CHALLENGES OF GEOARCHAEOLOGY AT URBAN HISTORIC SITES IN WESTERN WASHINGTON

Most urban archaeology in the United States is conducted in a regulatory context as archaeological monitoring of construction excavations to identify and evaluate potentially significant sites. This context imposes a number of constraints on the geoarchaeologist seeking to develop a broad geologic context for subsurface sediments and the artifacts within them: construction schedules demand speed and provide inadequate exposures, and current uses (such as active streets and ubiquitous pavements) further limit exposures. Additionally, decades or even centuries of continuous historic-era human actions have turbated or introduced many of the sediments at a site. In many cases, distinguishing among natural and artificial deposits is difficult.

Transportation projects in the Puget Sound region of western Washington State provide examples of the challenges for deciphering natural and cultural stratigraphy in urban historic archaeological sites. Two adjacent sites in the historic late 19th century downtown core of Tacoma are situated atop glacial sediments, but are characterized by very different stratigraphy. One, a commercial building lot, has seen numerous excavation and fill events during continuous use from 1889 onward; the situation is further complicated by petroleum and chlorinated solvent contamination. The other, an adjacent street, was filled as part of a late 1800s regrading operation, excavated numerous times for underground utilities and trolley track placement, and paved in multiple layers. Finally, one site in Seattle exhibits a sequence of artificial fill events atop Holocene estuarine and alluvial tideland sediments: a basal deposit of dredged estuarine and alluvial sediments from within a few kilometers of the site, followed by hydraulically sluiced glacial sediments from nearby uplands, followed by multiple layers of construction and sanitary fill.