ASK-A-GEOLOGIST by Jeff Wynn

Geoarchaeology, Part 3: finding graves. Also for KIDS.

One final example:

Stephen Potter, the National Park Service archaeologist, also asked me to investigate other NPS properties at Fort Washington and Marshall Hall Plantation in Maryland. There was a small, fenced cemetery on the latter property, complete with elaborate gravestones. However, investigators had gathered records suggesting a vastly larger number of burials there, including slaves and a group of small children killed by a Scarlet Fever epidemic in the 19th Century.

I thought I would Take My Kids to the Office:

Two of my kids, Valerie Wynn and Jared Wynn, with the Self-Potential (“SP”) measuring system we used at Marshall Hall Plantation, Maryland. Note the tiny formal cemetery behind the picket fence, and the Potomac River in the background.

Remnants of the Marshall Hall plantation in Maryland.

This time I again used a magnetometer — but added something different: Self-Potential. This just means measuring the voltage at different points on the ground-surface with respect to a base-station located a few meters off-site. Think: a fancy voltmeter with two fancy plug-into-the-ground electrodes, one moved along the ground in profiles.

Self-Potential (“SP”) has been used effectively by my friend Charlie Zablocki to map hidden vents in fresh volcanic flows on the Big Island of Hawai’i. Self-Potential is surprisingly complicated to actually do despite the simple equipment, however, because you must use special non-polarizing, silver-silver-chloride electrodes, and you must measure the voltage at the same elapsed time for each point after the electrode has been inserted into the ground. The method is so sensitive that clouds passing over the site can change the voltage values. To make data meaningful to non-geophysicists, I also passed a three-point filter through the data.

A typical example of combined geophysical profiles from nearby Fort Washington is here:

Self-Potential, reference topography, EM-31 conductivity, and magnetic field profiles for one line at Fort Washington Park. The fort was built to defend the river approach to Washington, DC, over 200 years ago. It is reassuring to a geophysicist to see that several different approaches all agree. Image from Wynn, 1990.

As mentioned earlier, I enlisted two of my children to help — they were excited to “do geophysics” with their Dad, especially as it involved a lunch including milkshakes at a fast-food place afterwards. The interpreted results are below. Note that there are several strong negative magnetic anomalies in the Marshall Hall Plantation cemetery. These turned out to be from cast-iron coffins that were popular among wealthier families (they cost ~50 times more than a pine coffin) from 1850 to 1888. The lost-wax casting process meant that the lids were cast upside down, thus there was a strong negative remnant magnetic anomaly above it when placed right-side up in the grave. This information allowed Stephen to state with some precision when these burials took place.

Interpreted results: Magnetic and Self-Potential profiling at Marshall Hall Plantation cemetery, Maryland. Note the several strong magnetic LOWS, especially in the visible-above-ground cemetery inside the picket fence. These are caused by cast-iron coffins popular among wealthy families in the mid-late 19th Century. Note also the evidence for MANY additional graves not seen without the geophysical survey.

SP involves using several non-polarizing silver-silver-chloride electrodes, a spool of wire, and a digital voltmeter — something anyone can afford to do in their own backyards.

Articles I published about how to do this can be found here and here.

Each of my children took a non-geophysics career path, however; both Val and Jared (shown above) climbed Mount Roraima in Venezuela with me, and both ended up with MBA degrees in Canada and the United States. I’m very proud of them.