Puget Sound and Coastal Geology | WA (2023)

Puget Sound and Coastal Geology

Puget Sound and Coastal Geology | WA (1)

Cape Flattery, Makah Reservation. Photo credit: Jessica Czajkowski, WGS.

Puget Sound and Coastal Geology | WA (2)

Point of Arches, Shi Shi Beach. Photo credit: Liz Thompson.

Puget Sound and Coastal Geology | WA (3)

(Video) Geological History of Puget Sound

Giant's Graveyard, Third Beach. Photo credit: Liz Thompson.

Puget Sound and Coastal Geology | WA (4)

Puget Sound from Pikes Place Market, Seattle. Photo credit: Dave Norman, WGS.

Puget Sound and Coastal Geology | WA (5)

Lummi Island and Mount Baker. Photo credit: Tim Walsh, WGS.

Puget Sound and Coastal Geology | WA (6)

Tacoma Narrows Bridge. Photo credit: Dave Norman, WGS.

The geology of Washington’s coast and Puget Sound is complex and beautiful. These attributes make Washington a great place to live, work and play. While many derive their livelihoods directly from these resources, we all depend on healthy coast and Puget Sound ecosystems, either directly or indirectly and whether we are aware of it or not.

(Video) Puget Sound’s Exotic Terranes | Nick on the Rocks

The majority of the state's population lives and works in the Puget Sound or other coastal areas. Because of this, the Washington Geological Survey has worked hard for over 125 years to provide the best-available science for planning and decision-making associated with the geologic hazards of these areas. We continue this legacy today, and the vast majority of our work is aimed at improving our understanding of geologic hazards.

Geologic Hazards

The most common hazards in the Puget Sound area and along marine coastlines of Washington include:

Clicking on these tiles will take you to a page where you can learn more about the specific hazard. Other hazards include Volcanoes and Lahars and Hazardous Minerals.

We publish a large variety of hazard maps. These can be found on our Geologic Hazard Maps page or in the Maps section of our Publications and Maps page. You can also find these data on the Geologic Information Portal.

What We Do

Survey geologists work extensively in the Puget Sound and coastal areas— including Washington’s outer coast and the Strait of Juan de Fuca—to increase our geologic understanding of these populated areas. We are proud to continue the legacy of pioneering geologic study that began around the turn of the 20th century when J. Harlen Bretz published a history of Puget Sound glaciation in 1913.

Geology – We have mapped thousands of square miles of geology in the Puget Lowland to provide a basic understanding of the geology underfoot. This critical information is used by planners, emergency managers, and developers as a basis for understanding the geologic risk of an area.
Active faults – We find and characterize faults throughout the region to learn which faults are active and how much risk they pose to society. Because the Puget Lowland is covered with thick forests and vegetation, finding faults can be difficult. Survey geologists increasingly use lidar in order to "see through" the trees to the ground surface. This new technology increases the visibility of active faults, but lidar coverage is not yet continuous everywhere.
Tsunamis – Tsunamis are a significant threat to the coastal areas of our state. We work with national and regional agencies to model and publish tsunami inundation maps. We then develop and publish tsunami evacuation brochures for areas of high risk. These evacuation brochures are available in published (paper) form, as an interactive online map, and as downloadable PDFs.
Landslides – We work with other state and local agencies to catalog existing mapped landslides through the Puget Sound region. To supplement this work we are developing a comprehensive and detailed landslide inventory based on the best-available lidar data. Lidar provides a huge advantage for mapping landslides. Landslides and landslide deposits that were masked by thick vegetation are often clearly revealed with lidar.
Seismic Hazards – We have developed and published a series of maps and reports about ground response to seismic shaking (earthquakes). These maps include: NEHRP site class maps, liqeufaction susceptibility maps, and seismic design category maps. We also offer a suite of datasets that show the predicted damage for several earthquake scenarios. You can find these seismic scenarios here. All these data are used by local, county, regional, and state planners and emergency managers. The products are also designed to help the public understand their risk from these hazards.
Lahars and Volcanic Hazards – We work with national and regional agencies to develop and publish maps of volcanic hazards. We have completed these maps for the area around Mount Rainier (including low-lying areas all the way to Seattle/Tacoma). Remaining areas near other volcanoes will become available as they are completed.
Multi-Hazard Analysis – We are developing a state-wide county by county assessment of all hazards. These data will become available as they are completed.
Subsurface Data – We collect and maintain a database of all publicly available subsurface information for Washington. These data include water wells, geotechnical boreholes, gravity and magnetic data, oil and gas wells, geothermal wells, and a depth to bedrock 3D model for the Puget Lowland.
Earthquakes and Faults – We maintain a database of all active faults and re-located earthquakes in Washington. These data help scientists and planners identify potential seismic hazards throughout the state.
Shallow Landslide Hazard Forecast Map – In cooperation with NOAA, we have developed a model based on recent and predicted rainfall data that may forecast hazards and may reduce losses from landslides.

