Puget Sound existed long before the Osceola Mudflow which approached the Sound, that collapse is on the north side (where you live) and was filled with lava from later eruptions (the rock you hit) around 2200 years ago..
FWIW: most of the region has top soil only a few inches deep and then one hits rock or hard pan. The best top soil many feet deep (formerly farms) was covered with 20 feet of gravel and paved over to make a business district in Renton.
Thank you for clarifying!