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Gradiometer Inventory in Southwestern Wyoming:
Looking for the Archaic Using Modern Technology

David Vlcek
BLM Pinedale Resource Area
William Current
Current Archaeological Research.

A paper presented at the 55th Annual Plains Anthropological Conference, Symposium on Geophysical Prospection Methods in the Great Plains: New Advances and Applications, November 19-22, 1997, Boulder, Colorado


Ephemeral archaeological sites containing heaths, bone and lithic debris are frequently preserved in eolian deposits in southwestern Wyoming. If surficial cultural material is not apparent, earthmoving activity can impact unrecognized sites, creating severe damage to archaeological sites. To prevent such "Unexpected Discoveries", magnetometer survey using a gradiometer was performed on several well pads. All anomalies were investigated and subsequent earth moving activity was monitored. Project construction has generally confirmed "no find" magnetometer survey, but we lack positive anomalies identifying buried sites. Our work to date will be summarized, comparing data and output. Future directions will be identified.

  Beginning in the early 1980's, archaeological investigations involving remote sensing strategies were infrequently performed in the Green River Basin. In 1983, the Frontier Pipeline project saw a magnetometer inventory of 48UT390, the Austin Wash Site, a Late Prehistoric Period antelope processing site subject to mitigative excavations. While the bone bed that was excavated was not subject to magnetometer inventory, areas adjacent to the bone bed were. Anomalies were tested, with one positive hit involving a prehistoric feature interpreted as a fire hearth. Another linear anomaly was tested and identified as an old filled in drainage. While results of the magnetometer survey were viewed as not very productive at the time, the survey did locate a buried cultural feature and correctly identified a natural one.

Magnetometry in the basin did not find another field application until 1985, when the Taliaferro Site, 48LN1468, was surveyed as part of the massive Exxon LaBarge project. Here, the excavation block was subject to a proton magnetometer survey encompassing some 800 square meters prior to onset of block excavations. The magnetometer survey has never been analyzed or compared to the block excavation results.

In 1989, an early historic period site, Fort Bonneville, was investigated as the senior author's (Sublette County Historic Preservation Commission's) contribution to the Wyoming Centennial celebration. Built in 1832, Fort Bonneville promised to provide an historic period archaeological site to validate magnetometer inventory on sites containing metal artifacts and large constructed features such as the rampart wall or internal room/wall construction. This remote sensing, nondestructive, non-intrusive strategy was central to the overall project research design. Budgetary and landowner constraints restricted our effort to only half of the 1832 Ft. Bonneville site. Placement of the magnetometer block was also limited by the dense tall sage growing on-site.

Magnetometer results at Ft. Bonneville were disappointing. We benefitted from Dr. George Frison's unpublished test excavations and placed the magnetometer block to overlap the fort's southeastern (uninvestigated) exterior wall. The wall was not present in our excavation units. Magnetometer anomaly testing, conducted by the senior author, identified only rodent burrows, not wall features. Additional excavations in non magnetometer survey areas did locate the fort's western wall, post molds, construction trench and the blacksmithing area, but location of these features outside our mag block did nothing to further remote sensing applications in the region. In retrospect, limiting the extent of magnetometer inventory due to an assumed budgetary restriction proved ill-advised.

No remote sensing initiatives were undertaken in the early 1990's in the region. The National Park Service's Remote Sensing training sessions at Fort Laramie and elsewhere again spurred interest in remote sensing applications in energy impacted regions of southwestern Wyoming. A series of archaeological discoveries had recently taken place in SW Wyoming, and managers and archaeologists alike were looking for ways to predict where archaeological sites lacking surficial expression might be located.

