Geology of Seattle and the Puget Sound

welcome to Seattle Washington it's an amazing place for geology what's beneath that noisy Stadium it's a long way down to bedrock the bedrock here has been buried by more than a thousand feet of muds sands gravels that were dumped here by ancient ice sheets distant volcanoes old lakes and rivers even by early residents of Seattle and there's an earthquake fault here a big one it's all right here there's lots to learn you're ready it's game time dig into most of Seattle's Hills and you'll find complicated sets of poorly sorted rocks and beds of sand and gravel that were dumped here by an ice sheet that came from Canada the Puget lobe advanced and retreated over Seattle at least seven times in the last two million years each ice advanced laid down a different generation of glacial till Canadian rocks strewn all over the Puget lowland stretching from the Olympic Peninsula clear over to the Cascade Range deposits that were first mapped in detail by Jay Harlen Bretz back in 1913 kind of hard not to notice a boulder like this right glacial erratics carried in ice from the north the most recent advance began 25,000 years ago in British Columbia ice crossed the border 19,000 years ago and made it as far south as Toninho Washington by sixteen thousand nine hundred years ago that was just yesterday geologically the glacier was 3,000 feet thick over Seattle yep I said 3,000 feet thick I that was three Columbia centers hi the weight of the ice caused the crust to sag by as much as 300 feet since 16,000 years ago Seattle has been ice-free and the crust is rebounded but the blanket of glacial deposits remain almost 75 percent of Seattle surface deposits were laid down by the most recent ice advanced bedrock here is hard to find only 3% of the Seattle area has exposed bedrock at the surface steeply dipping sand stones in a sea of loose glacial rocks Bluffs throughout the Puget Sound reveal complex ly interbedded glacial and interglacial deposits and Seattle's famous rain makes these deposits especially prone to landslides sometimes with tragic results as it flowed south the Puget lobe acted like a comb running over the landscape sculpting graceful elongated north-south oriented hills called drumlins Beacon Hill first Hill Capitol Hill Queen Anne Hill Mercer Island they're all drumlins one of the very few drumlin fields in all North America have you driven here in Seattle the traveling is easy going north and south along drumlin tops are through Valley bottoms but driving east or west here means you're going against the lay of the land when east-west streets are necessary steep climbs are in order interstate 90 cuts across the grain one of the most expensive stretches of our country's interstate system the freeway tunnels through drumlins and uses bridges to span ice age troughs between the drumlins as you leave Seattle on i-90 you skirt around the north end of one drumlin and then drive through the next one then a floating bridge over a trough then crossing another drumlin talk about obstacles if these tunnels were magically lined with glass walls we'd see blue clays at the days sordid sands and gravels in the middle and poorly sorted glacial till at the top during its march over Seattle the advancing Puget lobe a 3,000 foot high bulldozer was an on the move broad apron of rocks and dirt that blocked north flowing rivers as the Strait of Juan de Fuca was blocked by the Puget lobe the Puget Sound filled with a massive lake and for hundreds of years clay settled to the bottom at its peak the glacial lake was 120 feet higher than current sea level as the ice pushed south its outwash plain deposited rocks and sand over the clay eventually the glacier overrode these layers and when the climate warmed the ice dropped the poorly sorted rocks that it was carrying imagine Colombian mammoths roaming the scene another mammoth tusk was recently discovered beneath downtown Seattle a long ago tranquil Ice Age scene that now is just a little bit different Seattle shoreline in the 1850s was very different than today a winding freshwater Duwamish River Seattle's only River used to meet and mix with saltwater at the southern end of Elliot Bay fine sands and the Duwamish were washed to Seattle from the front of impressive volcanic mudflows lahars from a still active Mount Rainier on the horizon the most recent major lahar from Rainier was 2200 years ago Seattle's first white settlers arrived at Alki Point in November of 1851 surrounded by steep hills under a dense forest the trees up to 2,000 years old and towering up to 400 feet loved the climate and the soils a few months later the settlers took an Indian canoe a clothesline and a bunch of horseshoes out into Elliot Bay to see if it was deep enough to serve as a harbor Harbor passed the test and in 1853 plats were filed to establish a new town called Seattle down at the tide flats this place was mud and salt water a popular food gathering spot for local Native American tribes the mud flats only visible during low tide twice a day it was a quiet natural place with birds salmon and acres of clams it's not quite head anymore these football fans have been so loud they've registered on local seismic stations none of this was here in the 1850s and I'm not talking about the stadium I'm talking about there was no land here in 1895 just a few years after a devastating citywide fire seattle city leaders announced a bold new civic project to radically alter the city's rollercoaster topography hills would be removed and the dirt would be dumped on to the tidelands seattle needed new flat areas to grow horse-drawn wagons were lined up to accept dirt from neighboring hills and delivered the earth to the tide flats then more creative and efficient solutions emerged the hills were washed directly into the bay using sluicing equipment a mixture of soil and water that was shot in two large chutes for transport directly over land and into the bay no blasting was needed right they weren't dealing with bedrock eventually the regrade project was perfected to the point of using conveyor belts not everybody loved this hill moving business there were some holdouts folks that elected not to participate were left stranded on what became known as spite mounts all told sixty regrade dumped millions of tons of Earth from Seattle's Hills creating almost three thousand acres of new land interstate five that runs along the base of Beacon Hill traces the original saltwater shoreline Hills flattened rivers rerouted shorelines extended Seattle is one of the most dramatically engineered cities in the country if we drill at the 50-yard line in Seahawk Stadium we would encounter 40 feet of manmade fill in more than 1,000 feet of ice age sediments before we hit bedrock but at nearby Alki point that same bedrock is at the surface what's going on here the Seattle fault an east-west crack in the bedrock that runs from Bainbridge Island all the way to si quoi lurks beneath the Puget Sound and downtown Seattle the fault is produced at least four earthquakes in the past 3,500 years and there is concern for future magnitude 7 earthquakes here it's a thrust fault with a 35 degree dip to the south there's been more than 5,000 feet of offset on the fault over the last 15 million years my god how many earthquakes are we talking about here to produce that kind of offset plate tectonic forces responsible for past Seattle fault earthquakes continue to squeeze the crust here the San Andreas Fault an active Fault in California is visible from the space station due to millions of years of motion and erosion that have etched out an obvious gash across the land the Seattle fault might be just as active but our glacier wiped the slate clean less than 20,000 years ago and then dumped a thick layer of sand and gravel on top all of that loose material makes it tough to unravel an earthquake history the most recent earthquake on the Seattle fault struck in the year 900 AD a bedrock platform visible today at restoration point an alkine was formerly under the waves of the Puget Sound and is now high and dry above the tides all of the bedrock south of the fault jumped up suddenly 20 feet during the earthquake Seattle sits on loose glacial sediment and bay fill properly placed fill with the right materials can be strong but in South downtown the city founders put just about anything in there as fill without any engineering considerations sawdust woodchips demolished building materials asphalt cinders even garbage combined with the regrade held materials that's a 40-foot layer of fill this stadium sits atop the worst possible soils and earthquake country seismic waves get trapped in basins with soft sediment than our prone to shaking like a bowl full of jell-o the soft stuff shakes twice as hard as bedrock during earthquakes piles for the stadiums over 1700 steel pipes up to 90 feet long were driven 50 feet into highly compacted glacially overridden deposits this is a modern stadium built to current seismic standards one of the safest structures in the region looking to the future Puget Sound Bluffs will continue to fail as landslides future Mount Rainier mud flows could easily reach Seattle magnitude 9 mega quakes strike every 500 years off of Washington's coast is that a more likely seismic threat to the city than the Seattle fault that lies beneath there's so much geology here in Seattle but we're just getting started and our next episode of i-90 rocks let's jump on the freeway and head east the 30 miles between Seattle and the foothills of the Cascades thanks for watching

