Archaeo-Physics, LLC - geophysical survey and subsurface mapping

Fort Juelson

A Non-Invasive Geo-Spatial Investigation

Archaeo-Physics and the Otter Tail County Historical Society (OTCHS) are collaborating to conduct a non-invasive archaeological assessment of Fort Juelson. Light detection and ranging (LiDAR) and subsurface imaging methods will be used to assess the archaeological resources present on the hilltop. Data gained during the non-invasive investigation will be used to develop an archaeological research design and guide limited test excavations during a public archaeology program at the site during the summer of 2012. The project will ultimately lead to a National Register of Historic Places nomination for the site, a permanent exhibit at the OTCHS, and an online virtual archaeology exhibit.

Fort Juelson is a Black Hills war era defensive earthwork located in Tordenskjold Township, Otter Tail County, Minnesota. The fort was constructed in July of 1876 immediately after the Battle of Little Bighorn. The 7th Calvary’s defeat caused panic amongst Minnesota’s Euro-American settlers, and false rumors of hostilities in the area caused numerous settlers to leave the region and seek refuge elsewhere. A local storekeeper suggested the settlers band together and build a defensive fortification under leadership of the Civil War veterans, Hans Juelson and Berge O. Lee. A July 21, 1876 article in the the Fergus Falls Journal describes the fort as follows:

...this fort which was 120x100 feet, with a cross section wall through the center, ... and a curtain or shield in front of the northwestern entrance. The wall of this fort [were] from 4 to 4 1/2 feet high, squarely and compactly built, from the sods as cut from a 16-inch breaking plow. The hill-top, occupied by the work had less than half an acre of comparatively level ground, all of which was plowed up, and the sod used in the construction. The entire work is enclosed by a fence, to keep the cattle away. As seen from a distance, this is a more formidable earthwork, than we saw in the six month's service in the army of the Potomac, with the single exception of Yorktown, and in it 50 or 100 men under command of Captain Juelson, with arms and ammunition, provisions and water, could keep the entire Sioux nation at bay...

We suspect the hilltop on which the fort was built contains earlier Native American features and possible burial mounds as well. Thus far, our only historical evidence of this comes from Fergus Falls School District 104's The Educational Helper, Volume 41, No. 6 (1949).

"Along the chain of hills from south Turtle Lake to what we called Harm's Lake, probably German Lake, was an Indian trail. In the early days we could see the Indians with their conoes (sic) on their heads walking this trail. A space adjoining the fort on this trail had evidently been a burial ground for quite a few skeletons were uncovered when plowing the sod. These were all carefully reburied."

The non-invasive site assessment proceeded in five stages, beginning with a LiDAR data analysis and ending with a very high-resolution GPR survey of one of the suspected mounds. Although analysis is still in its early stages, the results of this investigation provide strong evidence supporting the presence of burial mounds on the hill top.

Phase I: LiDAR Data Analysis

A LiDAR analysis of the hilltop was conducted during the first phase of the non-invasive investigation. Ground penetrating radar (GPR), electrical resistance, and magnetic field gradient data will be collected over the hilltop during subsequent phases.

Figure 1: A LiDAR derived landscape overview.


Figure 2: A detailed exploration of LiDAR imagery from the hilltop (note: a FLASH PLAYER is required).

LiDAR point cloud data were processed using LAStools by Martin Isenburg.

Phase II: Ground Penetrating Radar (GPR) Survey

GPR data were collected over a 95 x 50 meter area in May of 2012. Data were collected every 5 cm in linear transects spaced at 50 cm, resulting in a final data sample density of 40 GPR traces per square meter. The GPR data identified numerous possible features of interest including: foot paths that may be modern, historic, or prehistoric in origin; subsurface architectural detail concerning the construction of the fort; and evidence of possible burial mounds and associated features on the hilltop.

A sequence of GPR images may be viewed by clicking the play button below (note: a FLASH PLAYER is required to view these data). The approximate locations of possible burial mounds may be viewed by clicking the appropriate button in the interactive graphic.

Figure 3: GPR data presented as an interactive graphic featuring a sequence of average amplitude plan view depth slice images. Click any of the overlay buttons to compare the various data images.

Phase III: Electrical Resistance Survey

Electrical resistance survey results are presented below. The image measures 95 meters east to west and 50 meters north to south. Mouse over the image to examine the resistance data after the application of a high-pass background subtraction filter.

Figure 4: Electrical resistance data collection at Fort Juelson. A reading was recorded every 50 cm across the site, resulting in an overall data sample density of 4 samples per square meter.

buck wise data collection

Figure 5: Electrical resistance survey results from Fort Juelson. "Mouse-over" the image to apply a high-pass background subtraction filter.

Fort Juelson electrical resistance data


Phase IV: Magnetic Field Gradient Survey

Results of the magnetic survey over a 95 x 50 meter area are presented below at a contrast level of ± 50 nT. Mouse over the image to examine a high-contrast version (± 5 nT). The very strong circular magnetic field dominating the central portion of the image was caused by a flag pole.

Figure 6: Magnetic field gradient survey results from Fort Juelson.

Phase V: High-Resolution GPR Survey over the Easternmost Mound

GPR data were collected over a 12 x 10 meter area centered on the easternmost suspected mound. A very high-resolution survey design was utilized, with a final data sample density of 160 GPR traces per square meter. The results of this survey clearly show what appears to be a central burial chamber lying just beneath the original ground surface near the center of the mound.

Figure 7: High-resolution GPR survey results from over the easternmost burial mound. The central burial chamber is shown in dark blue.

high-resolution GPR survey image from over the easternmost burial mound


On November 4, 2008 Minnesotans approved through referendum the Clean Water, Land, and Legacy Amendment to add Article XI to the Minnesota State Constitution. The amendment raised the state’s sales and use taxes 0.375% to fund programs meant to “restore, protect and enhance” wilderness lands, wildlife, water quality, state parks and trails, arts, arts education, arts access, and to preserve Minnesota’s history and cultural heritage. This research project is funded by a Historical and Cultural Grant from the Minnesota Legacy Amendment. more>>

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