A Petrographic Analysis of Extended Middle Missouri Ceramics from North Dakota

Article excerpt

This article summarizes a petrographic analysis often thin sections prepared from Riggs and Fort Yates ware sherds (ca. A.D. 1200 to 1450), the first such study ever conducted on prehistoric ceramics collected in North Dakota. The procedure identified, described, and estimated the percentage of observable aplastics (coarser grained inclusions) and examined the geometric relationships between the aplastics and the encompassing clay matrix (micromass). Grit tempering was used exclusively in the manufacture of each vessel represented. The large and abundant polymineralic grains (rock fragments) are granodioritic in composition and evince local tills as the most likely source for the temper. The mineral composition of the aplastics and the matrix is consistent with raw material resources readily available in central North Dakota. The overall abundance and coarseness of the tempering agent is likely added to mitigate the high shrink-swell capacity inherent in the montmorillonite clays that are prevalent throughout the region. The size, amount, and composition of the temper grains are also common to vessels manufactured for utilitarian (culinary) purposes. It was not possible in this study to distinguish Riggs from Fort Yates ware at the microscopic level.

Keywords: Ceramic petrography, geoarchaeology, archaeological geology, Middle Missouri ceramics, North Dakota archaeology

Petrographic microscopy is commonly employed by geologists to identify the mineral components in a rock and in the subsequent identification of the rock itself. A translucent slice of rock or mineral (i.e., a thin section) is mounted on a glass microscope slide and viewed in polarized light where distinguishing optical properties can be observed (Kerr 1977; Phillips 1971). These same principles and techniques can be effectively used to investigate clay-fired ceramics, essentially an anthropogenic sedimentary rock (Danson and Wallace 1956; Ferring and Pertulla 1987; Garrett 1986; Josephs 2002; O'Malley 1981 ; Shorten and Hendry 1979; Stoltman 1989, 1991, 1996, 2001; Williams 1983). Ceramic petrography is a powerful, yet tremendously underutilized, technique. Ceramic petrography can answer questions concerning raw material provenance, classification, function, manufacturing methods, and exchange of prehistoric pottery (Josephs 2002; Stoltman 2001). It is complimentary to, not in competition with or supplanted by, newer and more expensive "high tech" procedures such as neutron activation analysis, electron microprobe analysis, and acid extraction techniques, and its cost per sample is much lower (Josephs 2002; Stoltman 2001). With respect to these newer geochemical approaches used to determine the elemental composition of the ceramics, you cannot use chemical data by itself to determine the provenance of an artifact without, at least, knowing its mineral composition (Stolman 2001). Mineral identification is the primary goal of any petrographie investigation.

Kay et al. (2000:323) comment that "Middle Missouri subarea ceramic characterization is in its infancy," and that even though few studies have been undertaken to resolve this problem, those that have been accomplished "are sufficient to demonstrate the heuristic potential of chemical and mineralogical characterization techniques for the Middle Missouri subarea." This study examined ten thin sections prepared from Riggs (n = 8) and Fort Yates (n = 2) ware sherds (Table 1) for the purpose of investigating geologic (mineral composition and raw material provenance) and anthropogenic (classification, function, and manufacturing methods) properties evinced microscopically. This is the first study of its kind conducted on prehistoric ceramics from North Dakota and the first to incorporate micromorphological terminology to enhance the overall descriptions of the thin sections (Josephs 2005). The goal is to demonstrate the amount and quality of information that can be gleaned through petrographie analysis of clay-fired prehistoric ceramics, even from a relatively small and unprovenienced sample such as this. …