I had another good question from my Ocean 200 kids on Monday that I had to check for them. It was a question about the relationship between the glacial and tectonic processes in the Puget Sound area. Here’s what I gave them:
Around 20,000 years ago there was a glacier between the Cascades and the Olympics. It advanced and retreated periodically for a long time, before finally leaving for good about 13,000 years ago. Puget sound is actually in the forearc basin of the Cascadia subduction zone (not the backarc basin). The glacier did not cause this depression, rather, it filled the basin that was already there (ie. filling in a topographic low).
As it sat there, it pushed down all the land in the area, resulting in a relative rise in sea level (relative to the subsiding land). When the glacier retreated, the land slowly started to rise up again to reach isostatic equilibrium. This is called post-glacial rebound. It’s still happening today, but very, very slowly.
I hope this helps, and doesn’t have any outrageous flaws in it. Also: the sketch was done entirely on the iPad 🙂