Reconstructing a Historic Landscape

Geophysical Prospection at the Woodlands

By: Jason Herrmann, Kacie Alaga and Katie Breyer

Originally Published in 2020

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THE COUNTRY ESTATE of native Philadelphian William Hamilton (1745–1813), known as The Woodlands, included much of the land now occupied by Penn’s campus. This estate was notable in the Early Republic United States not only for the mansion at the symbolic center of the estate, but also for the cultivated landscape and botanical gardens designed by Hamilton to resemble English country estates. While some elements of Hamilton’s estate were used to structure the environment we encounter today, most of the historic landscape has been lost to urban expansion and development, and the core of the gardens was overwritten by the establishment of The Woodlands Cemetery in the mid-19th century.

A view of Center City, Philadelphia as seen from the Woodlands Cemetery.
Looking northeast from The Woodlands Cemetery toward downtown Philadelphia, across the Schuylkill River. The 18th-century Woodlands Mansion is visible in the lower left-hand corner of the image. Drone photo by Jason Herrmann.

Students in the Penn courses Introduction to Digital Archaeology (Fall 2019) and Geophysical Prospection
for Archaeology (Spring 2020) used digital methods to reconstruct the historic landscape at The Woodlands. They investigated the potential location of Hamilton’s greenhouse, which held his botanical collection and was home to what was at the time the largest collection of exotic plant species in North America. While our work has only just begun, our initial findings revealed some hints of the broader organization of the built landscape at Hamilton’s Woodlands and helped to inform strategies for longer-term investigations of the historic landscape.

A Brief History of the Greenhouse and Property

Born to a prominent Philadelphia family, Hamilton inherited 300 acres beyond the western border of the city upon his 21st birthday. It was here that Hamilton built his home, a classical villa with a two-story columned portico on its south face, overlooking a bend in the Schuylkill River. (For more on the history of The Woodlands, see Robert Peck’s article in this issue.) Struck by the picturesque principles of British landscape design and neoclassical architecture that he observed during a trip to England following the Revolutionary War, Hamilton returned to his home at The Woodlands determined to create his own version of an ideal English country estate. His campaign to remodel The Woodlands started with the addition of two wings to the original classical villa. Construction continued until 1789, and the resulting structure served as a precursor to the architectural trends of the Federal style that would go on to dominate in North America during the late 18th and early 19th centuries.

Watercolor painting of the Woodlands house as seen from a bridge across the river.
Watercolor of The Woodlands Mansion as viewed from the Gray’s Ferry Bridge by JP Malcolm, ca. 1792. This is the only potential depiction of the greenhouse while it was in use, thought to be the building at the far left. Courtesy of The Dietrich American Foundation.

A drawing of the plans for the Woodlands.
Detailed descriptions of the greenhouse itself are scant, and plans that date to the time of its use have yet to be found. In this drawing, created in 1846 to map the plan of the Cemetery, the greenhouse consisted of a long, narrow, rectangular building located between the Mansion and the carriage house to the north. The greenhouse appears to have run on a southwest-northeast axis, placing it parallel to the face of the Mansion.

Hamilton turned his attention to the surrounding grounds when the remodel of the Mansion was complete. One of the foremost objectives of British landscape design was to ensure the natural integration of the house with the surrounding grounds, and Hamilton dedicated the rest of his life to developing and achieving this goal. An eager botanist and plant collector, Hamilton imported plants and trees from across to the world to become part of his gardens. This interest spurred him to design and construct one of the first large greenhouses in the United States, where he amassed one of the most extensive collections of rare plants in the country, estimated by some to include 10,000 individual plants. The greenhouse, modeled on contemporary English greenhouses, measured 140 feet in length and was 1.5 stories tall. An internal cistern ensured that tropical aquatic plants could be supported within.

After his death, Hamilton’s estate was divided into parcels and sold off by his beneficiaries. The part of
the estate that held the Mansion and the greenhouse was sold to The Woodlands Cemetery Company of Philadelphia in 1840. The landscape cultivated by Hamilton was lost as the grounds were remodeled by the Cemetery Company. The greenhouse was demolished in 1854 to make room for the construction of a Carriage Shed, and the memory and location of this important structure was lost in the decades following its demolition.

A student walking in a straight line and holding a gradiometer.
A student uses a magnetic gradiometer to survey the South Survey area between the Carriage Shed and the Mansion.