Channel: hugefloods

Views: 191,861

Rating: 4.9307566 out of 5

Keywords: Seattle, Geology, Washington, Pacific Northwest, Puget Sound, Glacier, Ice Age, Seattle Fault, Seattle Earthquake, Seahawks, Glaciation, History, Seattle History, Video, Nick Zentner, Tom Foster, Drumlins, 2 Minute Geology, I-90 Rocks, Hugefloods

Id: oSSxdogrv1s

Length: 13min 11sec (791 seconds)

Published: Mon Mar 02 2015

Reddit Comments

[u/ilabean]

Love that Prof. Zentner's videos are getting shared again. I took his "Geology of National Parks" class at CWU ages ago and it's still one of my favorite classes ever.

[u/azzkicker206]

Fascinating. Thanks for the link.

Looks like that Wedgwood erratic has its own wikipedia page: https://en.wikipedia.org/wiki/Wedgwood_Rock

Wikipedia also has a list of other glacial erratics in King County: https://en.wikipedia.org/wiki/Glacial_erratic_boulders_of_King_County,_Washington

[u/matt2500]

I can't imagine how different the city would be without the regrades and the filling of the tide flats. The estuary of the Duwamish must have really been beautiful. I wish I could see what it was like.

[u/weegee]

Jesus Christ made Seattle under protest.

Jefferson James Cherry Columbia Marion Madison Seneca Spring Union University Pike Pine

[u/TacoTacoTacoTacos]

Prof. Nick is the man! This man knows the PNW like the back of his hand

[u/jbw976]

oh man i love this. it seems to be part of a larger series of "I-90 Rocks" videos, but for the life of me, I can't find a good listing of the videos in order (e.g., part 1 part 2 etc.)

All of Nick Zentner's videos can be found here: https://www.youtube.com/user/hugefloods/videos

Has anyone seen an up to date list for all the parts of the I-90 Rocks video series in order?

Bonus, here's another very relevant lecture for the area from Nick: Nick Zentner- Earthquakes: Will Everything West of I-5 Really Be Toast? https://www.youtube.com/watch?v=tW4D6OE7Qkc

[u/BaconSizzler]

Look down into the excavations of new buildings that are getting build in Belltown / Denny triangle and you might notice that there's not much of that glacial till and yellow sand near the surface that he was talking about. Instead, the excavations seem to hit lake bed clay and silt straight away. Is that due to the almost-century-old regrade project that removed Denny hill (and incidentally created sodo with it)?

[u/ntbc]

Great video. Thanks for sharing. And now everyone is going to call it "The Puget Sound" instead of Puget Sound.

Edit: "That sure is a pretty view of The Puget Sound from Queen Anne Drumlin."

[u/95percentconfident]

Thanks for sharing, those are cool. If you want to read some, Kruckeberg wrote the classic: The Natural History of Puget Sound Country.