Petrographic and geochemical source-modeling of volcanic lithics from archaeological contexts: A case study from British Camp, San Juan Island, Washington

Abstract

Classification and sourcing of vulcanic lithics is simplified through petrographic and geochemical analyses. In examining volcanic lithics, major element geochemistry is required for classification, trace element geochemistry is necessary for discrimination of materials from different sources, and rare earth elements are used in source-modeling. Materials such as those found in the debitage at the British Camp shell midden, San Juan Island (45SJ24), are ubiquitous in the Gulf of Georgia region of the Pacific Northwest. Previous archaeological reports over the past 100 years have classified this volcanic debitage as basalt and predicted a local source. Petrographic analyses of thin sections and geochemical analyses using ICP emission spectrometry have shown that these artifacts are formed of dacite rather than basalt. The analysis also shows that the major lithic material used for stone chipping during the entire temporal sequence at the British Camp site is invariably from the same distant source in the High Cascades, possibly as far as 200 km from the site. © 1996 John Wiley & Sons, Inc.