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Retreating glaciers create Puget Sound and Grand Coulee as the Ice Age ends about 15,000 years ago.

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About 15,000 years ago, the Vashon glacier begins to melt and recede from lands that will come to be known as the Puget Sound region and the Columbia Basin region. By 11,000 years ago, the glacier has retreated to the border of present-day Canada. During its advance, meltwater flowing under the ice sheet had carved out Lake Washington, Lake Tapps, Lake Sammamish, Puget Sound, and Hood Canal. The other major shaper of the land -- the pushing of the Juan de Fuca Plate underneath the North American plate, and the docking of terranes (fragments of continents) had already occurred long ago. 

The Vashon Stade

The Vashon stade was the last glacial advance and retreat to cover the region. It was the last of at least seven glaciations during the Pleistocene Epoch, which lasted from two million years b.p. (before present) to about 10,000 years b.p.

Extending as far south as the Olympia area to the west and the Spokane area to the east, the ice sheet, at its thickest, was 3,000 feet. In comparison, the Pacific Northwest's tallest skyscraper (as of 2012), Seattle's Columbia Center, is about 997 feet tall.

East of the Cascades

In Eastern Washington, the retreating Purcell Trench Lobe and Okanogan Lobe of the Cordilleran Ice Sheet formed temporary ice dams holding back billions of gallons of meltwater in Glacial Lake Missoula and Glacial Lake Columbia. As these dams weakened and burst, they unleashed titanic floods which scoured the Columbia Basin and created the Grand Coulee and the Channeled Scablands. Geologist J Harlen Bretz (1882-1981) first theorized these gargantuan gullywashers in the 1920s. Subsequent research has confirmed that the "Ice Age Floods" were among the greatest such events in the known history of the planet.

In the Okanogan Highlands and North Cascades, the retreating ice sheet left moraines and drumlins scattered across the landscape. Moraines are large hills of rock and sediment carried by ice and deposited as it melts. Drumlins are small moraines that were shaped by moving ice.

In glacier-cut valleys, lakes formed where moraines blocked creeks and rivers or where meltwater collected in glacier-formed depressions. Over time, many of these lakes filled with sediments and disappeared. Others, such as Lake Chelan, remain.

West of the Cascades

On the west side of the Cascade Mountains, two lobes covered the Puget Lowlands. Along the north side of the Olympic Mountains the Juan de Fuca Lobe moved west, and between the Olympics and the Cascades the Puget Lobe moved south. In the Puget Lowlands, the Puget Lobe shaped much of the topography below 3,000 feet. As the Puget Lobe advanced, it deposited hundreds of feet of Lawton Clay, Esperance Sand, and Vashon Till. Meltwater flowing below the ice carved out troughs that we know as valleys.

Many of the area's water bodies fill these troughs, including Puget Sound, Lake Washington, Lake Union, Lake Tapps, and Lake Sammamish. 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 5,600 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. This was an extension of today's Puget Sound that reached from present-day Elliott Bay to Commencement Bay. The Duwamish Embayment existed until eroded sediment built up the White-Green-Duwamish valley floor.

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 Past

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.

Sources:
Derek B. Booth and Barry Goldstein, "Patterns and Processes of Landscape Development by the Puget Lobe Ice Sheet," in Regional Geology of Washington State, edited by R. Lasmanis and E. S. Cheney (Olympia: Washington State Department of Natural Resources Division, 1994); Arthur R. Kruckeberg, The Natural History of Puget Sound (Seattle: University of Washington Press, 1991), 1-33; David D. Alt and Donald W. Hyndman, Roadside Geology of Washington (Missoula: Mountain Press Publishing, 1990); Astrida R. Blukis Onat and Roger A. Kiers, "Ethnohistoric and Geoarchaeological Study of the SR 520 Corridor and Archaeological Field Investigations in the SR 520 Bridge Replacement and HOV Project Including the Pacific Interchange and Second Montlake Bridge Option, King County, Washington," Appendix B, Section 106 Technical Report: Vol. I Archaeology, SR 520 Bridge Replacement and HOV Program, I-5 to Medina: Bridge Replacement and HOV Project, prepared by ICF International, (Seattle: Washington State Department of Transportation, 2011), 42-53; "Geology of Washington -- Okanogan Highlands," Washington State Department of Natural Resources website accessed September 14, 2012 (http://www.dnr.wa.gov/researchscience/topics/geologyofwashington/pages/okanogan.aspx); Kathy Goetz Troost, "Geomorphology and Shoreline History of Lake Washington, Union Bay, and Portage Bay Technical Memorandum" August 2011, prepared for the Washington State Department of Transportation and the Federal Highway Administration, available online (www.wsdot.wa.gov/NR/rdonlyres/6ADE9650-8EAF-474B-B38B-6AE3D69242EA/0/SR520_GeomorphologyReport_Final_Aug2011_Part1.pdf).
Note: This essay replaces an earlier essay on the same subject.

By Jennifer Ott, September 24, 2012

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