SHELL BEAD EXCHANGE
Shell beads and ornaments are relatively uncommon at most of the prehistoric archaeological sites in San Diego County, at least in comparison with sites in the Santa Barbara Channel area and coastal central California. Nonetheless, these artifacts do occur, and they have a potential to shed light on interregional exchange systems, as well as on chronological questions.
Shell bead typologies and chronologies that have been applied to San Diego sites have been based primarily on the work of Chester D. King (1990) and of James A. Bennyhoff and Richard E. Hughes (1987). Several studies by Hughes and his associates have discussed shell bead exchange networks within adjacent parts of California (e.g., Bennyhoff and Hughes 1987; Hughes and Bennyhoff 1986; Hughes and Milliken 2007). Lynn H. Gamble and Chester D. King (2011) examined shell bead and ornament evidence for interregional exchange in more than 23 San Diego County assemblages.
A detailed review and analysis of the archaeological occurrences of shell beads in San Diego area sites has been provided by Meg McDonald and James D. Eighmey (2008). Studies of substantial assemblages from individual sites include analyses of Southwest College's excavations at SDI-12,809 in the Otay River valley, reporting 245 beads (McDonald and Eighmey 2007; McDonald et al. 1993); SDI-2537 (Indian Hill Rockshelter) in southeastern San Diego County, with 131 shell beads and ornaments (McDonald 1986, 1992); and SDI-39 in La Jolla by Sarah Farmer and Douglas Joseph La Rose (2009), with a total of 269 beads. The continuation of shell bead use into the early historic period, possibly with radically changed patterns of exchange, has been examined by Carmen Zepeda (1998, 2004).
Olivella spp. spire-removed beads account for the great majority of the archaeological specimens. Other shellfish taxa that are represented include Acmaea sp., Cerithidea californica, Columbella major, Conus californicus, Dentalium neohexagonum, Donax gouldii, Glycymeris sp., Haliotis rufescens, Laevicardium sp., Megathura crenulata, Mytilus californianus, Oliva undatella, Septifer bifurcatus, and Tivela stultorum.
Identifying the source regions for the marine shells that were used to manufacture beads and ornaments has been the most widely applied method for reconstructing prehistoric bead exchange systems. Beads found at archaeological sites in the Great Basin and the Southwest have been attributable to source areas on the southern California coast and the Gulf of California. The San Diego region seems to have been peripheral to both of these major source areas. However, this area's involvement in interregional exchange or, less likely, in long-distance direct procurement, is attested by beads made from shells of Olivella dama, a Gulf of California species. Primarily on typological grounds, Farmer and La Rose (2009) argued that many of the shell beads recovered at site SDI-39 had come from the Santa Barbara Channel area and central California. Isotopic analyses offer another potential method for distinguishing between west coast and Gulf of California origins for shells, in cases in which either the species is not identifiable or else its natural range is not restricted to one region or the other (Eerkens et al. 2005, 2009).
Determining whether the archaeological sites at which shell artifacts are found were also the locations where the artifacts were manufactured is problematic, but it has been addressed in several ways. At the Elmore Site (IMP-6427), just east of San Diego County, Martin D. Rosen (1994, 1995) was able to document the production of beads and ornaments by identifying Olivella sp. manufacturing detritus at the site. Farmer and La Rose (2009:8) argued that SDI-39 may have been "a center for clam disk bead production in southern California," based on the recovery of Tivela stultorum disk blanks. They also argued that the relatively abundant spire-removed Olivella sp. beads at the site were locally manufactured: "The creation of spire-ground and spire-chipped beads is a fairly simple process, which does not require a lot of tools or materials and is not particularly labor-intensive. It would not make economic sense to manufacture and export large quantities of spire-removed Olivella biplicata beads from one coastal site to another" (Farmer and La Rose 2009:2). However, Olivella dama spire-removed beads, representing a species from the Gulf of California, were relatively numerous (n = 43) in the collection from SDI-12,809 (McDonald and Eighmey 2007:158).
PROSPECTS
Future archaeological investigations may be able to provide additional material, including small specimens and detritus, through the use of intensive recovery methods such as water-screening and sorting through very fine mesh. Chemical analyses looking at isotopes and possibly also trace elements may be able to shed light on the source areas for archaeological specimens. More systematic collecting, reporting, collating, and analysis of shell bead data may be able to clarify regional patterns in prehistoric bead production and use.