Editing Puget Sound (section)

== Geology ==
[[Image:Mt Rainier distant-600px.jpg|right|270px|thumb|Snow-capped peaks are a backdrop to many Puget Sound scenes; here [[Mount Rainier]] is seen from [[Gig Harbor]].]]

Continental [[ice sheet]]s have repeatedly advanced and retreated from the Puget Sound region. The most recent [[glacial period]], called the [[Last Glacial Period#Pinedale or Fraser glaciation (Rocky Mountains)|Fraser Glaciation]], had three phases, or [[Stadial|stades]]. During the third, or [[Vashon Glaciation]], a lobe of the [[Cordilleran ice sheet|Cordilleran Ice Sheet]], called the Puget Lobe, spread south about 15,000 years ago, covering the Puget Sound region with an ice sheet about {{convert|3000|ft|m}} thick near [[Seattle]], and nearly {{convert|6000|ft|m}} at the present Canada-U.S. border. Since each new advance and retreat of ice erodes 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, Washington|Olympia]] to near [[Tenino, Washington|Tenino]], and covered the lowlands between the Olympic and Cascade mountain ranges. About 14,000 years ago the ice began to retreat. By 11,000 years ago it survived only north of the Canada–US border.<ref name=kruckeberg18-23>{{harvnb |Kruckeberg |1991 |pp=[{{GBurl |id=yqtVmRNdHWwC |pg=PA18}} 18–23]}}</ref>

The melting retreat of the Vashon Glaciation eroded the land, creating a drumlin field of hundreds of aligned [[drumlin]] hills. [[Lake Washington]] and [[Lake Sammamish]] (which are [[ribbon lake]]s), [[Hood Canal]], and the main Puget Sound basin were altered by glacial forces. These glacial forces are not specifically "carving", as in cutting into the landscape via the mechanics of ice/glaciers, but rather eroding the landscape from melt water of the Vashon Glacier creating the drumlin field. As the ice retreated, vast amounts of [[Till|glacial till]] were deposited throughout the Puget Sound region.<ref name=kruckeberg18-23/> 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 lake]]s 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 (Washington)|Black Hills]], where it drained south into the [[Chehalis River (Washington)|Chehalis River]].<ref name=baum12-13>{{cite book |last1= Baum |first1= Rex L. |last2= Godt |first2= Jonathan W. |last3= Highland |first3= Lynn |title= Landslides and engineering geology of the Seattle, Washington, area |series= Volume 20 of Reviews in engineering geology |year= 2008 |publisher= Geological Society of America |isbn= 978-0-8137-4120-8 |url= https://books.google.com/books?id=neA6HWzDUVQC&pg=PA12 |pages= 12–13 |access-date= May 7, 2016 |archive-date= September 2, 2016 |archive-url= https://web.archive.org/web/20160902235434/https://books.google.com/books?id=neA6HWzDUVQC&pg=PA12 |url-status= live }}</ref> 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 {{ill|Chimacum Valley|ceb}}, 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.<ref name=baum12-13/>

As icebergs [[Ice calving|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 erratic]]s, 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 now Puget Sound filled alternately with fresh and with sea water. The upper level of the lake-sediment Lawton Clay now lies about {{convert|120|ft|m}} above sea level.
[[File:Seattle seen from Ferry on Puget Sound.jpg|thumb|The [[Downtown Seattle]] skyline, seen from a state ferry on [[Elliott Bay]]]]

{{anchor|Basins and sills}}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 Puget Sound|South Sound]], south of the [[Tacoma Narrows]], and the [[Main Basin]], which is further subdivided into [[Admiralty Inlet]] and the Central Basin.<ref name=sotn>{{cite web|title=Features of Puget Sound Region: Oceanography and P|url=http://your.kingcounty.gov/dnrp/library/2001/kcr762/PDFELEMENTS/SONR03.pdf|publisher=King County Department of Natural Resources|access-date=August 6, 2014|location=Seattle|year=2001|archive-date=December 22, 2014|archive-url=https://web.archive.org/web/20141222091921/http://your.kingcounty.gov/dnrp/library/2001/kcr762/PDFELEMENTS/SONR03.pdf|url-status=live}}</ref> 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 {{convert|175|ft|m|disp=or|abbr=on}} below the surface), and the one at the Tacoma Narrows (about {{convert|145|ft|m|disp=or|abbr=on}}). Other sills that present less of a barrier include the ones at [[Blake Island]], [[Agate Pass]], [[Rich Passage]], and [[Hammersley Inlet]].<ref name="Kruckeberg-1991-pp61-64" />

The depth of the basins is a result of the Sound being part of the [[Cascadia subduction zone]], where the [[terrane]]s accreted at the edge of the [[Juan de Fuca Plate]] are being [[subduction|subducted]] under the [[North American Plate]]. There has not been a [[megathrust earthquake|major subduction zone earthquake]] here since the [[moment magnitude scale|magnitude]] nine [[1700 Cascadia earthquake|Cascadia earthquake]]; according to [[Japan]]ese records, it occurred on January 26, 1700. Lesser Puget Sound [[earthquake]]s with shallow [[epicenter]]s, 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]].<!-- And if this source -- not a RS! -- says otherwise, it's wrong.  See refs at 'Seattle Fault'. --><ref>{{Cite news |date=June 1, 2004 |title=Folds at surface show ancient seismic stresses still at work in Washington |author-first1=Vince|author-last1=Stricherz|url=https://www.washington.edu/news/2004/06/01/folds-at-surface-show-ancient-seismic-stresses-still-at-work-in-washington/ |publisher=University of Washington |accessdate=March 26, 2024}}</ref>{{better source needed|date=March 2024}} 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.<ref>{{cite web|url=http://www.ecy.wa.gov/programs/sea/landslides/about/geology.html|title=Puget Sound landslides|publisher=Washington State Department of Ecology|url-status=dead|archive-url=https://web.archive.org/web/20060816083102/http://www.ecy.wa.gov/programs/sea/landslides/about/geology.html|archive-date=August 16, 2006}}</ref>