• What were the ice sheets and glaciers of the late Pleistocene Epoch?
• What were the effects of the ice sheets on the Pacific Northwest?
  • Effects in the Puget Sound area
  • Effects in eastern Washington and northern Idaho
• What is the glacial stratigraphy of the Puget Sound region?
• Web Links
• Glossary Terms

What were the ice sheets and glaciers of the late Pleistocene Epoch?

During the Pleistocene Epoch (approximately 2 million to 10 thousand years ago), a continental glacier called the Cordilleran Ice Sheet built up, advanced and retreated several times in the mountains of western Canada. It spread out into the surrounding lowlands, including the Puget Sound region and parts of northern Washington, Idaho and Montana. The ice sheets advanced and then retreated several times, according to the sequence of sediments that the ice sheets left behind. However, the most completely understood ice sheet advance is the one that occurred most recently, near the end of the Pleistocene epoch. The most recent advance obscured much of the evidence of the earlier advances.

Farther east, at about the same times as the Cordilleran Ice Sheet advanced, the separate Laurentide Ice Sheet spread from central Canada into other parts of the northern United States, from Maine and New York to as far west as central Montana.

The Cordilleran Ice Sheet reached its last maximum advance, and then retreated from the Pacific Northwest, between 20,000 and 10,000 years ago. The last Cordilleran Ice Sheet advance is called the Fraser glaciation. Several lobes of the glacier reached south into the United States. The Vashon lobe extended into western Washington and the Okanogan and Pend Oreille lobes into eastern Washington. The Vashon lobe covered the Puget Sound region with ice as much as a mile thick. It filled the area from the Cascades to the Olympics and south to Olympia. The Okanogan lobe extended across the Okanogan Highlands and onto the Columbia Plateau. The Pend Oreille lobe crossed into the mountains just north of Spokane.

Alpine glaciation also peaked late during the last ice age

• In the mountains of the Pacific Northwest, alpine glaciation-glaciers that built up on the local high peaks and moved down the local mountain valleys-reached their maximum extent earlier than the maximum advance of the Fraser glaciation. By 18,000 years ago, the alpine glaciers were retreating up the valleys and shrinking. The Cordilleran Ice Sheet was still moving south from Canada and had not yet reached its maximum.
• In the Pleistocene epoch, during the last major advance of the alpine glaciers in the Washington Cascade Range, some glaciers extended many miles farther down-valley than they do now, and many glaciers occupied valleys that now have no glaciers at all.
• More recently, during the Holocene epoch, the glaciers in the Cascade Range advanced up to a few miles farther down their valleys than they currently extend. Since the 1800s, glaciers in the Pacific Northwest have partly retreated.
• The alpine glaciers eroded and sharpened the shapes of the higher mountains of the Pacific Northwest. In ranges such as the Cascades, Wallowas and high parts of the Rockies, most hikes around jagged, steep-sided peaks and mountain lakes at the base of high cliffs are likely to provide vistas of landscapes created by alpine glaciers. Alpine glaciers continue to accumulate new ice and flow on some of the Northwest's highest and most snowed-upon peaks.

What were the effects of the ice sheets on the Pacific Northwest?

The Cordilleran Ice Sheet shaped Puget Sound and its surrounding landscape and altered the land surface of eastern Washington north of the Columbia Plateau, northern Idaho, and northern Montana. In places where the ice sheet flowed across bedrock, such as around Victoria in British Columbia, the San Juan Islands in Washington, and the Okanogan Highlands in eastern Washington, it smoothed the mountains and hills into more streamlined shapes and created roches moutonnées.

Effects in the Puget Sound area

• The ice sheet excavated the channels of Puget Sound, Lake Washington, Hood Canal, and other long, straight valleys in the Puget Sound area.
• In many parts of the Puget Sound lowlands, the flowing glacial ice shaped loose sediments over which the ice advanced into elongate hills or ridges.
• It distributed glacial drift across the landscape, including glacial outwash sediments that make convenient supplies of sand and gravel and provide aquifers to use as a water resource.
• The glacial advance caused lakes to form and deposited clay in them. The clay that adds to the landslide hazards around the Puget Sound area was deposited then.
• The glacial ice itself deposited the glacial till that mantles much of the Puget Sound landscape. The till is dotted with glacial erratics, many of which originated in Canada.

Effects in eastern Washington and northern Idaho

• The Cordilleran Ice Sheet left piles of glacial outwash around its margins in the northern part of eastern Washington. Katabatic winds, driven by the temperature contrasts between the ice and the land, picked up and moved the finer-grained sediments from the outwash. The winds spread the silt-sized sediment across lands south of the glacier as loess deposits, and locally creating sand dunes from the sand-size sediments.
• Lobes of the glacier dammed the Columbia River at the north end of Grand Coulee, and the Clark Fork River in northern Idaho, leading to the creation of the Grand Coulee and the Channeled Scablands.

What is the glacial stratigraphy of the Puget Sound region?

The Vashon lobe of the Cordilleran Ice Sheet left a stratigraphic sequence that is widespread in the Puget Sound area. Before the glacial advance, rivers meandered across the area through vegetated floodplains, working their way north through the Puget Sound region toward what is now the Strait of Juan de Fuca. Sediments deposited before the Vashon lobe advanced formed the Olympia Interglacial deposits. Olympia Interglacial deposits are mainly alluvium.

On top of the Olympia Interglacial alluvium, clay was deposited. Apparently the Vashon glacier had advanced southward far enough to dam the north-flowing rivers, backing up the water into lakes that shifted location as the glacier advanced southward. At the bottom of these lakes, the Lawton Clay accumulated.

As the glacier moved closer to the lakes, its apron of outwash spread across the lake margins. Outwash, mostly sand and gravel, was carried and deposited by meltwater flowing away from the glacier. The formation of outwash on top of the Lawton Clay is a sequence of sand and gravel beds called the Esperance Sand.

Finally, the glacial ice itself flowed onto and covered the land. The glacier deposited lodgment till beneath its base while it was present in the area. As the glacier retreated, it continued to deposit till, but the till deposited then was not as compressed as the lodgment till.

This entire till is known as the Vashon Till. Vashon Till occurs throughout much of the Puget Sound region, commonly as the first layer beneath the soil. The Vashon Till contains clay, silt, sand, pebbles, cobbles, and boulders. The pebbles, cobbles, and boulders are large enough to be identified, and most of them originated in the mountains of Canada, where the glacier accumulated and flowed from, beginning its process of erosion, transportation, and deposition of sediments.

Most of the pebbles and larger stones on the beaches of Puget Sound have been eroded from Vashon Till and Esperance Sand, and are thus a mixture of rocks from the mountains of British Columbia.

The following diagram shows the Vashon glacial stratigraphy that is common in the Puget Sound area.

Web Links

To see an animation of the Fraser glaciation visit the Olympic National Park site at:
http://www.nps.gov/olym/glacier/glaciate.htm

USGS information on ice sheets, particularly the Cordilleran Ice Sheet, with many details on the Puget Sound region and glacial Lake Missoula floods
http://vulcan.wr.usgs.gov/Glossary/Glaciers/IceSheets/description_ice_sheets.html

Glossary terms that appear on this page: continental glacier; alpine glacier; roches moutonnées; glacial drift; glacial outwash; clay; glacial till; glacial erratic; loess; alluvium