Geology of the Olympic Peninsula

The oldest rocks of the Olympic Peninsula date back about 50 million years, making the peninsula a relatively new addition to the west coast of North America. The peninsula has been shaped by incredibly powerful forces: the grinding intersection of pieces of the earth’s crust, the advance and retreat of ice sheets, and constant erosion by water.

When plates collide

USGS image

The science of plate tectonics describes the slow collisions of  huge pieces of the earth’s crust moving on molten rock. Offshore to the west, undersea volcanoes spewed out hot basalt lava that cooled to become the ocean floor of the Juan de Fuca plate. Floating ponderously on the magma of the molten core, that plate moved eastward, collecting sediment washed into the ocean by rivers on the North American continental-crust plate. About 34 million years ago, those two plates collided. In a process called subduction, the heavier Juan de Fuca plate moved under the North American plate. It is moving still.

During subduction, both original basalt and collected sediment of the Juan de Fuca Plate were scraped off and folded into layers onto the North American plate. The process formed today’s landscape, which began to rise above water about 12 million years ago.

Basalt on Mt. Constance

As the plates ground together, The Juan de Fuca plate’s native basalts formed into what’s called the peripheral horseshoe that rings the north, east and south parts of the Olympic Mountains. The mountain ridge along the east side of the upper Dungeness River headwaters, ranging up to Mount Constance at 7700 feet (2347 meters), contains some of those basaltic peripheral rocks.

The core rocks of the Olympic Mountains originated from the sediment collected by the Juan de Fuca plate and scraped off onto the continental crust plate. Blue Mountain, rising 6007 feet (1831 meters) above the northeast corner of the Gray Wolf basin, is a sedimentary formation.

Faults at Hurricane Ridge and Gray Wolf Ridge mark the processes that separate the core rocks and the peripheral basalt. Subduction continues to add folds of material to uplift the land, while erosion continues to carry away the oldest sediments from the top.


It has probably been less than 20,000 years since the Sequim area was covered by 4,000 feet of ice. During the Pleistocene ice age (about 1.8 million to 12,000 years ago) as many as six major glaciations moved over the Olympic Peninsula, with ice sheets coming down from western Canada, receding, and returning.

Alluvial sediments

Traces of the Possession Glaciation (about 80,000 years ago) are visible in the lowest layers of the bluff at Port Williams, east of Sequim. During the Fraser Glaciation (25,000-10,000 years ago), the ice sheet reached up the Dungeness River about 24 miles, nearly to Royal Creek, on its journey southward beyond what is now the city of Olympia and westward beyond Cape Flattery. Locally, Burnt Hill, Ned Hill, most of Deer Ridge and Bear Mountain were covered, with Mt. Zion and Maynard Peak nearly overtopped as well.

Hikers can see granite boulders above the 3600-foot level of Blue Mountain and Maynard Peak. Geologists conclude that the granite, which is not part of the peninsula’s native rock, was deposited by glaciers. The Gold Creek basin also contains glacial deposits, with other deposits just below the Forest Service road to the Upper Dungeness trailhead, along the canyon in the Three-O’Clock ridge area.

The Dungeness River’s wide valley may owe its shape to the glaciers: it was probably cut by glacial melt waters, perhaps even while the ice sheet remained overhead. A deep test well drilled northeast of Sequim showed more than 2,000 feet of glacial-related deposits above the bedrock. Other glacial material, carried downstream by the Dungeness River, can be found in the river’s alluvial deposits.

Deception Glacier

About 10,000 years ago, there occurred a global warming followed by several cooler periods. During one of these cool periods known as The Little Ice Age (1450-1890), many glaciers in the Olympics, including the Upper Dungeness Royal Basin glacier and the Gray Wolf glacier, re-established or extended their bases. Deception Glacier at the head of the upper Gray Wolf River is the main glacier still existing in the Dungeness River’s watershed.

[NOTE: much of the material in this article is adapted from Chapter 4 of  “Keys to an understanding of the natural history of the Dungeness River System” (1996, Welden & Virginia Clark).


