<P> 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 . </P> <P> 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 . </P> <P> 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 . </P> <P> 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 the southern tip of Bainbridge Island and under Elliott Bay . To the south, the existence of a second fault, the Tacoma Fault, has buckled the intervening strata in the Seattle Uplift . </P>

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