Although our blog has been silent since August 2011, we’ve been beavering away at many different projects related to Cooper’s Ferry and have much to share. Over the next few weeks, I’ll describe some highlights from the past few months’ work and will discuss the upcoming 2012 OSU Archaeology and Geoarchaeology Field School at Cooper’s Ferry. In this post, I’ll kick off our return to the blogosphere by discussing a recent publication that addresses a particularly wicked problem that plagues archaeological sites everywhere and how we’ve developed a solution for our own research.
In March of this year, our team published a paper entitled “A PXRF-based chemostratigraphy and provenience system for the Cooper’s Ferry site, Idaho” in the Journal of Archaeological Science (Volume 39, Issue 3, Pages 663-671). You can download a copy of our paper here: Davis et al 2012 JAS. In a nutshell, this paper presents the application of portable x-ray fluorescence (PXRF) technology in a new approach to characterizing the geologic layers in an archaeological site from their inherent elemental geochemistry. Knowing the difference between a site’s geologic layers allows you to correlate objects across horizontal space (i.e., when objects are contained in the same layer) and to tell when an object is in the wrong place as a result of having been moved vertically from one layer into another. This methodological approach, which is called chemostratigraphy, allows us to take geochemical measurements in the field to determine how artifacts and other materials are related to a site’s different sedimentary layers. Admittedly, this sounds pretty esoteric but being able to establish the spatial relationship between artifacts and disturbed or undisturbed sediments, or what is called associative context, is a critical part of our research program. To fully understand this topic, I’ll present a hypothetical scenario of archaeological site formation to illustrate how our quest for establishing associative context can represent a “wicked problem” at the Cooper’s Ferry site.
We begin our hypothetical scenario of site formation on a sand bar along the Salmon River. At one point, say 10,000 years ago, foraging peoples set up camp on this sand bar and in the process of practicing their daily activities, leave behind artifacts, food remains, and activity features such as fire pits. In this process, our hypothetical foragers create a site. Eventually, the foragers depart camp and the stone tools, bits of bone and shell, and charcoal-filled fit pits they leave behind on the sand bar get buried beneath silt deposited by a flooding Salmon River. At this point, our hypothetical site consists of two layers—a lower sand layer containing 10,000-year-old archaeological materials, which is overlain by an archaeologically sterile layer of silt. Foragers return to the site after a 1000-year absence and leave behind more artifacts, faunal materials and make a new fire pit feature, all of which are incorporated into the surface of the silt layer. This particular arrangement of sediments and the different traces of cultural occupation that they contain will stay preserved in their vertically layered stratigraphic sequence until site formation processes work to alter them. Site formation processes are those natural and cultural actions that work to alter the original state of an archaeological sequence by moving, removing or destroying items and sedimentary layers through time. At our hypothetical site, burrowing rodents enter our site just after 9,000 years ago and dig tunnels through its deposits. In the process of constructing their burrow networks, the rodents displace sediments and the objects they contain and push them vertically and horizontally from their original locations. Through this process, the burrowing rodents change parts of the site’s layered sequence, moving older deposits and objects into the younger layer and vice versa. If we were to excavate our hypothetical archaeological site, knowing how to identify and separate the effects of the burrowing rodents would be critical to our efforts to separate the 9,000 and the 10,000 year old cultural occupations from one another.
It is imperative that we are able to identify when an object has been moved from one layer to another as a result of any site disturbance processes. Being able to accurately determine whether an object is in association with disturbed or undisturbed sediments is a “wicked problem” that must be solved if we are to be able to accurately evaluate the archaeological record at Cooper’s Ferry. Lacking an ability to scientifically evaluate the associative context between an object and the sediments it was found with introduces an unacceptable level of uncertainty into archaeological research. When I say scientifically evaluate, I am speaking about the use of a method that generates empirical observations about the relationship between an artifact and its adjacent sedimentary matrix that can be subjected to statistical tests of correlation, discrimination, and significance. Such a scientific method of evaluation is different from that typically practiced in field archaeology, where archaeologists typically use their eyes and sense of touch to make observations about whether an artifact is “in place” or “disturbed.” In most cases, field observations are probably good enough to tell whether an artifact is in its original position or if it is within the confines of a rodent burrow or some other kind of post-depositional alteration. At the majority of sites, correctly guessing the associative context of an artifact in most cases is good enough; however, at an extraordinary archaeological site like Cooper’s Ferry, we’ll need to do better to satisfy the critics (and ourselves, for that matter).
As in the earlier hypothetical scenario, our wicked problem is that rodents have burrowed through many parts of the Cooper’s Ferry site and if we cannot accurately and consistently discriminate between mixed sediments and unmixed sediments, we’ll never be able to understand its archaeological record. Because the burrowing actions of rodents leave behind sedimentary signatures (e.g., infilled rodent burrows containing sediments with texture, color and geochemical attributes that contrast significantly with surrounding undisturbed sediments), they can be identified and addressed separately from undisturbed sediments. Some kinds of sedimentary disturbance are subtler and can elude detection by the naked eye. To avoid this problem, we’ll use our Olympus Innov-X Delta PXRF unit to measure the elemental geochemistry of sediments directly associated with all formed artifacts (e.g., bifaces, projectile points, cores), faunal materials and key samples (e.g., charcoal for radiocarbon dating). We’ll subject the geochemical data collected by the PXRF to a set of statistical tests that will evaluate how well the elemental composition of the measured sediments compare to our previously established chemostratigraphic framework. This approach will allow us to generate scientific assessments about the quality of associative contextual relationships between objects and sediments as we find them during the course of excavation. Furthermore, the use of our new PXRF method provides the first quantitative assessments of associative context—an entirely new class of archaeological evidence that takes our investigation of Cooper’s Ferry to a higher level.