Giant's Graveyard, Third Beach. Lummi Island and Mount Baker. Tacoma Narrows Bridge. 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. 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 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. 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. You can also find these data on the Geologic Information Portal. 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 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, years ago. 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. To the north, the San Juan islands are composed of metamorphic rocks accreted onto North America million years ago.
Unconformity on Fourth Beach between dipping sediments below and flat sediments above. Photo credit: D. 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. 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.
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. Skip to main content. Public Land Survey Office.
Assessing Species Vulnerability. In the Puget Lowlands, the Puget Lobe shaped much of the topography below 3, feet. Meltwater flowing below the ice carved out troughs that we know as valleys. Some of the meltwater moved across the direction of the ice flow, creating east-west oriented valleys like the cleft through which the present-day Lake Washington Ship Canal runs.
The receding ice left moraines at its margins, outwash deposits left by meltwater streams, and exposed drumlinoid ridges that largely run parallel to each other. As the ice sheet receded, meltwater formed Glacial Lake Russell. This drained via the Chehalis River Valley and the Chimacum Valley until the Juan de Fuca Lobe receded and marine water entered the lowlands and filled the ice-sheet-carved troughs.
The ice sheet had exerted enormous pressure, pushing the land down, but over time the land rebounded. In the Puget Lowlands, this action, accompanied by the gradual deposition of sediment at river mouths, slowly raised the elevation of the valley floors. In the White-Green-Duwamish valley, the massive deposition of mud and rock by the Osceola Mudflow about 5, years ago punctuated and accelerated this process. Marine water retreated and was replaced by freshwater in all of the troughs except for the Duwamish Embayment.
The level of Lake Washington slowly rose about 40 feet as the Cedar River alluvial fan at its mouth built up over time. A terrace below the lake's surface indicates the ancient shoreline. Peat bogs along the shoreline and the remnants of drowned forests found below the water line indicate formerly dry areas inundated by the lake's rise.
The Pleistocene Epoch was characterized by glacial stades and by warmer interglacial periods. The archaeological record shows that humans moved into this fluctuating landscape during the Pleistocene and the fossil record indicates that the animals that lived during interglacial periods and just after the Vashon glacier retreated include the horse, the bison, caribou, woolly mammoths, and the mastodon.
The barren land left by the glaciers was gradually filled by primeval forests dominated by Douglas fir, ponderosa pine, and other evergreen species in Western Washington and in the higher elevations statewide, and sagebrush steppe in the lowlands east of the mountains.
Derek B. Lasmanis and E. Alt and Donald W. Blukis Onat and Roger A. Note: This essay replaces an earlier essay on the same subject.
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