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Fidalgo
An Island Through Time

Fidalgo Island Field Trip: Geology / Science 111
North Seattle Community College
J. Figge 2009
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Fidalgo
An Island Through Time

Fidalgo Island Field Trip: Geology / Science 111
North Seattle Community College
J. Figge 2009
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© John Figge 2009
Fidalgo
An Island Through Time
J. Figge 2009
Figure 1 (Above) The west coast of Fidalgo Island, from Washington Park
Figure 2 (Above left) Sunset from Fidalgo Island. Burrows (right) and Allen (left) Islands in the foreground.
1
Introduction
Ancient Islands and the Geology of the Pacific Northwest
In his classic textbook on the Geology of the Northwest, Bates McKee (1980) never even mentioned the subject of
ancient Pacific island groups. He never wrote about island volcanoes, and only talked about the Pacific Basin in passing. In over 350 pages of text, he never once made a connection between these subjects and the geology of the northwest. Accordingly, one might easily assume that this is not an important part of the geologic story here.
McKee wrote his textbook just as the modern theories of “plate tectonics” were emerging, theories which quite literally revolutionized the science of geology. Since that date, we have learned that the geologic story of this region is a
much more exotic tale than we could have ever expected, and a more intriguing story than we could have imagined.
In the end, a big part of that story includes ancient volcanic island groups of the North Pacific basin. As it turns out,
much of the Pacific Northwest is floored in rocks of these origins. These are an important component of our regional
geology, and an important key to understanding its history.
As prevalent as these rocks are, they are rarely preserved as coherent or representative sections. Repeated episodes of
mountain-building, deformation and magmatism have taken a toll on these ancient rocks, and most have been broken
down into dismembered units. Most are not particularly durable rocks, and have not weathered the passage of time
very well. Where they are preserved, they typically exist as regional-scale features, spread over large expanses of the
landscape. They don’t lend themselves well to casual observations on an afternoon field trip.
One very convenient exception to this rule, and a very scenic one at that, is found on Fidalgo Island, west of Mount
Vernon. Fidalgo Island is the southernmost of the San Juan Islands, and is connected to the mainland by State Route
20. It is the remains of a Pacific island which developed in Jurassic time, during the rise of the dinosaurs. It is a fairly
small and compact section of representative rocks, conveniently tilted on its side so that one can traverse through
the entire thickness of the unit. It is something on the order of 6 km thick, and contains most of the components of
the original volcanic island. It is a convenient, representative, and very scenic venue for exploring the geology of the
region.
2
Figure 3 (Above) Burrows Bay from Fidalgo Head (Washington Park)
3
Table of Contents
Introduction........................................................................................................................................................2
The Fidalgo Island Field Trip.............................................................................................................................5
Island-Building 101............................................................................................................................................7
Oceanic Crust and Hot-Spot Volcanic Islands........................................................................................7
Volcanic-Arc Islands..............................................................................................................................8
Ancient Volcanic Islands in the Evolution of the Pacific Northwest................................................................10
Exotic Terranes.....................................................................................................................................10
Continental Arcs...................................................................................................................................12
The Strange Tale of Ancestral Fidalgo Island..................................................................................................13
Ancestral Fidalgo Island......................................................................................................................13
The Later Evolution of the Melange Belts...........................................................................................16
Sculpting the Modern Landscape.........................................................................................................17
Summary..............................................................................................................................................18
The Geology of Fidalgo Island........................................................................................................................19
Modern Day Fidalgo Island.............................................................................................................................22
Fidalgo Island Field Trip...............................................................................................................................26
1: Washington Park: .........................................................................................................................27
Mantle Rocks of the Basal Fault
2. Marine Heights...............................................................................................................................29
Diorite of the Mature Volcano
3. Marine Drive..................................................................................................................................31
Gabbro of the Early Volcano
4. Red Rock Quarry...........................................................................................................................33
Marine Sediments Deposited Around the Island
5, Rosario Beach.................................................................................................................................35
Rocks from a Different Thrust Sheet
Lunch
6. Deception Pass................................................................................................................................37
Sedimentary Rocks of the Submarine Island Slope
7. Mount Erie Road............................................................................................................................39
Volcanic Rocks of the Early Volcano
8. Mount Erie......................................................................................................................................41
Plutonic Rocks of the Mature Volcano
Summary..........................................................................................................................................................43
Credits and References.....................................................................................................................................47
A Last Word.....................................................................................................................................................49
4
Vancouver Island
Bellingham
San Juan Islands
Strait of
Juan De Fuca
Fidalgo
Island
Whidbey Island
20
20
Mt. Vernon
Puget Sound
Everett
5
Seattle
The Fidalgo Island Field Trip
Fidalgo Island has always been one of the more popular geologic field trips in the Puget Sound region. Starting in the
late 1970’s, geologists described these rocks as part of an ocean-floor assemblage, known as an “ophiolite complex.”