Puget Sound Geology

The Puget Sound is part of the larger geologic province of the Puget Lowland. Both were sculpted by the thick and extensive glaciers that advanced south to just beyond Olympia. Glacial till (sediment deposited directly by the ice) and outwash (sediment deposited by meltwater in front of the glacier) make up most of what is found at or near the surface. These glacial sediments were deposited during the last 2 million years by numerous glacial advances, the most recent of which was around 15,000 years ago.

(Video) Coho, Rockfish, Lingcod, & Whales! Kayak Fishing the Washington Coast

The glacier covered the area in several thousand feet of ice. As glaciers move, sediment is scraped off the ground and transported on top of, within, beneath, and in front of the ice. This created many of the long and narrow hills and lakes we see today. Glaciers are capable of lifting, mixing, and moving rocks from the size of very large boulders to clay. The meltwater streams that flow in front of advancing and retreating glacial front can also move significant amounts of sediment.

Though glacial sediment covers a great deal of the Puget Lowland, isolated exposures of bedrock are found throughout. The southern area of the Puget Lowland is partly covered with ancient lava flows, similar to those found at Mount Rainier and Mount St. Helens. To the north, the San Juan islands are composed of metamorphic rocks accreted onto North America 160 million years ago.

For a longer explanation of Puget Sound’s geology, see the Puget Lowland geologic province page:

Washington Coast Geology

Unconformity on Fourth Beach between dipping sediments below and flat sediments above. Photo credit: D. Norman.

The Pacific coast of Washington is characterized by river and alpine glacier sediments above basalt and marine sedimentary rocks that were accreted to the continent.

The southern coastline lined with sandy sediment that works its way from the mouths of the rivers. This sediment forms beaches and sand spits like Long Beach, Ocean Shores, and Westport. Sediment from the Columbia River migrated north on ocean tides and formed the Long Beach peninsula, which is still actively growing.

The northern coast is made up of basalt from lava flows on the ocean floor that have been accreted onto the continent over the last several million years. For more information on how this works, see the Olympic Mountains page.

(Video) The Puget Sound Model: Tides and Currents

Extensive work has been done by Survey geologists to characterize the geology of coastal regions. Particularly, the work of Weldon Rau has provided a comprehensive overview of the geology of Washington's outer coast. See the Resources section below to download his publications.

Resources and More Information

Foraminifera, Stratigraphy, and Paleoecology of the Quinault Formation, Point Grenville-Raft River Coastal Area, Washington
Geology of the Washington Coast Between Point Grenville and the Hoh River
Washington Coastal Geology Between the Hoh and the Quillayute Rivers
Glaciation of the Puget Sound Region
The Puget Lowland earthquakes of 1949 and 1965—Reproductions of Selected Articles Describing Damage (1986)
Thickness of Unconsolidated Sediments, Puget Lowlands, Washington (1974)
Models of Bedrock Elevation and Unconsolidated Sediment Thickness in the Puget Lowland (2014)
Geologic Information Portal
(Video) Geology of Seattle and the Puget Sound


What is the geology of Puget Sound? ›

Puget Sound Geology

Both were sculpted by the thick and extensive glaciers that advanced south to just beyond Olympia. Glacial till (sediment deposited directly by the ice) and outwash (sediment deposited by meltwater in front of the glacier) make up most of what is found at or near the surface.

What type of landform is Puget Sound? ›

Puget Sound itself was scoured out by glacial action. Its deep north-south channels and basins are the glacier's legacy. The hills of Seattle and Tacoma are north-south trending drumlins, or hills of glacial debris. Elevation in the Puget Trough ecoregion averages 450 feet, but ranges from sea level to about 2000 feet.

