The Seattle to Everett Railroad Revetment, Puget Sound, Washington

摘要

In the U.S. Pacific Northwest, the modern landscape of the Puget Sound region was largely shaped by the Vashon glaciations of 15,000-20,000 years before present (BP). Puget Sound is characterized by drowned river valleys, coastal bluffs, numerous small river mouths and deltas, and relatively low wave energy (in contrast to the open Pacific coast). Much of the sediment exposed in river valleys and along the coastal bluffs is glacially derived, consisting of lake-bed clays, outwash sands and gravels, coarse-grain till and glacial marine drift, and interglacial fluvial (riverine) deposits (Shipman 2010). About half of Puget Sound's 4,000 km of shore consists of bluffs and small barriers, with the remainder comprising bedrock shores, several large river deltas, and hundreds of sheltered estuaries and barrier lagoons. Beaches on the sound consist of a wide mixture of grain sizes, but are dominated by gravel and coarse sand. Natural beaches in this region typically comprise a steep, coarse-grained beach face and a gently-sloping, sandy low-tide terrace (Finlayson 2006).