Accordingly, most references describe this as the “Fidalgo Ophiolite.” More recently, geologists have recognized that
only the bottom-most sections here look like oceanic crust. Most of rocks are much more typical of a volcanic island
group, As a whole, it offers what appears to be a relatively complete cross-section from the ocean floor to the island
surface. As such, it represents an ideal locale to illustrate the evolution and composition of these important geologic
features.
In addition to providing a unique geologic setting, Fidalgo has numerous attributes which contribute to its popularity
as a destination. It lies only a hour north of Seattle on Interstate 5, and is conveniently accessed by State Route 20
heading west out of Mount Vernon. It lies in the rain shadow of the Olympic Mountains, receiving barely half the precipitation as does Seattle. It features three major parks (Washington Park, Mount Erie, and Deception Pass State Park),
which afford public exposures of these unique rocks.
Figure 4 (Above) View of the northern Puget Sound and the San Juan Islands, showing the regional setting of Fidalgo Island.
Base image from Google Earth.
Figure 5 (Right) Fidalgo Island from the south, illustrating the major geographic features. Base image from Google Earth.
5
But most of all, the island setting offers idyllic venues on the Puget Sound, on the San Juan Islands, and of the mountains to the east and west. It holds a strategic position at the head of the Strait of Juan De Fuca, between the islands
of the Puget Sound to the south, and the San Juan Archipelago to the north. Along its western shores, the vegetation
reflects the winter storms which rake the coast from off the Strait. It’s coastline is marked by numerous bays and
points, each offering a different perspective on this unique setting. The island hosts a number of substantial lakes, in
surroundings which vary from natural to pastoral. Finally, the center of the island is marked by a substantive mountain
(Mt. Erie), which affords a commanding view on this islandic landscape.
Finally, the trip offers a visit to a unique and historic maritime community, one which dates back over a century.
Anacortes is the home of much of the Alaskan fishing fleet, a terminal for the Washington State Ferries, Washington’s
major port for Alaskan oil, and a center for services throughout the San Juan Islands. Since 1935, it has also served as
the gateway to Whidbey Island, via the spectacular Deception Pass bridge. That highway extends the length of Whidbey Island, to the terminal for the Clinton - Mukileteo ferry.
For these reasons, and others, the Fidalgo Island Trip has always been among the most popular of local geologic excursions.
Cypress Island
Guemes Island
Washington Park
Anacortes
March
Point
Fidalgo Bay
Burrows Bay
Fidalgo Island
20
Campbell Lake
Pass Lake
Rosario
Head
Bowman Bay
Deception
20
Hoypus Point
Pass
Whidbey Island
6
Mid-Ocean Ridge
(Spreading Center)
“Hot Spot” Volcano
Oceanic Crust
New oceanic
plate is formed,
drivng adjacent
plates laterally.
Linear “curtain” of
magma rises to the
surface
Single plume
of magma
rises to the
surface
Figure 6 (Above) The formation of oceanic plates at mid-ocean ridges, and the eruption of hot-spot volcanoes. Both develop
from gabbroic / basaltic magmas, rising from deep in the earth.
Island-Building 101
Oceanic Crust and Volcanic “Hot Spot” Islands
In large part, islands are typically composed of volcanic rock, the crystallized product of magma (aka “liquid rock”)
where it has erupted on the surface of the planet. In the ocean basins, that surface is a thin (7-10 km) section of ocean
crust, which overlies denser rocks of the Earth’s mantle. This oceanic crust is being continuously generated along the
great “mid-ocean ridges,” where magma rises to the surface along linear fissures.
