GEOMORPHIC MAP UNITS AND LANDSCAPE EVOLUTION IN THE PUGET LOWLAND

Systematic analysis of high-resolution lidar topography of the Puget Lowland (http://pugetsoundlidar.org) has allowed definition of a rich suite of geomorphic map units, based on criteria of mappability, inferred genesis, and utility for historical analysis.

In the central and southern Lowland, the master landform is a proglacial outwash plain (Booth, 1994) formed in front of the advancing, latest Pleistocene (Vashon age) ice-sheet. Subsequent subglacial processes carved an anastomosing network of large troughs and decorated the plain and troughs with sub-parallel flutes. The fluted subglacial surface has mappable variations in the aspect ratio of flutes and their surface texture (smooth, rippled, or lumpy). Kame-kettle complexes, eskers, and hilltop channels locally record late-glacial dead ice conditions. In the northern Lowland, cross-cutting flutes record significant late-glacial change in ice-flow direction, from south to west-southwest and west.

Glacial surfaces were dissected, and locally filled, by hillslope, fluvial, deltaic, littoral, and subaqueous processes. Nested sets of fluvial-deltaic-littoral flats record a history of falling base level during deglaciation and subsequent glacio-isostatic rebound. Extensive entrenched post-glacial valleys at sea level in some southern locales suggest relative sea-level rise, perhaps by fore-bulge collapse, though eustatic sea-level rise could also explain these landforms. Relative ages of some hillslopes are evident from their relation to adjacent fluvial or littoral flats. Most of this history is latest Pleistocene and earliest Holocene.

Many deep-seated landslides are clustered, suggesting strong stratigraphic control on their occurrence. With scattered exceptions, topography gives no indication of landslide ages more precise than Holocene.

Identifiable late Holocene surfaces are valley-bottom fluvial flats, active littoral surfaces, and associated hillslopes. Raised relict littoral flats, associated hillslopes, and a couple of landslides, all in the vicinity of the Seattle and Tacoma faults, are also late Holocene.