Puget Sound Native Tree
Botanical Garden
Geology of
Bainbridge Island
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The Puget Sound Native Tree
Botanical Garden was originally
known as Dolphin Place
​The Pacific Northwest's landforms, like the Olympics, Cascades, and Puget Sound, are
built by terranes (chunks of oceanic crust, island arcs) sticking to North America's edge via accretion over millions of years as the oceanic Juan de Fuca plate subducts beneath the North American plate. This process, occurring in phases since the Jurassic, scraped off marine sediments and volcanic layers, creating mountain belts and adding significant land, forming the diverse geography seen today.
Key Processes & Features:
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Accretionary Prism: Sediments and scraped-off oceanic crust pile up at the continental margin, forming coastal ranges like the Olympics and Coast Ranges.
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Terrane Docking: Exotic blocks (terranes like the Okanogan, Insular) carried on subducting plates collide and weld onto the continent, adding land and forming mountain belts like the North Cascades.
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Volcanic Arcs: Subduction melts the mantle wedge, creating magma that forms volcanic arcs, most notably the active Cascade volcanoes.
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Forearc Basins: Low-lying areas between the coastal ranges and the volcanic arc, like the Puget Sound/Willamette Valley, are filled with sediments from erosion.
Major Terranes & Events:
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Jurassic (200-170 Ma): Early terranes, like the Intermontane Belt (Okanogan, Quesnellia), docked, adding significant land and ending earlier subduction zones.
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Cretaceous (150-100 Ma): More terranes added, contributing to the Okanogan terrane's collision and growth of the continent.
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Cenozoic (50 Ma): The North Cascades terrane and others (Olympic, Insular) docked, shaping the modern PNW coastline and islands (Vancouver Island, San Juans).
Resulting Landforms:
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Coastal Ranges: Olympic Mountains, Coast Range (from accretion/scraping).
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Inland Mountains: North Cascades (from terrane collisions).
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Volcanoes: The Cascade Volcanoes (from subduction).
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Lowlands: Puget Sound, Willamette Valley (forearc basins).
In essence, the PNW is a collage of pieces from ancient oceanic islands, seamounts, and microcontinents plastered onto North America, creating its complex and mountainous landscape.
That being said, the most recent geological activity in Washington State has been the eruptions and building of the Cascade volcanoes and the last Ice Age.
The Pleistocene Epoch, spanned roughly 2.6 million to its final cold phase ending about 11,700 years ago, marking the start of our current warm "interglacial" period.
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The photo to the left is an example o a small glacial erratic (loose) rock showing scraping scars resulting from a glacier.
The photo to the left is the rocky beach on the South end of Bainbridge Island. The majority of the local beaches are gravels, sands, and mud from the constant erosion of glacial sedimentary remains.
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The bedrock of this beach likely remains exposed due to the active Seattle fault which is nearby and the tectonic forces that are driving these rocks upwards.
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Investigations reveal that this massive sedimentary rock has been tilted from horizontal by the many historical earthquakes, including one 1100 years ago that in one event lifted the interie beach and Restoration Point approximately ten feet towards the sky.
To the left is a photo of one the few fossil types found in Puget Sound region. They are contained in a marine sandstone formation adjacent to the western edge of one of the terranes that acreted against the North American Plate many million years ago.
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This specimen contains the remains of sea shells, agatized worm casings and carbonized wood, likely 25-40 Million years ago.