Fort Raleigh Construction Project: Geometric Analysis & Virtual Imaging
Archaeologists have found some clues to the location of Sir Walter Raleigh's "Virginia" colonies, but no evidence for the settlement of the remains of the colonists has yet been found. The most important piece of verifiable evidence is the small Elizabethan earthwork fort, locally called "Old Fort Raleigh," which became the Fort Raleigh National Historic Site. The earthwork was excavated and reconstructed over 50 years ago by the U.S. Park Service.
New research by First Colony Foundation is underway, and this fort and its reconstruction are being re-evaluated by FCF. I asked a Mercer University mathematics major, Daniel Brown, to calculate the amount of dirt taken out of the fort ditch and the amount that would have been used to make the fort's earthen walls. His computations and geometric drawings serve as the basis for this project: a computerized presentation of his analysis and 3D imaging of the fort construction. A future project will use this imaging as the basis for alternative virtual reconstructions of the fort.
To determine why there was a small earthen fort with no nearby settlement, we need to understand its function. What was its purpose? To mount cannon to control the waters off the north end of Roanoke island? To protect Thomas Harriot's 1585-6 industrial and scientific activities taking place west of the fort? To protect a now-lost landing spot east of the fort? To form part of a chain of small "sconces" linking the landing and/or Harriot's worksite with Ralf Lane's main fort and settlement? To house and defend the eighteen men who made up the military "holding party" that came, fought, and fled in 1586? Or was it to serve the new, civilian settlers of 1587, who later became the Lost Colony? To find an answer, we must carefully analyze all the evidence for "Old Fort Raleigh" and its construction.
The first step is to compare how the archaeological features of the fortification were interpreted on paper and how they were reconstructed on the ground. Daniel Brown used his calculations to help me, an archaeologist with limited mathematics skills. Daniel and I worked out three ways of addressing the problem: assessment, methods, and procedures.
Assessment meant what questions did the archaeologist want answered. How was it built? How was it restored? Are other reconstructions plausible? To answer these questions, we noted that there were some problems: how to treat the west opening, the odd-shaped south bastion (projecting defense), and possible interior structure(s). We also wondered whether 16th-century cannons would fit the other, more normal bastions.
Methods meant mathematical measurements to compare the amount of soil dug from the fort ditch to the amount that was used in the fort wall (bank). We agreed that we would base the analysis on the measured drawings from the official archaeological report. There, the fortification had been carefully measured on the excavation maps (plans). But the report noted that there were some irregularities in the depth of the ditch, so we decided to make some basic assumptions. We would use one standard depth of the excavated ditch and one standard height of the reconstructed wall. Given other variances in length and breadth of the angled ditch and wall base, we would need to use a maximum-minimum approach. It meant more work for Daniel, but it produced a more accurate result.
Procedures that Dan used were breaking down the multi-angled fort ditch into a few standard geometric units (segments), employing the same formulas to determine volumes for each kind of segment, and then adding up the results for all the types of segments to reach a total volume for the ditch. The same steps were followed for the wall, and then the two numbers could be compared, to check the accuracy of the reconstruction.
Before we begin the analysis, we should note that the First Colony Foundation and the National Park Service use the internationally standard metric system for all measurements. But here you will find only feet and their decimal parts. Why? Because this was used in the original excavation and in the restoration of the earthwork. Maintaining the original system makes the mathematics easier by avoiding the tedious conversion of all the measurements.
A second point we need to mention is that the archeologists found evidence of what is called a parapet step, a strip just inside the wall where the soil was raised about a foot high to let a soldier step up to fire his musket, then step back down to keep his head hidden by the top of the wall. The archaeologists deduced that only the topsoil of the fort's interior had been removed to build the parapet step. This means that it was taken from an area untouched by the wall construction. As such, it does not affect our analysis.
Students! If we present you with the raw data and instructions on how to work out the formulas, will your answers match ours? Let's see...
Eric Klingelhofer, FCF