The extensive dunal complexes overlooking the Green River in the LaBarge Platform were undergoing extensive petroleum development in areas of dense prehistoric hunter-gatherer campsites. Conflicts between unrecognized, buried prehistoric sites and bulldozers were common; one site (48SU595) had recently been the scene of a large discovery of Archaic-aged hearths and a human burial dated at 2550+60 BP. Despite traditional limited test excavations (which were negative), over forty archaeological features were located when the three acre well pad was constructed. The energy company involved (ENRON) was supportive of development of remote sensing strategies to prevent the reoccurrence of discoveries akin to that of the GRBU 165-35 well pad. With Enron's encouragement, Bill Current of CAR first enlisted the gradiometer and professional services of Dr. Lou Sommers, and we conducted gradiometer surveys of several potential sites in eolian deposits.

We began our work at 48SU595, where several gradiometer blocks were surveyed just north of the feature concentration. Preliminary shovel testing of anomalies located darkened soils thought to be prehistoric features (an encouraging development), but delays in the NAGPRA aspects of the project had slowed down our archaeological research. Magnetometer blocks were surveyed in several additional portions of this large, 400 acre dune mass proposed for continued energy development. Anomalies were tested, results proved negative, and subsequent construction was monitored by archaeologists. The construction monitors all proved negative behind areas "cleared" by magnetometer surveys.

Our magnetometer work moved north into the sand sheets, shadows and other and other eolian deposits of the Deer Hills, where Archaic-aged sites with poorly-expressed surface assemblages were common. Buried sites like 48SU1106 and 48SU1753 had been heavily impacted by energy development, and we felt it imperative to test whether similar buried sites could be located and protected using gradiometry.

48SU1106 is a "Benchmark Site" containing a thirty meter long stratified archaeological deposit of Middle Archaic and Early Archaic (Pine Springs and Opal Phases, Vlcek, 1997) components C-14 dated at 3,870, 4230, 6040 and 6,170 years ago. A magnetometer block was placed adjacent to the known archaeological materials exposed in the pipeline trench, however anomaly testing is lacking . Other well pads in the Deer Hills "cleared" by gradiometry were monitored, always with negative result. Lou Sommers has retained the bulk of our Deer Hills magnetic work output and we haven't integrated this effort into the overall remote sensing project.

In the winter, 1996-97, Bill Current was trained in gradiometer use and software manipulation by Lou Sommers and then purchased his own FM 36 Fluxgate Gradiometer from Geoscan. Armed with his own state-of-the-art gradiometer and dozens of energy developments threatening archaeological sites in dunal and other eolian deposits, we continued our own independent "mag work" in earnest.

An oil and gas boom was about to happen in an archaeologically poorly understood region called "Yellow Point", located on the northern edge of the Little Colorado Desert between the Big Sandy and Green Rivers in SW Wyoming. In the fall of 1996, we unexpectedly discovered a completely buried archaeological site along Sand Draw that ultimately proved to contain an Early Archaic Period (Opal Phase) housepit, and camp debris, 48SU2094. The housepit dates to 6600 radiocarbon years ago, while the activity area produced dates of 6600, 6080 and 5990 years ago. 48SU2094 represents the earliest known housepit in Wyoming.

Sediments containing the archaeological materials at 48SU2094 consist of a coarse-grained tan "sand" that we thought was eolian in origin, but which we now feel are Monte-Leckman and San Arcacio-Saguache soils. Such sediments represent ancient terraces or fans, perhaps deposited from overbank flooding of ancestral Sand Draw (Eckerle, per comm; ERO, 1988). Would the band of coarse-grained sand 1/2 mile wide along Sand Draw contain additional Archaic sites? Could these sites be located by magnetometer inventory? We had located two surface Folsom Points and numerous Late Paleoindian lanceolates during the 1997 field season, so our research orientation also included consideration of Paleoindian occupations. Where could Paleosols be preserved? Extensive energy development and supportive companies like Enron, McMurry Oil, Amoco Production, Ultra Petroleum and others provided us the projects and support we needed to find out.

San Argacio/Saguache soil deposits were known to contain buried archaeological materials, and we decided to apply the gradiometer survey to several areas slated for energy development within this soil zone. Too, the 1997 field season experienced a series of Unexpected Discoveries in the Sand Draw area, two of which resulted in the identification of a 6410 year old hearth cluster surrounded by what we think are post molds at 48SU2317 and a 6600 year old house pit at 48SU2324.

