Notes on Salish Sea Landscape 20,000 Years Ago

For my shoreline bluff talk I started of with this image.  

 

The Puget lowlands before the last ice age

Dark blue in the upper part is sea water

Light blue is fresh water

Ralph Haugerud and Harvey Greenburg developed a video (youtube) of the Vashon Glaciation in Puget Sound and the first image is of a time right before the arrival of the Puget and Juan de Fuca ice lobes. The video uses elevation estimates, estimated sea level at various times during and after the last glacial period and estimated ice extent during the last glaciation.

The beginning of the video as shown above raises the question of What did the low lands between the Cascade Range and the Olympics look like before the arrival of the last ice sheet?

Today this low land area has deep trenches filled with sea water. In the image above, the deep trenches are shown, but are lakes.  The sidebar of the video notes that the deep trenches may not have been there. Possibly there were deep Puget Sound like trenches before the last ice age but they may have been oriented differently.

But another aspect of the image above is that the deep trenches are show as lakes. A low land plain extends between the Strait of Juan de Fuca on the northwest and Puget Sound on the south cutting Puget Sound (and Hood Canal) off from the sea rendering the deep Puget Sound trenches to fresh water lakes.

A look at sediment loads in our rivers today suggests that this is not an unreasonable concept.

Estimated sediment loads (USGS)

Note that the Skagit River has a massive load of sediment relative to all the other rivers draining into the Salish Sea in Washington State. The Skagit is the largest river, but it also drains from the highest mountains and includes two large strato volcanoes. The Nooksack to the north of the Skagit is a much smaller river, but it is draining out of steep, glaciated, and very high terrain and has a volcano as well. And of course there is a very big river just to the north without a sediment figure - the Fraser River.

While many of the steep bluffs along the shores of the Salish Sea are underlain by glacial related sediments from the last glacial period there are bluffs that are underlain by thick sequences of non glacial sediments.

100-foot thick sequence of pre ice age alluvial river sediments 

One of several peat layers embedded within alluvial sediments suggesting non glacial sediment