Puget Sound Region - Physical Geography

Physical Geography

The United States Geological Survey (USGS) defines Puget Sound as a bay with numerous channels and branches; more specifically, it is a fjord system of flooded glacial valleys. Puget Sound is part of a larger physiographic structure termed the Puget Trough, which is a physiographic section of the larger Pacific Border province, which in turn is part of the larger Pacific Mountain System.

Puget Sound is a large salt water estuary, or system of many estuaries, fed by highly seasonal freshwater from the Olympic and Cascade mountain watersheds. Puget Sound is connected to the Strait of Juan de Fuca to the north at Admiralty Inlet, between Point Partridge on Whidbey Island and Point Wilson on the Olympic Peninsula. A second connection is Deception Pass, between West Point on Whidbey Island and Rosario Head on Fidalgo Island.

Continental ice sheets have repeatedly advanced and retreated from the Puget Sound region. During the most recent glacial period, called the Fraser Glaciation, which had three phases, or stades. During the third, or Vashon Glaciation, a lobe of the Cordilleran Ice Sheet, called the Puget Lobe, spread south about 15,000 years ago, covering the Puget Sound region with an ice sheet about 3,000 feet (910 m) thick near Seattle, and nearly 6,000 feet (1,800 m) at the present Canada-US border. Since each new advance of ice scours away much of the evidence of previous ice ages, the most recent Vashon phase has left the clearest imprint on the land. At its maximum extent the Vashon ice sheet extended south of Olympia to near Tenino, and covered the lowlands between the Olympic and Cascade mountains. About 14,000 years ago the ice began to retreat. By 11,000 years ago it survived only north of the Canadian border.

The Vashon Glaciation scoured the land, creating a drumlin field of hundreds of aligned drumlin hills. Lake Washington and Lake Sammamish (which are ribbon lakes), Hood Canal, and the main Puget Sound basin were carved out by glacial forces. As the ice retreated, vast amounts of glacial till were deposited throughout the Puget Sound region. The soils of the region, less than ten thousand years old, are still characterized as immature.

As the Vashon glacier receded a series of proglacial lakes formed, filling the main trough of Puget Sound and inundating the southern lowlands. Glacial Lake Russell was the first such large recessional lake. From the vicinity of Seattle in the north the lake extended south to the Black Hills, where it drained south into the Chehalis River. Sediments from Lake Russell form the blue-gray clay identified as the Lawton Clay. The second major recessional lake was Glacial Lake Bretz. It also drained to the Chehalis River until the Chimacum Valley, in the northeast Olympic Peninsula, melted, allowing the lake's water to rapidly drain north into the marine waters of the Strait of Juan de Fuca, which was rising as the ice sheet retreated.

As icebergs calved off the toe of the glacier, their embedded gravels and boulders were deposited in the chaotic mix of unsorted till geologists call glaciomarine drift. Many beaches about the Sound display glacial erratics, rendered more prominent than those in coastal woodland solely by their exposed position; submerged glacial erratics sometimes cause hazards to navigation. The sheer weight of glacial-age ice depressed the landforms, which experienced post-glacial rebound after the ice sheets had retreated. Because the rate of rebound was not synchronous with the post-ice age rise in sea levels, the bed of what is Puget Sound, filled alternately with fresh and with sea water. The upper level of the lake-sediment Lawton Clay now lies about 120 feet (37 m) above sea level.

The Puget Sound system consists of four deep basins connected by shallower sills. The four basins are Hood Canal, west of the Kitsap Peninsula, Whidbey Basin, east of Whidbey Island, South Sound, south of the Tacoma Narrows, and the Main Basin, which is further subdivided into Admiralty Inlet and the Central Basin. Puget Sound's sills, a kind of submarine terminal moraine, separate the basins from one another, and Puget Sound from the Strait of Juan de Fuca. Three sills are particularly significant — the one at Admiralty Inlet which checks the flow of water between the Strait of Juan de Fuca and Puget Sound, the one at the entrance to Hood Canal (about 175 ft or 53 m below the surface), and the one at the Tacoma Narrows (about 145 ft or 44 m). Other sills that present less of a barrier include the ones at Blake Island, Agate Pass, Rich Passage, and Hammersley Inlet.

The depth of the basins is a result of the Sound being part of the Cascadia subduction zone, where the terranes accreted at the edge of the Juan de Fuca Plate are being subducted under the North American Plate: there has not been a major subduction zone earthquake here since the magnitude nine Cascadia Earthquake; according to Japanese records, it occurred 26 January 1700. Lesser Puget Sound earthquakes with shallow epicenters, caused by the fracturing of stressed oceanic rocks as they are subducted still cause great damage. The Seattle Fault cuts across Puget Sound, crossing just north of Vashon Island and dipping under the city of Seattle. To the south, the existence of a second fault, the Tacoma Fault has buckled the intervening strata in the Seattle Uplift.

Typical Puget Sound profiles of dense glacial till overlying permeable glacial outwash of gravels above an impermeable bed of silty clay may become unstable after periods of unusually wet weather and slump in landslides.

The Puget Sound region has been called "Ish River country", perhaps most famously by poet Robert Sund, owing to its numerous rivers with names ending in "ish", such as the Duwamish, Samish, Skokomish, Skykomish, Snohomish, and the Stillaguamish. The ish ending is from Salish language meaning "people of".