The hills and lakes of Seattle, Washington are a direct result of multiple Puget Lobe advances during the Ice Age. Beneath the drumlins, outwash, glacial troughs, and scattered glacial erratics lies the Seattle Fault, an active fault which has produced numerous magnitude 6 or higher earthquakes since the Ice Age.
Interstate 90 exposes much of this geology in its first few miles heading east from downtown. The freeway begins on old tidelands that were filled by early residents of Seattle. Hills composed of soft glacial deposits were moved and dumped into Elliot Bay to make new land for a growing city. Today’s SODO district – including stadiums for the Seattle Seahawks and Seattle Mariners – sits on filled tidelands that are especially prone to seismic shaking during the next big earthquake in the Puget Sound.
Tom Foster (https://hugefloods.com/) and Nick Zentner (Central Washington University) have been hiking together in Washington for years. ’Geology of Seattle’ is part of an “I-90 Rocks” video series.
Learned the reason everything in the Puget Sound is so north-south oriented: it’s a field of drumlins formed by the glaciers that at one point lay 3000′ thick over Seattle!
I always figured our yard was an old stream bed because of all the rocks but it’s totally glacial till 😂
It’s shocking how much of Seattle proper is built on fill 😳 South Seattle west of I-5! So, like, a lot (including the stadiums). As a born and raised Californian with a deeply rooted awareness of earthquake danger, my mind turns to liquifaction 😳
So much interesting geology in Washington! Snoqualmie Pass is different from most mountain passes, lacking a steep cirque on either side because of ice age glaciers weighing down the peaks (probably also explains why it’s a relatively low pass at 3000′). And a diverse blend of rocks – sandstone uplifted, magma chambers exposed, volcanic deposits, lots of cool stuff.
