ABSTRACT Regional-scale processes of tectonism, late Quaternary marine transgression, and patterns of aeolian deposition and erosion largely control the geoarchaeological character of the Oregon coast. Dramatic changes to the landscape of the Oregon coast since the Last Glacial Maximum drove the evolution of terrestrial and marine environmental processes which in turn conditioned the location and nature of prehistoric human activities. Due to the geologic complexities of Oregon's coast, archaeological investigations must address a broad range of geological factors that worked to greatly modify the ancient coastal landscape. In many ways, the modern Oregon coastline bears little resemblance to that associated with prehistoric coastal peoples prior to 3000 years ago, requiring geoscientific perspectives to reconstruct the late Quaternary environmental context. Through the integration of geologic concepts and information, geoarchaeology offers an effective means of finding early sites in the modern coastal landscape and in the now-submerged paleocoastal landscape.

ABSTRACT Investigation and design-build construction of the Highway 20 realignment through the Oregon Coast Range provides new insight into paleo-landslides of the Tyee Formation and their slope stability. They are widespread, often extending outside of current drainage basins, and much of their morphology has been almost completely hidden by surficial processes. Radiocarbon tests indicate that some of the slide features are older than the testing limits, while other results range from approximately 18,000 to 40,000 yr B.P. The depth of erosion suggests that the paleo-slides may be as old as Pliocene. Geotechnical models of the paleo-slides, needed to analyze potential construction impacts, are developed from subsurface explorations, construction outcrops, radiocarbon testing, monitoring of geotechnical instruments, and geomorphology revealed by light detection and ranging (LIDAR). The process of predicting landslide boundaries (head scarps, toes, lateral and basal shear zones, etc.) for stability analysis of specific landslides has revealed details of their geologic evolution. This field trip provides background on (a) the investigations that have exposed numerous giant paleo-landslides, (b) findings and interpretations of the age of the landslides and (c) methods that are being employed to mitigate landslide reactivation.