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Prehistoric Site Distributions in West Central Alabama: Results of the 2011 Survey of the I-85 Extension and GIS Modeling

By Eric Poplin and Scott Butler

Alabama Archaeological Society

Scott Butler presented the results of a 2011 archaeological survey of the I-85 extension by Brockington at the 2013 Alabama Archaeological Society annual meeting on January 12th, 2012. This project included the use of a Geographic Information System (GIS) site predictive model, and an intensive survey of a preferred alignment for the proposed extension of I-85 through several west central Alabama counties. The results of the survey were then added to the previous GIS model to create a more in-depth model of site probability across the landscape. The work was done in support of the preparation of an Environmental Impact Statement by the Alabama Department of Transportation (ALDOT).

Transportation engineers have a myriad of factors to consider when planning new road projects, including the potential impact on local communities, wetlands, endangered species, and cultural resources, along with access issues and engineering concerns. Another concern is cost, and unsurprisingly, it is far more cost effective to avoid significant archaeological sites than to include them in a preferred alternative. For this reason, Brockington was involved with the early planning stages of the I-85 project. We created a predictive model in GIS that would help the project engineers select corridor alternatives that minimized potential to impact archaeological sites.

After the selection of several corridors, Brockington personnel examined 125 miles of the preferred extension of I-85, which crosses both the Alabama and Tombigbee Rivers. The survey resulted in the identification of 108 archaeological sites and 49 isolated finds. These included 76 distinct prehistoric components and 48 multi-component sites. It is important to note that 61 percent of the survey corridor was within the Alabama River watershed, compared to the remaining 39 percent lying within the Tombigbee watershed. Interestingly, a full 76 percent of all identified sites were located within the Alabama watershed, which suggests the relative importance of this waterway to past peoples.

Dr. Eric Poplin, a Senior Archaeologist in Brockington's Charleston Office, then examined behavioral factors in the locations of the prehistoric sites. Dr. Poplin modeled formulas to define behaviors associated with Early Archaic, Middle-Late Archaic, Early-Middle Woodland, and Late Woodland through Historic occupations, which correlate with the temporal periods of the sites found during the 2011 survey. The models of site potential generate a GIS surface that has a value for 30-meter interval grid points across the study area. The values range from 1 (highest) to 99 (lowest) and imply the probability that someone would select that point to live on, or work at, in the past.

These models emphasize sedentism and access to transportation routes as one moves forward in time (i.e. sedentism increases over time). Therefore, it is expected that earlier sites will have a higher value on the 1-99 scale than later sites. Dr. Poplin found that, along the surveyed I-85 preferred corridor, the site values ranged from 1-42. As predicted, the Early Archaic sites had higher values than the Middle-Late Archaic sites, and these were higher than the Woodland-Mississippian sites. By indexing the sites in this way, and including information about the components identified at different sites during the survey, Dr. Poplin was able to create clusters of values to explore site distribution and functions.

By refining the previous GIS model, Brockington created a more in-depth GIS predictive model of different types of sites. In the future, we will have a better understanding of the likelihood that a large multi-component and/or significant site will be found at a particular location. This information can then be utilized by transportation planners, and others, to try to avoid negative impacts to significant archaeological sites, and reduce project costs.

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