The type of magma which erupts along the mid-ocean ridges forms dark rock which is rich in metals like iron and
magnesium. When it cools slowly deep beneath the surface, it forms a greenish rock with large crystals (a plutonic
rock) called “gabbro.” When that same magma erupts on the surface, it cools rapidly to form a black microcrystalline
(volcanic) rock called “basalt.” Where these rocks erupt to form new oceanic crust, they cool into a particular arrangement which is typical of such an assemblage. That arrangement is called an “ophiolite complex,” and it is the signature
characteristic of the oceanic crust.
Where gabbroic / basaltic magmas erupt along the linear fissures of the mid-ocean ridges, they accumulate new oceanic “plates” which are driven away from the ridges. This process does not produce volcanic islands, just flat ocean
crust. In places, however, that same magma rises to the surface at a single point, known as a “hot spot.” There are
several hundred such points on the globe. Here, basaltic lava erupts and builds up on the ocean floor, until it finally
rises as an island. This is how the Hawaiian Islands have been formed, from a “hot spot” in the middle of the North
Pacific Basin. Because the location of the hot spot is relatively fixed, while the oceanic plate is in motion, it produces
a chain of volcanic islands, getting older in the direction of plate motion. Above the Hawaiian Hot Spot, the Pacific
Plate moves to the northeast.
7
Gabbro
Basalt
Figure 7 (Above) Gabbro (left) and Basalt (Right). These rocks crystallize from the same magma, and are made up of the same
minerals. Gabbro crystallizes at depth in the crust, and forms large crystals over a long period of cooling. Basalt erupts on the
surface, and crystallizes rapidly. Acordingly, it is made up of very small crystals. Rocks which crystallize at depth are called “plutonic” varieties, while those which erupt on the surface are called “volcanic” varieties.
Figure 8 (Below) The formation of volcanic-arc islands above subduction zones. Partial melting of the subducting plate generates
magma which rises into the oceanic plate above, forming a chain of island volcanoes.
Volcanic-Arc Islands
Most volcanic islands are not, however, the result of “hot spots.” Instead, they develop where two oceanic plates collide, forcing one underneath the other in a process known as “subduction.” As the descending plate is forced to depths
of 75-100 km, it begins a process of partial melting. From that point, magma rises into the overlying plate above,
and erupts on the surface as a volcano. Over time, that lava builds up to emerge as a volcanic island. Because these
develop along the length of the subduction zone, a linear chain of volcanic islands develops by this process. This belt
is known as a volcanic “island arc”.
Deep Ocean
Trench
VolcanicArc Islands
Plate A
Plate B
Subducting
Oceanic Plate
Partial melting of
subducting oceanic
plate
8
Mature-stage
Andesite Stratovolcano
Mature-stage
Diorite Plutons
Sediments eroded
off growing volcano
Original
Basalt shield volcano
Basalt
Gabbro
Dikes of younger rocks
cut older gabbro.
Original Gabbro
magma chamber
Mantle
Figure 9 (Above) Generalized cross-section of an island-arc volcano.
The entire western margin of the Pacific Basin is ringed with volcanic island chains produced in this process. The
Aleutian Islands of Alaska, the Islands of Japan, and the innumerable island chains of Indonesia and the south Pacific
have developed where oceanic crust of the Pacific Plate is being forced beneath other oceanic rocks to the west.
Because the melting rock in the oceanic plate is basalt, the resulting magma is initially also basaltic in composition.
Young island groups are composed of basalt lavas, just like hot-spot volcanoes. This hot, thin magma spreads quickly,
accumulating into a broad, low-profile feature called a “shield volcano.” Most volcanic islands start out by building a
broad shield volcano as their base.
Over time however, the island groups grow in thickness. As this happens, the magma has a longer chance to cool as
it rises to the surface. As a result, some of the iron and magnesium- rich minerals are left behind. The type of rock
produced in this setting is called andesite when it erupts on the surface, and diorite when it cools at depth. These are
the rock types of a mature volcanic island chain. Andesite is much cooler and thicker than basalt, something broadly
resembling toothpaste in its consistency. It usually doesn’t flow far, so it starts to build up a steep summit cone.
Island chains often form from repeated episodes of volcanism, separated by periods during which they are eroded by
the forces of water and gravity. On the central portions of the island, rocks are reduced to gravel-sized particles, while
finer sediments (e.g. sand) accumulate along the beaches. Still finer particles accumulate in the shallows around the
island, including volcanic ash from periodic eruptions. These sediments are destined to become conglomerates, sandstones and mudstones, in this case all of volcanic origins. In the deeper waters along the edge of the island, reef-type
environments develop in warm-water settings. Eventually, these features become limestone deposits.
