Formation of PUget Sound


The geological history of Puget Sound is quite complex.  The following description of that history is paraphrased from a 2008 technical report to the Puget Sound Nearshore Partnership by Hugh Shipman of the Department of Ecology.  The report is entitled “A Geomorphic Classification of Puget Sound Nearshore Landforms”. Please consult the original report for more details.

The Puget Lowland, in which Puget Sound lies, is the result of tectonic processes related to the subduction of the Juan de Fuca Plate beneath the western edge of North America. These processes have established the broad-scale topography and geology of the region.  Superimposed on the underlying geology is the earth shaping action of the Vashon glaciation (15,000-20,000 BP).  Ice sheets advanced across Puget Sound, depositing large volumes of glacial sediment. This left a distinct north–south grain to the region’s hills and valleys, which are generally superimposed on a broad outwash plain about 330 feet in elevation.

Meltwater flowing southward beneath the ice is believed to have scoured the major troughs that define Puget Sound today.   Most of the sediment exposed on the edges of river valleys and along the coastal bluffs is glacially derived.  The resulting landscape was modified during the last 10,000 years, after the ice sheets melted, by water erosion of hillsides and subsequent deposition. The major rivers, which flow from the Cascades and Olympics, carried sediment into their lower reaches, building alluvial valleys and deltas.   In addition the growth of many of these large river deltas has been influenced by eruptions and mudflows associated with the Cascade volcanoes. Also streams drain hundreds of small watersheds located entirely within the Lowland and often within a few miles of the coastline; they cut into the erodible glacial sediments, forming small valleys and ravines, and redeposit material in narrow floodplains and at the marine shoreline.

Finally the shoreline has also been altered due to changes in sea level.  These changes are caused by several processes including global sea level changes, rebound of the land as the ice sheets melted and local tectonics.  Specifically, in the last 5,000 years, sea level has remained relatively constant in northern Puget Sound, whereas in southern Puget Sound, submergence has occurred.

The behavior of the Puget Lobe of the Vashon glacier is shown in the below diagram (origin unknown).