Magnetic Prospection

Students carried out geophysical surveys on the north and the south side of the original greenhouse site with the intention of mapping parts of the greenhouse foundation that may have extended beyond the Carriage Shed, as well as any other landscape features that connected the greenhouse to other parts of the built environment on the Hamilton estate.

Our approach was different from that taken by the previous investigators, who, guided by historic accounts and documents, used targeted investigations to verify and sample features where they are likely to be preserved. In contrast, we sought to create a continuous map of the landscape surrounding the Mansion through geophysical prospection, which relies on mapping variations in the physical properties of buried materials to “see” underground without digging. Such a map provides several advantages to traditional excavation. First, we were able to map changes in the soils and archaeological deposits near the subsurface very quickly compared to excavation. Another advantage is that we could see the spatial relationships between the features we detected and gain a degree of context that could otherwise be achieved only through extensive excavation that would be expensive in terms of time and labor. This broader picture can also be used to reevaluate findings and interpretations from earlier investigations.

An aerial photo of the Woodlands, with survey areas outlined in green.
Magnetic survey areas at The Woodlands Cemetery surround the predicted location of the greenhouse.

We used a magnetic gradiometer to survey the grounds surrounding the location of the greenhouse; this is a sensor that measures local variation in magnetic fields across an area. Past human activities left traces through unintentionally distributing magnetically enhanced materials across the landscape. The most common source of magnetic materials in urban environments is pieces of ferrous metal, primarily litter, but we frequently find architectural installations and conduits for utilities. Iron-rich rocks used in construction are also detectable; in our case, curbstones and foundation stones contributed to the map of magnetism surrounding the greenhouse. Firing enhances the magnetism of naturally occurring iron in soils, so places where kilns, hearths, and fireplaces were located will feature a more intense magnetic signature, as do the bricks and ceramic products from these ovens. That means architectural and landscape elements built from brick can be mapped with magnetometry. Finally, traces of naturally occurring iron compounds in topsoil or organic-rich deposits are also detectable through magnetic survey, even when not fired. Earth-moving activities, such as clearing the ground of topsoil, digging a trench or pit, mounding soil, or cutting through topsoil layers in the creation of a path or route, are all things that can be seen in maps of magnetic intensity.

One limitation of data from the magnetic gradiometer is that it is difficult, if not impossible, to estimate the depth of features, as one can do with ground-penetrating radar (GPR). Therefore, features from different episodes of the development of the landscape surrounding the Hamilton greenhouse, including magnetic influence from above ground features, are conflated into a two-dimensional plan map of magnetic intensity. The maps of magnetic intensity that we produced are a record of the complex history of the landscape surrounding Hamilton Mansion, but it is up to us to use what we know about the landscape to interpret the results.

A student walking in a straight line while holding a gradiometer, while two others watch.
Survey with the magnetic gradiometer in the South Survey Area. Remains of the Carriage Shed are visible in the background.

An aerial view of a section of the Woodlands, with a smaller section showing magnetic intensity near the mansion.
Map of magnetic intensity in the North Survey Area.

An aerial view of a section of the Woodlands, with a smaller section showing magnetic intensity near the stables.
Map of magnetic intensity in the South Survey Area. Magnetic intensity is recorded in nanoteslas (nT).

The Results of Our Work

The first area that was surveyed lies south of where the greenhouse stood, and where most of the previous investigations were carried out. The magnetic gradiometry results are dominated by evidence of the magnetic signature of modern installations, namely two utility poles positioned on the bottom right of the map. These include steel support cables whose magnetic field is strong enough to mask out an area almost 10 m in diameter. There is also a buried utility line, documented in prior excavations, that has a very strong magnetic field and can be seen leading almost directly southward from the utility poles. Other evidence of modern interference is the speckling across the survey area. Each one of these points likely represents the position of a small amount of ferrous metal litter: bits of nails, fasteners, or staples are most common. A cluster of these anomalies was concentrated surrounding the area where a possible corner of the greenhouse foundation was exposed through excavation.

