Archaeologists are often pretty lousy geologists. Nowhere is this more apparent in the use of visual cues to classify rock types. Accurate classification of rock types is an enormous clue to the mobility of past people, to ancient territories, as well as trade and exchange networks. Even so, very few archaeological studies on the Northwest Coast have looked at geochemical characterization of raw materials, other than for obsidian at least. The best work done to date is probably Nicole Smith’s M.A. thesis on the Richardson Island site.
It is therefore nice to see some other comparable work being done. I don’t know much about the Punchaw Lake (near Prince George) project beyond this poster which has been put online in the form of a PDF conference poster by Lorenz Bruechert (abstract). I understand Bruechert did his MA at UNBC on this geochemical study and also there is a much earlier 1978 MA thesis by Patricia Montgomery available online. It seems Bruechert used the 1970s excavations as a study sample, and the ICP-MS and other work was contracted out to ACME (not that one), a geochemistry consulting company (note: Nicole did the geochem work herself at UVIC so bonus points there).
In any case, Bruechert finds that a sample of the debitage is geochemically sortable into six groups, all of which are closely-related trachydacite or dacite. There is as much internal differentiation at Punchaw Lake in the geochemistry of this rock type as there is between any two typical sites on the Plateau. Hence, he suggests three distinct sources with “tens” of kilometres between them. As the Punchaw Lake site finds itself on the Alexander Mackenzie trail, this is not that surprising to see diversity of raw material (though one could argue there should be even more diversity). Nonetheless, the point remains that, as the picture below shows, all the material tested by Bruechert would have been classified visually as basalt (lower left on the graph); whereas in fact, none of it is basalt. It was such findings at Richardson Island which helped allowed Nicole to postulate an “experimental” phase of technological innovation – in her case, a variety of materials including rhyolites, argillites and varvites had all been variously mixed and matched in ways that did not reflect their geochemical origins.