Pyrometamorphic Marbles: An Excursion into Forensic Archeology

During a cultural heritage survey conducted by the National Forest Service, numerous (22+) small (2cm), fine-grained “clay marbles” were discovered at a prehistoric human habitation site in Blue Mountains of eastern Washington. The marbles were found adjacent to a cryptocrystalline projectile point associated with the Harder Phase (2500-250 yrs. BP) of human settlement in the Pacific Northwest. Their archeological significance is of some interest. Were the spheres geologic (i.e. sedimentary concretions), biogenic (digestive animal “bezoar” concretions), or were they anthropogenic? Initial visual observations led one of the authors (Williams) to conclude that they were likely natural sedimentary concretions. SEM/EDS revealed a composition consistent with the presence of quartz, feldspars, clays, and metal oxides. To determine the mineralogy, X-ray Powder Diffraction was performed in the UI X-ray Diffraction Laboratory. XRPD scans revealed that the spheres contain a significant non-diffracting component. The mineralogy of the spheres was interesting and unexpected: silica polymorphs (quartz and tridymite) and aluminosilicate polymorphs (sillimanite/mullite). One explanation for the assemblage is contact metamorphism of sediments by much older basaltic lava flows observed at the site location. Given their regular shape, and association with prehistoric artifacts, another possible explanation exists-anthropogenic pyrometamorphism. Mullite is a common component of firebricks, clay pottery, etc., which closely resemble mullite “buchites”. A preliminary conclusion is that the “marbles” may represent fired anthropogenic artifacts. The archeological significance of these items is unknown at present, however ceramic marbles have been used historically for various purposes (e.g. a type of bottle stopper). Also, small stone balls have been found at other prehistoric sites in the western US and interpreted as possible gaming devices. Forensically, the episode is instructive. Initial interpretations based on incomplete information may overlook critical data, and that one significant analytical observation, taken in context, can lead to very interesting and unexpected conclusions.