Prairies and the Puget Ice Lobe

A few Washington State our prairies

Scattered around various parts of western Washington Sate are prairies. Early English and American explorers noted large prairie areas in the low lands of Western Washington. There are prairies on Whidbey Island and the San Juan Islands, but the big prairie areas are in southwest Washington with a fair number in what is now Thurston County south of Olympia. I labeled the satellite image above with some of the prairies that are designated on USGS topographic maps. Except for two Fort Lewis prairies named for military divisions, all the above labeled prairies are in Thurston County. 

These Thurston County prairies are associated with glacial outwash from the Puget ice lobe. Large rivers of water flowed across these areas depositing gravel as the rivers flowed toward what is now the Chehalis River valley. Consider all the rivers coming off the west side of the Cascade range, all the rivers coming off the east side of the Olympic Range, and all the ice melt from the Puget ice lobe itself - that is a lot of water. The Geologic Map of the Maytown 7-5-minute Quadrangle, Thurston County, Washington (Logan, Walsh, Stanton and Sarikan, 2009) has a spectacular side map to help sort all the various water routes out - and that map only contains some of them channels.

Goldstein, Pringle and Futornick have noted that some of these water routes are associated with huge floods that are associated with volcanic mudflows coming off of Mount Rainier along some of the outwash channels http://www.centralia.edu/academics/earthscience/pringle/pubs/Tanwax_NWSA_poster2010.pdf.

I started thinking more about this area of our state when I had a project in the Black Hills southwest of Olympia and realized the outflow routes were a lot more complicated than I had realized. Lots of geologists have been working on figuring this area out and as can be seen LiDAR has clarified some of the water routes or pehaps made us realize how complicated the outwash channels that developed as the ice lob retreated from the area approximatley 17,000 years ago.

There are a lot to these prairie landscapes of southwest Washington State. The geology is a big part of the story for some of the prairies. But there is a lot more that I will be posting on over the next few weeks or months depending on my own interest and whims. In the meantime here is one of the prairie's from above and one of my favorite prairie songs.

Mima Prairie