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The Geology of the Olympic Peninsula

by Welden and Virginia Clark

The Evolution of the Olympics

This is interesting and unusual territory, only beginning to be understood in the light of the plate tectonics research of the last half of the 1900s.  The Olympic Peninsula is a ‘young’ addition to the west coast of North America, with oldest rocks dating back only about 50 million years.  The prime mover in the system has been the Juan de Fuca Plate of oceanic crust moving toward the east from a ‘mid-ocean rise’ of magma deep in the earth.   The oceanic plate, being more dense, dives under the North American continental-crust plate in a subduction zone and trench that started about 34 million years ago at this location.  As the oceanic crust slab slants down deeper under the continent, the process produces basalt flows, Cascade volcanoes, granite batholiths, and major earthquakes.

Simplified diagrams of the Olympics are illustrated in Figure 1.  The ‘peripheral horseshoe’ basalts form the present north, east, and south portions of the Olympic Mountains.  One explanation suggests they were seamounts on the oceanic crust, scraped off to the continental crust from the subducting slab.  Another theory identifies them as remnants of an exotic, island terrane (subcontinental landmass) that docked to North America.  Other recent research suggests they were extruded in place about 50 million years ago from fissures in a forearc basin, concurrently with sediments (the Blue Mountain Formation) being deposited in submarine canyon flows from ancestral rivers.  The peripheral basalts were probably more widespread over the Peninsula before the uplift of the core rocks and subsequent erosion displaced them.

The ‘core rocks’ of the Olympic Mountains are mostly sediments from the subducting oceanic crustal slab, scraped off and ‘underplated’ on the bottom of the continental crust.  This stacking of successive scrapes thus continually thickens and raises the older, top surface.  Arching of the subducting plate under the Olympics adds to the uplift, while erosion eats away the oldest, top sediments.  The top surface at Mount Olympus was underplated by subduction off the oceanic slab about 17 million years ago.  It is about 30 km (18 miles) above the subducting slab now. Another 10 to 12 km of the underplated sediments from the subducting slab have probably have been eroded off the top, together with some of the pre-existing peripheral basalts.

The Olympic terrain probably began emerging above water about 12 million years ago as a consequence of the continuing uplifting.  The broad dome of the Olympic Mountains has since been dissected by glaciation and river erosion. The present topography of deep canyons and valleys carries rivers radiating to the Pacific Ocean, the Strait of Juan de Fuca, Hood Canal, and inner Puget Sound.


Glaciation and Climate Change

The global climate began to slowly cool about 50 million years ago.  By about 10 million years ago the Antarctic ice sheet had formed and high mountain glaciers were forming.  As many as six major glaciations, with ice sheets coming down from western Canada, impacted the Olympic Mountains and lowlands during the 2 million years of the Pleistocene ice age.  The latest, the Fraser Glaciation, lasted from about 25,000 to 10,000 years ago.  The traces of the earlier ones are not evident here, only the last two.  The Possession Glaciation, about 80,000 years ago, is identifiable in the lowest visible layers of the bluff at Port Williams.


The Vashon episode of the Fraser Glaciation

The consequences of the Fraser Glaciation are evident almost everywhere in our local terrain.  The earliest episode of the Fraser was most evident in mountain glaciers.  The round-bottom canyons such as upper Cameron Creek likely date from this, though the lower portions of the canyons have since been cut into a V-shape from subsequent stream flows.  The most obvious effects for us come from the latest episode of the Fraser glaciation, called the Vashon Stade, roughly 17,000 to 13,000 years ago.