Some subtle features in the magnetic map in this section of the survey could represent parts of the landscape as it existed when Hamilton was in residence at The Woodlands. First is a sign of the submerged brick path, documented through excavation, that led from the Hamilton Mansion cryptoporticus, or covered gallery, toward the greenhouse. Also apparent are two square anomalies that are aligned with each other near the center of the survey area. These are located in an area where we know soil was mounded to create level ground for the road to the Mansion after the grounds were being converted to a cemetery, and it remains possible that these features are associated with this later construction. However, it seems unlikely that the deposition of soil for grading would have been so systematic, and even more unlikely that the mounding of soil would have been so regular and on the same orientation as the Mansion facade, rather than following the natural slope or the contour of the road. A more likely interpretation is that these anomalies are evidence for the piles of soil that were temporarily placed around trenches during archaeological excavation. An alternative interpretation is that these squares represent the position of planting beds, a known part of Hamilton’s garden landscape at The Woodlands.

A student walking in a straight line and holding a gradiometer.
Survey with the magnetic gradiometer in the North Survey Area.

A student walking in a straight line and holding a gradiometer.
Survey with the magnetic gradiometer in the North Survey Area, north of the Stables with headstones in the background.

The map of magnetic intensity for the survey north of the location of the greenhouse and Stables also exhibits features that are interesting in terms of their spatial relationship to what we know about the 18th-century landscape. First, we can see the evidence for the magnetic field that surrounds the headstones along the northern border of the survey area. These headstones are fixed to their bases with iron rods, causing strong magnetic anomalies along the northern edge. There are also clusters of magnetic highs—one toward the middle and one in the southwest—that would be difficult to interpret as anything other than buried detritus, probably a mix of ferrous metal rubbish and fragments of brick and stone. At the southern edge, we see the influence of additional hardware in the picnic tables used by stewards and visitors to the cemetery.

An interesting element is a line that cuts from west to east across the survey area. This subtle line likely represents a relict boundary separating one activity area from another that is preserved in the soil. Perhaps it is the edge of a field plot, where plowing was different on one side from the other, or the edge of a cleared area that existed sometime in the past. A second, more tenuous boundary lies approximately 10 m to the south; further investigations are necessary to secure this interpretation. Like the square features in the southern survey area, these boundaries are oriented roughly parallel to the Hamilton Mansion facade.

A diagram of magnetometry interpretation situated amongst the buildings.
Interpretation of the magnetic gradiometry results with predicted position of the greenhouse structure.

But what does this tell us about the greenhouse itself? While not direct evidence for the placement
of the greenhouse, the east-west linear feature in the magnetic gradiometry data does match the synchronized orientation of these disparate features and reveals an overarching structure to the landscape, one that likely guided the placement of the greenhouse itself.

More investigations by the Center for Analysis of Archaeological Materials (CAAM) in collaboration
with The Woodlands staff are planned to verify the existence of a broader organization of Hamilton’s gardens and buildings and to find more evidence from the greenhouse. These investigations will involve student training as we apply complementary geophysical techniques to the landscape on and around the greenhouse. Initial results from an earth resistance survey, a geophysical method that is primarily sensitive to variation in soil moisture, has proved to be a promising source of new data for revealing the landscape that is William Hamilton’s legacy.

Jason T. Herrmann, Ph.D., is the Kowalski Family Teaching Specialist for Digital Archaeology in the Center for the Analysis of Archaeological Materials (CAAM) at the Penn Museum and a lecturer in Penn’s Department
of Anthropology. Kacie Alaga is an M.A. student in the AAMW program who studies ways in which imperial Roman conquest in the Eastern Mediterranean and North Africa are manifested in the built environment. Katie Breyer is a graduate student at Bryn Mawr College; she is interested in spatial and landscape archaeology, urban development, and mortuary practices in the Roman world. Special thanks go to the staff of The Woodlands, Jessica Baumert, Emma Max, and Robin Rick; Fall 2019 Introduction to Digital Archaeology students Shaashi Ahlawat and Taré Floyd; and to students in the Spring 2020 course Geophysical Prospection for Archaeology: Joseph Bacci, Chelsea Cohen, RJ Hakes, and Autumn Melby.


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Cite This Article

Herrmann, Jason, Alaga, Kacie and Breyer, Katie. "Reconstructing a Historic Landscape." Expedition Magazine 62, no. 3 (September, 2020): -. Accessed April 18, 2024.

This digitized article is presented here as a historical reference and may not reflect the current views of the Penn Museum.

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