First, I would like to apologize for the quality of this image. I sent this postcard off to a person in Thailand via Postcrossing, before I started this series of blog posts. Thankfully this person put the card up on their postcard wall, where I retrieved this image. So I do not have the card in front of me, to tell you about the details of the geology of Washington.
However: the first page of the teacher resources PDF http://www.dnr.wa.gov/Publications/ger_teacher_packet.pdf (published by the WA Dept. of Natural Resources) has a great map. Print it out and brighten up your office!
I can tell you though that the southern and central parts of the state, in various shades of orange and yellow, are underlain by thick layers of Columbia River basalt. This nice graphic from the US Geological Survey explains it well. The eruptions ended 6 million years ago. A rich soil has formed over much of this area, making it great for farming.
Not feeling too well today, so will sign off here. Happy PFF!