Why are Washington's coastal areas and the Puget Sound important? ›

Beaches and bluffs of the Puget Sound region provide critical nearshore habitat functions and values for the region's fish and wildlife. Coastal bluffs are the primary source of beach sediment along the Puget Sound shore, and their natural erosion is essential for maintaining beaches and associated nearshore habitats.

How is Puget Sound formed? ›

Water from the melting ice carved valleys as it moved under the glacier. This is how Puget Sound, Lake Washington, Lake Union, Lake Tapps, and Lake Sammamish were formed. After the glacier melted and it became warmer, the bare land gradually was covered with forests and other native plants.

What is unique about Puget Sound? ›

It's Home to Unique Marine Life

Marine life is abundant in Puget Sound. On the small (animal) end of the scale, it's home to more than 3,000 types of invertebrates, including clams, oysters, sea urchins, and octopus. On the larger end, 13 types of sea mammals live in the sound.

Why is Puget Sound unique? ›

Puget Sound is the second largest estuary in the United States. Its numerous glacier-carved channels and branches are fed by freshwater from 19 different river basins that flow down from the Olympic and Cascade Mountains to the wetlands, salt marshes, and bays of the sound.

What is the coastline of Puget Sound? ›

The coastline around Puget Sound is 2,143 km (1,332 miles) long. It would take about 18 days to walk the whole shoreline if it were passable—or legal—everywhere. Note: this distance refers to Puget Sound proper and does not include the San Juan Islands or the Strait of Juan de Fuca.

Is Puget Sound inland or near coastal? ›

(11) "Puget Sound" means Puget Sound and related inland marine waters, including all salt waters of the state of Washington inside the international boundary line between Washington and British Columbia, and lying east of the junction of the Pacific Ocean and the Strait of Juan de Fuca, and the rivers and streams ...

What type of estuary is Puget Sound? ›

Estuaries carved by glaciers, such as Puget Sound, are known as fjord estuaries. They are prominent in areas where the glaciers once loomed, including Alaska and Scandanavia in the Northern Hemisphere and Chile and New Zealand in the Southern hemisphere.

Why does that water go directly into Puget Sound? ›

Puget Sound is an estuary, a semienclosed body of water in which salt water from the nearby Pacific Ocean mixes with fresh water runoff from the surrounding watershed. In general, the denser salt water sinks deeper and moves toward the land, while fresh water forms a surface layer that moves seaward.

Why is Puget Sound so important? ›

Puget Sound is critical to our environment, culture, and economy. About two-thirds of the state's population lives in the Puget Sound region. Our nation's second largest marine estuary faces a number of challenges related to population growth and development, as well as habitat loss and multiple sources of pollution.

Is Puget Sound considered the ocean? ›

Puget Sound (/ˈpjuːdʒɪt/ PEW-jit) is a sound of the Pacific Northwest, an inlet of the Pacific Ocean, and part of the Salish Sea.

Why are there no waves in the Puget Sound? ›

Most beaches in Puget Sound and Hood Canal are isolated from the Pacific Ocean by topography. As a result, waves in these more protected waters are generated locally by wind blowing over the irregular channels and basins of the fjord system; they receive little to no swell from the eastern Pacific.

What is the geology of Seattle? ›

Seattle's geology has been shaped by multiple processes with movement of materials caused by glaciers, rivers, volcanoes, earthquakes, landslides, coastal deposition and erosion, and human activities.

Why is it called sound in Puget Sound? ›

In areas explored by the British, the term "sound" was applied to inlets containing large islands, such as Puget Sound. It was also applied to bodies of open water not fully open to the ocean, or broadenings or mergings at the openings of inlets.

What is the deepest spot in the Puget Sound? ›

The deepest part of Puget Sound is off Point Jefferson, five miles northwest of Seattle, where the sea floor drops to 930 feet.

How deep is the water in Puget Sound? ›

About Puget Sound

The average depth is around 450 feet with a maximum depth of around 930 ft. It is subdivided into three deep basins connected by shallow sills, Whidbey Basin (mean depth-206'), South Sound (mean depth-121') and the Main Basin (mean depth-323').

What is the difference between the Puget Sound and the Salish Sea? ›

The Puget Sound region includes the area within the United States while the Salish Sea region encompasses the entire shaded area. Additional areas that influence circulation patterns or that eventually drain into the Salish Sea estuary are marked by boundaries extending further into Canada.