The Vashon ice sheet originating in British Columbia moved down through Georgia Strait on a base of advance outwash sands and gravels, then proceeded south through the Puget Lowland to below the present city of Olympia.  It also extended out the Strait of Juan de Fuca to beyond Cape Flattery.  The western edge of the Puget Lobe crossed over the northeastern Olympic Peninsula (nearly 4,000 feet thick over present Sequim) and up the Dungeness River canyon about 24 miles, nearly to Royal Creek.   Burnt Hill, Ned Hill, most of Deer Ridge, and Bear Mountain were overtopped, and Mt Zion and Maynard Peak nearly so.  Glacial erratic granite boulders were left above the 3600 foot level on Blue Mountain and Maynard Peak.  Evidence suggests an ephemeral glacial lake dammed by the glacier at near 3300 ft elevation.  The Gold Creek basin leading to Bon Jon Pass shows deposits of advance outwash, glacial tills, glacial lake beds, and outwash from the glacier.  The Forest Service road to the Upper Dungeness trailhead sits just above the glacial deposits along the canyon in the Three-O’Clock Ridge area.

In the lower watershed between the foothills and the Strait the Vashon ice sheet left all the usual glacial evidence over essentially the whole Sequim/Dungeness peninsula.  A cliff-forming top deposit of Everson glaciomarine drift on some of the bluffs (such as along the downstream part of McDonald Creek) was apparently deposited from glacier breakup in rapidly warming climate and rising sea levels.  The wide river valley of the lower Dungeness was presumably cut by glacial water outflows, possibly while the ice sheet remained overhead, as reportedly happened for the deep basins in the Puget lowland.

The Sequim/Dungeness peninsula has apparently been formed by at least four ice sheets, the Vashon being the latest.  A deep test well drilled northeast of Sequim showed over 2,000 feet of glacial-related deposits above the bedrock.  The alluvial deposits along the river valley contain much reworked glacial drift from further upstream.   There is no known native granite rock in the Olympics, so all of the gray/white speckled stuff is exotic, brought by the ice sheets.


Changing climate since the Pleistocene ice age

Global warming occurred rapidly at 10,000 years ago, but was later followed by several cooler periods.  Another milder warming, known as the Medieval Optimum, occurred from 1100 to 1300 (thought to be near 1980 temperature levels).  During the Little Ice Age of 1450 to 1890, many glaciers in the Olympics, such as the Upper Dungeness/Royal Basin and the Gray Wolf, extended or re-established themselves in their respective basins.  Deception Glacier at the head of upper Gray Wolf River is the main one still existing in the watershed.  The warming trend since the late 1800s, as discussed in another article, Dungeness River Flows, may cause the complete melting of this last remnant of the Little Ice Age.


The Northeast Olympics and the Dungeness Watershed

Figure 2 is an aerial depiction of the northeast Olympics, looking northeast toward the Gray Wolf and upper Dungeness basins.  It shows the deeply dissected terrain, especially the canyons of the Elwha and Dosewallips Rivers that border the Dungeness/Gray Wolf system. 

The mountain ridge along the east side of the upper Dungeness headwaters is part of the basaltic peripheral rocks.  It ranges up to Mount Constance at 7700+ ft elevation.  Mount Deception at the southwest corner of Royal Basin is the second highest in the Olympics at nearly 7,800 ft (Mount Olympus, west of the Gray Wolf basin, is the highest at nearly 8.000 ft.).  Blue Mountain at the northeast corner of the Gray Wolf basin represents the sedimentary Blue Mountain Formation.  The Hurricane Ridge and Gray Wolf Ridge faults identify the faulting and uplift processes separating the core rocks and the peripheral rocks.

The eastern Olympics are dryer and more rugged than the western rainforests.  The upper basins of the Dungeness only get about 60 inches annual precipitation, compared to 240 inches west of Mount Olympus, as is illustrated in Figure 3.  Most of the wet-season storms come off the Pacific Ocean from a west-southwest direction, so the multiple ridges of the Olympics provide the rainshadow effect that characterizes the Dungeness Watershed (see the section on climate for more discussion of the rainshadow.

NOTE: much of the material in this article is adapted from Chapter 4 of  “Keys to an understanding of the natural history of the Dungeness River System”, (1996, Welden & Virginia Clark).