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Puget Sound Under Ice  

Imagine Puget Sound under a mile of ice. 20,000 years ago, glaciers covered everything in between the Olympics and the Cascade mountains and spread as far south as Olympia. The ice over Seattle was higher than five Space Needles (3,412 feet.)

Puget Sound under Pleistocene ice.

Glaciers advanced from Canada and retreated four or more times. Over a few million years, Puget Sound was carved and scoured by glaciers.

A glacial horizon

  • Heavy Pressure
    Glaciers containing up to 2,383 cubic miles of ice pushed over the Puget lowland. In some places, the heavy pressure of the ice sunk the earth's crust.

  • Meltwater On The Move
    As glaciers advanced and retreated, melting ice created streams and lakes. A massive melt water lake formed and swelled 120 feet above today's Sound. The lake drained via the Chehalis river and formed a large valley.

  • Deciphering Drift
    As giant slabs of ice melted, huge icebergs carrying mud, sand, and gravel drifted over the flooded Sound. Glacial and marine sediments settled, covering most of north Puget Sound with a layer geologists call "glaciomarine drift."

    After the Ice Retreated

    When the ice finally retreated to the north, it left behind deeply gouged channels, north-and-south oriented passages and bays.  
    Weather, waves, and gravity reworked the glacial sediment, molding landforms and shorelines like frosting on a cake. The results are the beaches and bluffs that now edge the Sound.
    After the ice melted and the sea returned.

    What Glaciers Left Behind

    Glacial erratic. Photo by Wolf Bauer.

    Glacial Rocks

    Glaciers bulldozed and scattered materials. Many beaches along the Sound display rocks of mixed sizes.
    Scrapes in bedrock. Photo by Wolf Bauer.

    Glacial Scrapes

    The power and pressure of glacial action remains etched on bedrock.

    Glacial layers, Discovery Park bluffs, Seattle, Washington.

    Glacial Layers

    Many shoreline bluffs reveal layers of glacial activity in soils and rocks. These layers can slide when saturated.
    Glacial Layers Of A Puget Sound Bluff:
    Discovery Park, Seattle.
    Glacial action revealed in layers. 1. Vashon Till
    Mixed unsorted, boulders, rocks, sand, silt, and clay, deposited by the glacier.

    2. Esperance Sand
    Loose, yellow-tinted sand deposited by glacial meltwater and mountain streams.

    3. Lawton Clay
    Dark blue-grey clay and silt which accumulated at the base of a huge freshwater lake created by glacial meltwaters.

    4. Kitsap Formation
    Sediments that accumulated in lakes and streams before the last glacier pushed over the Puget Sound lowlands.


    Shorelines Shaped In A Flash

    • Ages ago, the earth's crust uplifted and folded, forming the mountains flanking the Sound.
    • The last glacier retreated about 13,000 years ago, leaving behind the basic shape of the Sound.
    • Beaches, deltas, spits and other sedimentary features along the Sound were formed in the last 5,000 years - a flash of time geologically.
    • Weather, waves, and gravity are still shaping Puget Sound shorelines.

    Related Topics

    Slides, Factors leading to slides along Puget Sound.
    Buyer Beware, Evaluate slope hazards before buying property.
    Understanding Erosion, What to consider on land loss.

    Related Links

    Puget Sound Landslides, Department of Ecology. This site includes slope stability maps, property owner tips, and a slide show.

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    Comments? E-mail: Shellyne Grisham