Carved by Ice

20,000 years ago, Puget Sound was covered in a mile-high sheet of glacial ice. The ice sheet over Seattle alone was 3,412 feet high. As the glaciers advanced and retreated over many Ice Ages, the Sound was carved and scoured into its present form.

As the ice melted it created a huge fresh water lake whose water level was 120 feet above today’s high tide level. Eventually ocean sea levels rose and filled in the deep valleys carved during the glacial period.

You can see traces of these glaciers as scrapes carved into the exposed bedrock around the Sound, giant boulders deposited along beaches and the millions of tons of glacial sediments that are the bluffs lining Puget Sound’s shores.

 Geological History:

"Geology of Puget Sound" (Pacific Science Center)
"Nestled between the Cascade and Olympic mountains in northwest Washington, the Puget Sound basin covers more than 16,000 square miles of land and water. The basin's surface area is roughly 80 percent land and 20 percent water. Puget Sound offers a breadth of landscapes unique in the world..." (more)

"Tectonic plate motions, crustal blocks, and shallow earthquakes in Cascadia" (USGS)
"What causes earthquakes in the Pacific Northwest? Motions of earth's tectonic plates cause earthquakes along plate boundaries..." (more)

Glaciation

"Puget Sound Under Ice" (Washington State Dept. of Ecology)
"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)..." (more)

"Frasier Glaciation" (Olympic National Park)

"The Fraser glaciation lasted about 10,000 years and consisted of 3 stades (periods of ice expansion) and 2 interstades (ice recession)..." (more)

"Reconstructing the last (Vashon) continental glaciation of the Puget lowland" (Pacific Science Center)
A twenty second movie recreating the event. (view)

"Glacial History of Puget Sound" (University of Washington)
A glacial advance map. (view)

The Shoreline

"Puget Sound Shorelines: Tour - Land and Sea" (Washington State Dept. of Ecology)
"Along Puget Sound, exploring a labryrinth of inlets, coves, and bays, we witness the work of natural forces on the terrain: rocky headlands, steep bluffs, forested slopes, deltas, estuaries, salt marshes, tide flats, spits, and beaches made of rocks, silt, and sand..." (more)