Ground Truth: Recent Investigations of Ground Penetrating Radar Anomalies by the MHT Archaeological Research Program

By Dr. Zachary Singer (MHT Research Archaeologist)

The theme for Maryland Archeology Month 2022 is “The Future of Studying the Past: Innovative Technologies in Maryland Archeology”. One suite of innovative technologies that is being highlighted is remote sensing: methods which allow archaeologists to detect cultural resources buried beneath the ground surface. Remote sensing technologies have transformed how archaeologists study the past. Today, with the aid of high precision GPS receivers and Geographic Information Systems (GIS) mapping software, the coordinates of potential archaeological resources can be precisely located via remote sensing, recorded and plotted in GIS, and then surgically examined through excavations pinpointed to their exact locations.

Over the years, the MHT Office of Archaeology had dabbled with these technologies, carrying out some limited survey with magnetic susceptibility technologies or partnering with others for such work. Beginning in 2019, MHT Chief Archaeologist, Matt McKnight, began a push to acquire additional equipment with which to undertake a more rigourous terrestrial remote sensing research program. The remote sensing technologies deployed by our office include a magnetic susceptibility meter (MagSusc), a fluxgate gradiometer, and a ground penetrating radar (GPR) system. We also utilize a high-precision GPS system capable of pinpointing a location on the Earth’s Surface accurate to within 7 millimeters (or about 1/4 inch).

MHT archaeologists have assisted with remote sensing surveys on archaeological sites throughout Maryland and identified many intriguing anomalies suggestive of archaeological features. However, as is always the case with remote sensing data, these potential features are just that: potential features. Without archaeological ground truthing through excavation it is not possible to conclude with absolute certainty what the various anomalies identified via remote sensing represent. Fortunately, our office has collaborated with many members of the Maryland archaeology community to ground truth (or physically excavate) some of the intriguing anomalies identified via our remote sensing surveys to determine their forms, functions, and ages. Below, we present a sampling of some of these exciting ground truthing results.

Barwick’s Ordinary (Caroline County)

MHT archaeologists carried out a geophysical remote sensing survey at the Barwick’s Ordinary Site on the Choptank River in Caroline County during the summers of 2019 and 2020 to examine a field where the owners of the property had encountered colonial artifacts during a prior landscaping project. The primary objectives of remote sensing at the site were to obtain detailed imaging of the subsurface features believed to be yielding the artifacts recovered on the property. Magnetic susceptibility, gradiometry, and GPR surveys on the property revealed several anomalies suggestive of buried architectural elements.

Annotated results of the MagSusc, Gradiometer, and GPR remote sensing surveys at Barwick’s
Ordinary.

In the fall of 2020, with assistance from ASM volunteers, locals, and Professor Julie Markin of Washington College, a few small test units were excavated to ground truth the anomalies at Barwick’s. The results confirm that the site contains well-preserved, artifact rich, mid-late 18th century archaeological features. Come participate in additional ground truthing excavations this summer at the Annual Tyler Bastian Field Session, which will take place at Barwick’s Ordinary from May 20-30, 2022.

Dr. Matt McKnight ground truthing a GPR anomaly, which was revealed to be the corner of a likely 18th-century privy at the Barwick’s Ordinary site.

Calverton (Calvert County)

In the summer of 2020, MHT archaeologists conducted a ground penetrating radar survey at the 17th-century Calverton Site in Calvert County in an area located within 10 meters of the eroding edge of Battle Creek. The creek is slowly destroying the site and the goal of the GPR survey was to identify anomalies in the portion of the site most at risk of loss from shoreline erosion. The GPR survey would later be investigated via ground truthing using traditional archaeological methods.

Annotated results of the GPR remote sensing survey at Calverton, highlighting the location of a shaft anomaly, which ground truthing determined to be a 17th-century
cellar (CLICK IMAGE TO EXPAND).

Seven likely anthropogenic features were identified in the GPR survey at Calverton. Eight test units were excavated by Applied Archaeology and History Associates during the summer of 2020 to assess these GPR anomalies. The excavations resulted in the identification of ten cultural features, which yielded late 17th- and early 18th-century artifacts including tobacco pipes, a Charles I sixpence coin (1639-1645), and sherds of tin-glazed earthenware. The largest and most artifact-dense feature related to the colonial occupation of Calverton was an in-filled cellar.

Photo of the 17th century cellar feature after it was bisected to ground truth the GPR
anomaly.

Maiden’s Choice (Washington County)

In the spring of 2021, MHT conducted a GPR survey at the Maiden’s Choice I site in Washington County to search for buried domestic structures. The GPR survey revealed the presence of an anomaly suggestive of a subsurface foundation remnant roughly 40 ft east-west by 20 ft north-south, and with an apparent chimney remnant (roughly 5 X 5 ft) near the center. In the fall of 2021, MHT collaborated with the Western Chapter of the Archeological Society of Maryland to excavate three test units to ground truth these GPR anomalies. The ground truthing excavations uncovered remnant rubble stone foundations with artifacts recovered from the plowzone dating primarily to the late 18th and early 19th centuries.

3D models of the excavation units that revealed the remains of a late 18th century
foundation.

Domestic artifacts were present such as furniture tacks, buttons, clay marbles, handwrought nails and coins including a 1776 Spanish half-reale, a pierced 1796 half dime, and a “draped bust” American half cent (1800-1808). A Napoleon Bonaparte First Consulate German jetton was also recovered. A jetton is a commemorative token or medal and this one likely dates to the years 1799-1804, before Napolean was coronated as Emperor. The fall 2021 excavations suggest that this site is a domestic site associated with the Barnes-Mason family that occupied the Maiden’s Choice property after 1773.

Fortunately, in the three examples discussed above, ground truthing of remote sensing anomalies resulted in the discovery of artifact rich archaeological features. However, this is not always the case. Remote sensing anomalies can also be caused by natural occurrences like bioturbation from plant roots and animal burrows. Accordingly, although it is tempting to jump straight from remote sensing results to archaeological site interpretation, the step of ground truthing cannot be skipped. Excavations will always be necessary to determine whether remote sensing anomalies are in fact the remains of
exciting archaeological features or less exciting gopher holes.