This research formulated a recent geologic history of the Clatsop Plains dating from 3500 years BP to present. Rese arch consisted of geomorphic mapping, near-surface stratigraphic evaluation, carbon dating and subsurface int erpretation of available data. 2 The Plains were formed from a series of arcuate shaped foredune ridges and interdunal flats oriented sub-parallel to the coastline. Groups of dune ridges and associated interdunes are differentiated by similiar morphology separated by wave

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... ut discontinuities which truncate dune forms. Ages of these prehistoric shorelines range between 3500 years BP, at the mountain front, to 110 years BP, some 1 to over 5 km west. Dune ridges within a group diverge from one another in two distinct patterns: Type A -where the ridge crests are sinuous and hummocky and the westernmost or youngest ridge acts as a main stem from which older truncated ridges branch and Type Bwhere the ridge crests are subparallel, continuous, and curvilinear and most ridges extend along much of the Pla.ins length.