survey line (bottom). The area where the clay loam liner is present in auger 9 shows a sharp decrease in amplitude in the GPR and resistance data, whereas the area below auger 8 where the clay loam liner is missing exhibits increased amplitudes in both datasets.

auger 6, we suggest that the thinning and removal of the clay loam liner in the center and southern portion of the pit are products of disturbance associated with the Camp Sherman occupation. Also, there is evidence of removal or thinning of the clay loam liner along the eastern edge of the pit. In that area, recent earthwork reconstruction may have affected original features of the site. Reconstruction of the pits was completed based on maps created by Squire and Davis, but not necessarily with regard to the recorded depths of the pits. For future research, increasing the GPR antenna frequency may improve subsurface imaging, depending on the depth of the stratigraphic units that are being examined. For this research, while the 400 MHz frequency antennas provided satisfactory imaging with approximately 6 cm vertical resolution, the shallowest portion of the subsurface data had to be filtered out because they were masked by antenna interference. Higher frequencies up to 1 GHz should provide higher resolution of the stratigraphic units, and may be able to image the targets of

Fig. 7. Cross plot of GPR RMS amplitude vs. resistance observed at auger locations. Augers that encountered the clay liner are shown as open circles. Augers without the clay liner are shown as solid circles.

interest at this site, which are only at a maximum depth of 75 cm. The GPR RMS data indicate that there are additional remnants of clay loam liner across the pit where the resistance data is unclear, particularly in the areas where the resistance data increases to a more neutral value on the color bar between 0 Ω and 4 Ω. This difference is attributed to the higher resolution of the radar method; thin layers of small contrast with respect to the background will be missed in resistance measurements (Sharma, 1997). At Hopewell sites in the Scioto River valley, pits do not typically occur in close proximity to enclosure walls. However, at Mound City, four of the pits are immediately adjacent to the walls and four other pits are within 6 m–10 m of the walls. Erosion of the loose sand exposed in the pits probably would have threatened the walls; hence the builders may have emplaced clay-rich liners in the pits to stabilize them (Lynott, 2015). Also, the pits at Mound City are unique in that they display some symmetry, with one pit at each corner and two pits surrounding the gate entrances on the east and west side of the enclosure. This evidence, along with the many artifacts and burials that have been found in the pits at the site, suggests that the pits at Mound City were intentionally designed by the Hopewell, perhaps with the additional purpose as water retention ponds to enhance the architecture and function of the site. At the Shriver circle, a Hopewell site in south-central Ohio, Burks and Cook (2011) reported a prehistoric clay-rich liner above the unconsolidated gravels in the ditches along the northern side of the enclosure. A similar clay-rich liner was found on the bottom and sides of the ditch surrounding the embankment wall at the Hopewell Mound Group (Lynott, 2006). Hence, the clay loam liners at Mound City do not appear to be unique and may occur in ditches and borrow pits at Hopewell sites throughout the U.S. The application of non-invasive geophysical techniques, in particular ER and GPR, at locating and defining soil stratigraphic units within earthworks without the need for extensive excavation is important. Prehistoric earthworks have been documented across the Midwestern, Eastern, and Southeastern United States, with cultural affiliations ranging from the Middle Archaic (ca. 3750 BC) to Mississippian (1000–1700 AD) (Anderson, 2002). Geophysical methods make it possible for future researchers to explore and identify the complex history and structure of

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Fig. 8. (left) GPR root mean square (RMS) amplitude map showing amplitudes less than 50 overlain onto the corresponding resistance and auger data. (right) Elevation map (m) of BP2 showing the predicted distribution of the remnant clay lining as interpreted from the GPR, ER, and auger data. The numbers in black are thicknesses of the clay liner based on auger data.

these earthen monuments with limited to no excavation necessary, preserving the history and cultures of the ancient mound builders. Acknowledgments The NPS provided support for this project, and we are especially grateful to NPS staff at the Hopewell Culture National Historical Park. We are also grateful for student research grants from the Society of Exploration Geophysicists and Geological Society of America that aided in this research project. Thanks also to the two anonymous reviewers, who provided comments that greatly improved our manuscript. After this research was completed, the Hopewell archeology community suffered a great loss by the sudden passing of one of the authors, Dr. Mark Lynott. Mark left a rich legacy in his contributions to Hopewell archeology and was an influential mentor and friend to so many. He is missed greatly and we feel very fortunate to have had the opportunity to work alongside him and learn from him during this project. References Abrams, E.M., 2009. Hopewell archaeology: a view from the Northern Woodlands. J. Archaeol. Res. 17, 169–204. Anderson, D., 2002. The Evolution of Tribal Social Organization in the Southeastern United States. In: Parkinson, W. (Ed.), The Archaeology of Tribal Societies. International Monographs in Prehistory, Ann Arbor, Michigan, pp. 246–277. Annan, A.P., 2005. Ground-Penetrating Radar. In: Butler, D.K. (Ed.), Near-Surface Geophysics. Society of Exploration Geophysicists, Tulsa, Oklahoma, pp. 357–434. Benson, B.B., 2012. Geophysical Investigations of the Mound City Borrow Pits, Ross County, Ohio (Master's Thesis) Department of Geology, University of Kansas. Bongiovanni, M.A., Vega, M., Bonomo, N., 2011. Contribution of the resistivity method to characterize mud walls in a very dry region and comparison with GPR. J. Archaeol. Sci. 38, 2243–2250. Brady, K., Weinberger, J.P., 2010. Recent Investigations at the Mound City Group. The Newsletter of Hopewell Archeology in the Ohio River Valley Vol. 7, pp. 25–34 (http://www.nps.gov/mwac/hopewell/v7n2/three.html). Brown, J.A., 1994. Inventory and Integrative Analysis: Excavations at Mound City,Ross County, Ohio. Report to the National Park Service Vol. 4, pp. 135–137. Brown, J.A., 2012. The Archaeology of a Renown Ohio Hopewell Mound Center. Midwest Archeological Center Special Report No. 6. National Park Service, Lincoln, Nebraska. Brown, J.A., Baby, R.S., 1966. Mound City Revisited. The Ohio State Archaeological and Historical Society, Columbus, Ohio.

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