Why Puget Sound is not a fjord? ›

WHAT IS PUGET SOUND? Puget Sound is a deep fjord estuary that lies within the broader Salish Sea. As a saltwater body, it is far from being uniform: each basin, carved by retreating glaciers more than 10,000 years ago, varies in its physical, chemical, and biological properties.

What is the Puget Sound region known for? ›

The region is also home to numerous ports. The two largest and busiest are the Port of Seattle and Port of Tacoma, which, if combined, comprise the third largest container port in North America after Los Angeles/Long Beach and New York/New Jersey.

Why is Puget Sound an estuary? ›

Estuaries are places where fresh water and saltwater mix. Puget Sound is the largest estuary, by water volume, in the contiguous United States. Puget Sound encompasses many other estuaries--including large ones, such as the Duwamish River Estuary, and small ones, found in sheltered bays, inlets, and lagoons.

How big do waves get in Puget Sound? ›

Researchers went back to the drawing board, continued to study data and storm events, and now believe that the maximum waves the region may face could approach or even exceed 50 feet.

What are 2 ways to define the Puget Sound boundaries? ›

Puget Sound is defined by Washington State Legislature as, “all salt waters of the state of Washington inside the international boundary line between Washington and British Columbia, and lying east of the junction of the Pacific Ocean and the Strait of Juan de Fuca, and the rivers and streams draining to Puget Sound as ...

Why is it called the Salish Sea? ›

The name Salish Sea was proposed by Bert Webber in 1989 to reflect the entire cross-border ecosystem. Both Washington State and British Columbia voted to officially recognize the name in late 2009. The name honors the Coast Salish people, who were the first to live in the region.

Is the Puget Sound fresh or saltwater? ›

All estuaries, including Puget Sound, are par- tially closed-off waterbodies where freshwater from rivers and streams mixes with salt water from the ocean.

Is there sewage in Puget Sound? ›

Wastewater is used water that is affected by domestic, industrial, and commercial use. Wastewater pollution is a major source of contamination to Puget Sound.

Why is the water in Washington so blue? ›

Lake Crescent is known for its brilliant blue waters and exceptional clarity, caused by low levels of nitrogen in the water which inhibits the growth of algae.

Where does most of the sea water in Puget Sound enters through? ›

Deep, dense saltwater enters Puget Sound from the Strait of Juan de Fuca through Admiralty Inlet, part of it flowing south into the Main Basin and part flowing north up into Whidbey Basin.

Has there ever been a great white in Puget Sound? ›

Additionally, they're not found in Puget Sound, but they do occasionally approach the coastal waters of western Washington. The only two recorded shark attacks in Washington State are attributed to great white sharks. Both attacks occurred in Gray's Harbor, in the southwest part of the state.

Are there any sharks in the Puget Sound? ›

The Sixgill Shark is found all over the world including Puget Sound. They have been observed as deep as 6,000 feet but are commonly found around 300 feet. They are a rare sight in Puget Sound, so if you're diving and you see one, don't feel scared ... feel lucky!

Can people swim in Puget Sound? ›

While Puget Sound is cold and best for activities like paddle-boarding, kayaking, and fishing you can still find swimmable beaches near the city. Alki Beach Park has 2.5 miles of sandy beach, but the cold waters mean that you'll mostly find sunbathers hanging out here.

Are there whales in Puget Sound? ›

The waters of Puget Sound are home to a diverse array of marine life, including several species of whales. You can see both resident and transient Orcas, sometimes known as killer whales, in addition to Humpback Whales, Gray Whales, and Minke Whales.

Can a tsunami happen in Puget Sound? ›

Tsunamis can be generated in Puget Sound by both landslides and earthquakes. The most frequent cause of Puget Sound tsunamis is landslides. The 1949 Olympia earthquake triggered a landslide in the Tacoma Narrows that caused a 6 to 8-foot tsunami that affected nearby shorelines three days after the earthquake.

What are the three 3 causes of tsunamis within Puget Sound? ›

Washington has three major earthquake sources that have the potential to cause tsunamis: subduction zone earthquakes, deep (Benioff Zone) earthquakes, and shallow crustal fault earthquakes.

Why is Puget Sound so polluted? ›

CSO occur during rain storms when runoff combines with raw sewage, overflowing pipes and carrying pollutants, harmful pathogens, and excess nutrients directly into Puget Sound. This is a threat to the biodiversity of marine wildlife as it degrades/damages the marine habitat.

