Explaining the monopoly in shell-bead production on the Channel Islands: drilling experiments with four lithic raw materials

Significant political and economic developments among the Chumash of southern California were catalyzed in part by the emergence of an intensive, specialist-driven shell-bead industry during the second millennium CE on the Santa Barbara Channel Islands. The production of millions of beads depended in turn on the availability of lithic microdrills of standardized form and materials. Channel Islanders quarried a particular stone type, a blocky Monterey Formation chert, from multiple outcrops situated close to the eastern shores of Santa Cruz Island. Rich archaeological assemblages document the lithic and shell byproducts of these intertwined production systems, each of which endured for several centuries (CE 1150-1819). Islanders invariably chose Island chert for making microdrills: hundreds of thousands of specimens recorded to date are of this material. Furthermore, nearly every microlith in all of Chumash territory (post CE 1150) was produced on the islands; the large populations on the mainland did not participate in microlith making or bead making after CE 1150-1200. We argue that this pattern had its roots not only in the patchiness of key resources and shifting regional social relationships, but also in the physical properties of available raw materials. Here we experimentally assess the properties of Santa Cruz Island chert alongside three important mainland raw materials-Grimes Canyon fused shale, Coso obsidian, and Vandenberg chert-that potentially could have been tapped to make microliths. We test the proposition that Island chert outperforms other lithic materials in drilling efficiency and drill use life. Our experimental results from 108 drilling trials reveal sharp distinctions in performance characteristics across the four materials. We infer that the process by which Islanders became the more-or-less exclusive manufacturers of shell-bead currency in southern California was facilitated by both the efficacy and physical properties of the Island cherts and the propitious locations of the outcrops.