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Site name: Fort Clark Trading Post
Type of survey: Magnetic gradiometry; Instrument: Geoscan Research FM-36 fluxgate gradiometer; Prospection depth: up to 1.5 m; Sampling interval: 0.5 x 0.25 m; Area surveyed: 12,600 m2; Dates of survey: 6/00 & 7/01.
Comments:
The Fort Clark Trading Post was named after William Clark of the Lewis and Clark
expedition. It is located in central North Dakota and was one of several important
trading centers on the upper Missouri River. Founded by the American Fur Company
and supplied by steamboats from St. Louis, it was sited about 200 m south of
the the Mandan village of Mit-tutta-hang-kush
in 1830 or 1831. Many first-hand written accounts exist about this site. Its
visitors included Prince Maximilian of Wied, and the artists Catlin and Bodmer
who left depictions of this trading post in paintings (first figure, by Bodmer).
The magnetic survey results (second figure) include three principal zones of
interest. First, the outlines of the centrally placed trading post are apparent,
with a number of walls and rooms clearly visible. Fort Clark was burned in 1860
and, although its remains were used as a fuel source for steamboat traffic,
the firing may have left some of the large magnetic signature seen. A small
amount of rubble on the site indicates, however, that foundation stones were
composed of a magnetic sandstone and numerous iron artifacts undoubtedly litter
the area, contributing to the signal. Although the Missouri River presently
flows about a mile from the site, during the 19th century it flowed adjacent
to the bluff edge along the bottom of the figure, as seen in Bodmer's depiction.
Easy access to the steamboat landing may have been achieved using the draw to
the lower left. A comparison of the magnetic findings against an 1860 eye-witness
sketch of the trading post is given in the third figure.
The second zone of interest is the Euro-american cemetery, located on the high
bluff edge immediately across the draw (south). A number of graves are
suggested in the data (detail in figure), some of which are visible as
depressions in the surface.
The third component of the magnetic data (second figure, above) is known as Garreau's lodge and enclosure. Garreau was a full-blooded Arikara who took the name of his step-father, an employee of the trading post. The locus of this circular, earth-covered lodge is readily seen, as is its entryway to the east, central hearth, a number of probable sub-floor cache pits (see Huff Village), and a plethora of iron artifacts. Also visible is a faint suggestion of the surrounding enclosure. A depiction of circular, earth covered Mandan lodges by Catlin can be seen at Mit-tutta-hang-kush Village.
Type of survey: Magnetic Susceptibility; Instrument: Geoonics, Ltd. EM-38B; Prospection depth: about 0.5 m; Sampling interval: 0.5 x 0.5 m; Area surveyed: 3,400 m2; Dates of survey: 6/00.
Comments: These data were acquired, and gratiously made available by Jami
Lockhart of the Arkansas Archeological Survey, using the in-phase component of
data logged with the EM-38B. Unlike the magnetic gradiometry survey, which
utilized a passive remote sensing technique, the MS survey employed an active
method that injected a signal into the earth and measured subsurface responses
to the signal. While the MS data parallel the magnetic findings many new details
and subtle features are also revealed, including better definition of a number of
likely rooms, room blocks, and other features.
Type of survey: Resistivity; Instrument: Geoscan Research RM-15 & MPX-15; Prospection depth: 0.5 & 1.5 m; Sampling interval: 0.5 x 0.5 m; Area surveyed: 5,600 m2; Dates of survey: 6/00 & 7/01.
Comments: The resistance data (left) perhaps more clearly reveal some
of the individual walls of the trading post and multiple rooms are suggested.
The data may be compared with extant historical maps, including a sketch prepared
by Prince Maximilian in 1833-4 (right). Confusing the picture, however, are
a number of linear trenches excavated throughout the site by archaeologists
in the 1970s, that particularly show up in the 50 cm depth data. The 1.5 m depth
data indicate multiple lines of deep palisade trenches indicating changes in
the trading post's size through time.
Type of survey: Electromagnetic Conductivity; Instrument: Geonics, Ltd. EM-38B; Prospection depth: 1.5 m; Sampling interval: 0.5 x 0.5 m; Area surveyed: 3,400 m2; Dates of survey: 6/00.
Comments: Electromagnetic (EM) conductivity data were acquired by Jami
Lockhart of the Arkansas Archeological Survey using the out-of-phase (or quadrature)
component of the data logged with the EM-38B. These data were obtained simultaneously
with the magnetic susceptibility data. While soil resistivity and conductivity are
the theoretical inverse of each other, the results of these surveys include important
differences stemming from the very different instrumentation employed to obtain each
data set and variations in the soil volumes evaluated.
Type of survey: Ground penetrating radar; Instrument: Geophysical Survey Systems, Inc., SIR-2000; Prospection depth: about 1.5 m; Sampling interval: 0.5 x 0.02 m; Area surveyed: 30 x 20 m; Dates of survey: 7/00.
Comments: A small section of Fort Clark was surveyed by GPR in 40
transects, each 30 m long focused on a portion of the fort's perimeter
room blocks. Interpolation methods were then employed to generate a
time slice between the profiles at 10-15 nS (TWTT), yielding this image.
Clearly visible are the principal alignments of some of the trading
post's walls, but only a subset are revealed in comparison with the resistance
survey findings.
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(updated: 11/01)