What kind of rock is under Seattle? ›

Four Mile Rock (also Fourmile Rock) is a round granite erratic, approximately 20 feet (6.1 m) across, in the intertidal zone below Seattle's Magnolia Bluff and 60 yards offshore.

What kind of rock is Seattle built on? ›

Seattle's geologic record begins with Eocene deposition of fluvial arkosic sandstone and associated volcanic rocks of the Puget Group, perhaps during a time of regional strike-slip faulting, followed by late Eocene and Oligocene marine deposition of the Blakeley Formation in the Cascadia forearc.

Is Seattle on a tectonic plate? ›

Seattle lies near to the boundary between the North American tectonic plate and Juan de Fuca, a large oceanic plate.

Why is Puget Sound so productive? ›

A Pacific Northwest icon, Puget Sound is the second-largest estuary in the United States. Its unique geology, climate, and nutrient-rich waters produce and sustain biologically productive coastal habitats.

What is the difference between a sound and a sea? ›

In geography, a sound is a smaller body of water typically connected to a larger sea or ocean. There is little consistency in the use of "sound" in English-language place names.

Is the Puget Sound man made? ›

On the westernmost end of the Puget Sound is the Hood Canal. It's not actually a man made canal. In fact, it is the only fjord in the lower 48, which means it's an exceptionally long and deep inlet between steep sides.

What is the geology of Palouse Washington? ›

Geology. The peculiar and picturesque loess hills which characterize the Palouse Prairie are underlain by wind-blown sediments of the Palouse Loess that covers the surface of over 50,000 km2 (19,000 sq mi) on the Columbia Plateau in southeastern Washington, western Idaho, and northeastern Oregon.

What is the geology of Seattle and the Seattle area? ›

The city of Seattle, Washington State, lies within the Puget Sound Lowland, an elongate structural and topographic basin between the Cascade Range and Olympic Mountains. The area has been impacted by repeated glaciation in the past 2.4 m.y. and crustal deformation related to the Cascadia subduction zone.

Why is Puget Sound so deep? ›

The deep and complex troughs that make up Puget Sound were carved by glaciers, most recently about 10,000 years ago. The Sound has remarkable patterns of water circulation that support its thriving ecosystem, and which give rise to water quality problems such as hypoxia.

What created the hills of the Palouse? ›

During this Ice Age, strong winds blew across the Palouse, carrying large volumes of silt. This silt, ground to a fine powder by glaciers, is known as loess. Most of the world's best agricultural soils are in loess. When this silt settled, it formed the rolling, dune-like hills of the Palouse, with their rich topsoil.

What is the oldest rock in Washington state? ›

In eastern Washington, between Spokane and Pullman, find Steptoe Butte, home to the oldest rocks in the state. The rocks at Steptoe are 400 million years old, and from the top of this drivable destination, you'll enjoy sweeping views of the Palouse, formed by massive floods 15,000 years ago.

How was the Palouse formed? ›

The hills were formed over tens of thousands of years from wind blown dust and silt, called "loess", from dry regions to the south west. Seen from the summit of 3,612 foot high Steptoe Butte, they look like giant sand dunes because they were formed in much the same way.

Is Seattle on a subduction zone? ›

Potential Hazard to Seattle

The Cascadia Subduction Zone is close enough to Seattle to cause serious damage to the city. A Cascadia Subduction Zone earthquake could be as severe as a 9.0 magnitude, resulting in shaking so strong that one would barely be able to stand during the earthquake.

Why is there no bridge across Puget Sound? ›

Due to the depth of Lake Washington (214 feet) as well as the softness of the lake bed. The soft lake beds can be attributed to the glacial till that covers the Puget Sound region, and it's why we have quite a few floating bridges in the area, such as…

What tectonic plates are under Seattle? ›

Geologic Setting

Earthquakes as large as magnitude 7 originate in the subducted Juan de Fuca plate, mainly beneath the Puget Sound region. Moderate and large earthquakes occur also on faults within the overriding North America plate, on both sides of the Cascades.

Is the mountain in Seattle a volcano? ›

Mount Rainier is a strato volcano with the potential to erupt again.


1. New maps show how a tsunami could impact Seattle and inland Puget Sound
(KING 5 Seattle)
2. How the Puget Sound was created
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3. The Puget Sound Rocks!
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4. Whale sightings spike in Washington’s waters
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5. Tsunami wave simulation for Washington State
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6. Marine Life Identification, Puget Sound, WA 🐟 🐠
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