Sunday, August 4, 2013

Expanding the Boundaries of Fort St. Joseph

During the 2012 field season, Dr. William Sauck of Western Michigan University’s Department of Geosciences graciously visited the site to expand the geophysical survey in the floodplain at Fort St. Joseph. He employed a magnetic gradiometer which has the potential to detect items underground that have magnetic readings, such as iron, pottery, and even burned stones! The data were then analyzed and organized using computer programs, and allowed us to isolate areas of interest. There are some magnetic anomalies adjacent to the river to the west of the dewatering system. Needless to say, we were very interested in the results and eager to ground truth the survey data.
However, the area that we were interested in exploring is very prone to flooding with the groundwater only 20 cm below the surface. This posed a serious logistical challenge: how can we recover sediments from the occupation level if we cannot dig through the wet, soupy mess? Well, challenge accepted.
Field school veterans put the PVC corer to use.
Neil, a local volunteer, and I devised a system to extract soil through the watery mess. By using a 3 inch PVC pipe, a valve, and a little ingenuity, we were able to create a vacuum in the PVC pipe that held the soil in the pipe, creating a compact core that could be easily placed in a bucket to be screened by opening the valve and releasing the vacuum pressure. After a successful test run of the PVC corer, we were confident that we could dig shovel test pits in the area of interest along the river.
On Thursday (8/1), a small group of field school alumni and community volunteers came together at the site to dig a series of shovel tests pits (STPs) along a transect parallel to the edge of the river. The transect was 140 meters long with STPs at 10 m intervals.. The STPs were 50 x 50 cm, about two shovel widths wide on each side. The field team dug through the alluvium, a layer of organic material built up through flooding and natural processes, with a shovel until a soil change was visible. Once the plow zone was identified, the corer was utilized to recover soil to a depth of 1 meter. A total of 14 STPs were dug along this transect.
The results were interesting, exciting, and confusing. STPs 1 through 4 yielded a few 18th century artifacts, whereas STPs 5,6,7,8, 10, 11, 13, and 14 were relatively sterile, with small amounts of calcined bone and seeds. STP 9 contained nearly 50 burnt seeds, while STP 12 contained lead shot similar to other specimens found elsewhere on the site near 18th century buildings.

So, what does this mean? Well, a lot! The results of the magnetic gradiometer survey and the artifacts recovered from STPs 1 through 4 suggest that Fort St. Joseph extends 40 meters further to the west, nearly doubling the size of Fort St. Joseph. However, we must not jump to conclusion: further excavations must take place to truly know the western boundary of the site. But the evidence suggests that the Fort St. Joseph site extends outside of our dewatering system! Who knows what might lie to the South under the dump? Stay tuned as the Project continues to explore various aspects of colonial life in the St. Joseph river valley–Alexander Brand


Anonymous said...

Tell it like it is!

ME said...

Forgot to mention the poison ivy

Toby said...

This is awesome!

enochdavis said...

In my view the magnetic gradiometers have so many advantages Horizontal magnetic gradient (HGRAD) and total magnetic field can be recorded at the same time.
HGRAD will do a better job of mapping near surface magnetic bodies than total field.
HGRAD provides more geometric information than the total magnetic field.
HGRAD is more sensitive to smaller bodies and objects to one side of the flight line, making it possible to define the body better when only two flight lines cross the object.
HGRAD can be used to improve gridding.