A PETROGRAPHIC ANALYSIS OF EXTENDED MIDDLE MISSOURI CERAMICS FROM NORTH DAKOTA

This poster presents the results of a petrographic analysis of Extended Middle Missouri (ca. 1200 to 1450 A.D.) Riggs and Fort Yates ceramics. 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 suggest till as the most likely source for the temper. The majority of the clay matrix in each of the samples is highly organic as evidenced by its dark brown to black coloration, the opaqueness of which produces an undifferentiated birefringence fabric in cross-polarized light (XPL). The mineral composition of the aplastics and the micromass is consistent with raw material resources readily available in central North Dakota. The overall abundance and coarseness of the tempering agent is common to Extended Middle Missouri wares and is likely added to overcome structural weaknesses inherent in smectitic clays which are prevalent throughout the region. The size, amount, and composition of the temper grains is also common to vessels manufactured for utilitarian (culinary) purposes.