Geophysical Surveys of Stratigraphically Complex Island California Sites: New Implications for Household Archaeology

Article excerpt

Ground-penetrating radar and other geophysical techniques are known to produce useful data when deposits are crisply structured, as in the case of sub-surface masonry walls or large ditches. New studies of Californian coastal sites find the methods are effective in tracing the less sharp distinctions that define clay and sand house floors within these large and dense hunter-gatherer middens.

At a time when archaeologists, indigenous peoples interested in protecting their pasts, and preservationists are increasingly concerned that disturbances to archaeological sites be minimized, geophysical techniques obviate the need for large-scale exploratory excavations and permit highly informed and focused sub-surface studies. Such techniques may reduce significantly the expense and field effort in archaeological projects at sites of virtually any size and degree of internal complexity. A number of published works have shown that these applications can effectively trace major buried architectural features such as masonry walls, moats, cisterns and the like.

Recently, we have found geophysical methods also useful at small but unusually dense and stratigraphically complex residential sites of hunter-gatherer groups in southern California. The dense midden deposits characteristic of many coastal Chumash sites range from 1 to 5 m deep and are packed with well-preserved marine food refuse, many artefacts and very little soil. Contents include masses of fish, bird and mammal bone, shells, grinding implements, flaked debitage and tools, shell ornaments and beads, bead-making refuse, fishing gear, woven bags and mats, bone tools, asphaltum and much more.

Excavations reveal that directly within these dense deposits are found hearths, craft workshops, other features and - importantly for the present discussion - Chumash house floors. The positions of the last houses to be occupied at undisturbed sites on the region's offshore islands [ILLUSTRATION FOR FIGURE 1 OMITTED] are often marked by shallow or deep depressions at site surfaces; earlier houses are likely obscured by overburden. Chumash residences are the focus of current field research on the California Channel Islands, where the senior author is exploring inter-household variability in craft production, distributions of exotic goods and other indicators of differential wealth and status at dozens of well-preserved house depressions dating to the late 1700s. It was not clear before we initiated this project whether subtle features such as 2-3-cm thick clay house floors - or non-clay (e.g. sand) floors - buried within middens might be detectable by remote sensing instruments at these dense sites. Yet identifying floors is crucial to efficient excavation of houses, and augering is not always adequate to detect their position and depth.

Here we report preliminary results from the application of two geophysical survey techniques at Channel Islands sites with multiple houses. We present initial results and interpretations derived from these field studies that may be useful to researchers working in other areas of the world, including advances in data visualization, detection of features and reliability and replication of techniques. Certain of the sub-surface anomalies detected await further field-testing; here we focus on the data pertaining to houses.

Stein and colleagues (see Dalan et al. 1992) have previously demonstrated the utility of resistivity and seismic refraction methods for interpretations of site-formation processes and microstratigraphic analyses of shell middens in coastal Washington. We focus on two different techniques, ground penetrating radar and caesium vapour magnetometer, and highlight their potential to advance our understanding of household archaeology at exceptionally dense midden sites.

Cultural setting

The Chumash groups occupying the mainland and offshore islands of the southern California coast enjoyed one of the richest maritime settings along the Pacific Rim. …