Our magnetometer surveys were conducted in standard 20 x 20 meter grids, walking half meter wide transects and taking eight samples per meter walked or 16 samples per square meter. After every two grids were scanned, the data was transferred to a laptop computer on-site. The data were processed with Geoscan Research software GEOPLOT ver. 2.02. Processing, utilizing standard filters such as zero mean traverse, clipping, compression, low passes, etc. is conducted to correctly identify the statistically significant anolamies (probability and confidence). After analysis in GEOPLOT, the data is then imported into contouring software such as SURFER. Between GEOPLOT and SURFER, a wide variety of map style can be output to hard copy.

In the Yellow Point area, magnetic survey, anomaly testing and followup construction monitoring again produced a series of negative finds. To date, we have conducted magnetometer surveys on some 13 well locations, and have not located a single prehistoric feature underneath areas "cleared" by gradiometry. Yet we do not have the opposite side of the coin: magnetic inventory, positive anomaly testing resulting in the location of buried archaeological deposits. Continued work at 48SU595 near LaBarge and especially the Meridian Site, 48SU1731 may provide us with some positive results.

At the Meridian Site, road construction impacted a 320 meter long series of Archaic features and occupation debris C-14 dated to between 3800 and 5600 years ago. In 1997, we placed 3200 square meters of magnetometer grids over the road cut in areas where features were NOT exposed, with the hope of using these areas to test for positive anomalies. The magnetometer results show numerous anomalies we think are buried features, both large and small, that may run the archaeological gamut from simple hearths to large housepits. Now that a new and more archaeologically sensitive energy company has purchased the Meridian gas lease (Ultra Petroleum), we are working with a supportive energy company and proactive work at the site in 1998 is greatly anticipated.

Conclusions: We have just begun. We need to use the gradiometer in consort with other remote sensing strategies such as resistivity or conductivity on sites where we know buried deposits exist. We need to test more, and to conduct more remote sensing inventories in different soil types. Software needs to be refined to allow for greater flexibility in mag grid placement. We need to listen to our expert geologists, geoarchaeologists and soils specialists to learn how to make the technology work for us. Funding is being generously supplied by supportive energy companies and there's plenty of work to be done. We need to be more driven by archaeological project areas and "pure research" and less by where the petroleum companies wish to construct their developments. We feel we have the tools; the future is bright.


ERO Resources Corporation
1988 Burma Road Soil Survey. Prepared for the United States Department of the Interior, Bureau of Land Management, Pinedale Resource Area and Rock Springs District by ERO Resources Corporation, Golden Colorado.

Gardner, A. Dudley, David E. Johnson and David Vlcek.
1991 Archaeological Investigations at Fort Bonneville. Cultural Resource Management Report No. 51, Archaeological Services of Western Wyoming College, Rock Springs.

Schroeld, Alan R
1985 Archaic and Late Prehistoric Adaptation in Southwestern Wyoming: The Frontier Pipeline Excavations. Bureau of Land Management, Cultural Resource Series No. 3, Cheyenne.

Smith, Craig and Steven D. Creasman
1988 The Taliaferro Site: 5000 Years of Prehistory in Southwest Wyoming. Bureau of Land Management, Cultural Resource Series No. 6. Cheyenne.

Vlcek, David
1992 "Archaeological Investigations at Fort Bonneville". Paper presented at the Rocky Mountain Fur Trade Symposium, Pinedale. Ms. on file.

1997 "McKean or Not McKean...Is That the Question? A Reevaluation of Fourth to Fifth Millennium Prehistoric Occupations in the Upper Green River Basin". Paper presented at the third biennial Rocky Mountain Anthropology Conference, Bozeman, MT. Ms. on file.

Vlcek, David and Gary Wilson
1995 "Fur Trade Era Sites in Southern and Western Wyoming" Paper Presented at the 53rd. Annual Plains Anthropological Conference, Laramie. Ms. on file.

Last updated: 01-13-2011