During periods of eruption, flows often extend over sediments accumulated between eruptive periods. Around the
volcanic vents, extensive debris fans often form. Where they extend into the water, they contribute to submarine
landslides which flow to the ocean floor. Elsewhere, andesite magma erupts in blocky flows which build up a conical
edifice. Eventually, it assumes the classical lines of a composite stratovolcano. In many cases, that steep-sided stratovolcano is built on an older, low-profile shield volcano.
9
Figure 10 (Above) Diorite (Left) and Andesite (Right). Diorite is the plutonic variety, with large crystals indicating prolonged
cooling at depth. Andesite is the volcanic variety, with small crystalls reflecting rapid cooling on the surface. Andesite is a much
thicker, pastier magma than basalt, and accumulates to form stratovolcanoes. Diorite cools deep beneath such volcanoes.
Because andesite is a much thicker, more viscous material, it doesn’t release dissolved volcanic gasses as easily as
does basalt. In andesite eruptions, those gasses are often released explosively. These explosive events produce volcanic ash, sand-sized blast debris, cinder-sized and boulder-sized ejecta, and a number of other eruptive materials.
Sometimes these are released as glowing pyroclastic debris avalanches, which sweep down the volcano slopes. At
other times, they rain down over region as a whole.
These are the processes by which most island chains accumulate. As along as magma erupts at a rate faster than the
elements break down the island, they continue to grow. As long as the adjoining oceanic plate is being subducted
beneath them, a continued supply of magma is assured. When that process stops, the processes of weathering and erosion become dominant. Eventually, the island will be eroded back down to a level beneath the waves.
Ancient Islands in the Geologic Evolution
of the Pacific Northwest
Exotic Terranes
The Pacific Northwest is a geologically young province, having been accumulated entirely over the last 200 million
years. It starts on the edge of the ancestral North American Continent (a point not far west of Spokane), and extends
west to the Pacific margin. From roughly the Columbia River on the south, it extends north to include most of British
Columbia and Alaska, as far north as the Brooks Range. All of the rocks in this region have been added to the edge of
the continent, added over the last 200 million years.
This process had its origins in the breakup of the ancestral “supercontinent” of Pangea, a single accumulation of all
the continents which developed about 300 million years ago. About 200 million years ago it started to break up, along
a line where the Atlantic Ocean now stands. For the last 200 million years, the eastern and western hemispheres have
been moving apart, as the Atlantic Basin expands between them. Since that date, North America has been moving
slowly to the west.
10
Future Pacific
Northwest
Panthalassa Ocean
Tethys Sea
Pangaea
Figure 11 (Above) The supercontinent of Pangaea, as it appeared about 200 million years ago. Note the absence of land in the
Pacific Northwest. This entire region has been added to the continent over the last 200 million years.
As North America has been moving west, the oceanic plates of
the Pacific Basin have generally been moving to the east. On
that oceanic plates however, there were a succession of large
volcanic-island belts to the west of North America. As the continent moved westward, and the oceanic plate moved eastward,
those island-belts eventually arrived along the edge of the continent. When this happened, those island belts were added to the
edge of the continent. This is a process known as “accretion,”
and it has added over 500 km of new land to the margin of the
continent over the last 200 million years. This is the major process by which the continent has grown over this period.
The first and largest of these island groups to be added were the
Intermontane Islands. These islands were accreted as a large belt
of terranes about 180 million years ago, in Mid-Jurassic time.
The Intermontane Belt in Washington extends from Kettle Falls
on the northern Columbia River as far west as Winthrop (roughly, Wenatchee). It is the remains of a volcanic island chain that
developed over Late Triassic to Mid-Jurassic time, from roughly
220 to 180 million years ago.
Figure 12 (Left) The Geologic Time Scale, courtesy of the US Geological Survey. The events surrounding the evolution of Fidalgo Island
start in Mid-Jurassic time.
11
The next island belt to arrive on our shores was
known as the Insular Belt. It was accreted to the
continent about 115 million years ago, and makes
up the western margin of British Columbia. It was
an older and more diverse group of islands than
was the Intermontane Belt, but these rocks are still
typical volcanic island and ocean-floor types. In
Washington, they make up part of the “core” rocks
of the modern North Cascades.
Intermontane
Belt
The next belt of rocks added to the continent was
what is known as the “Melange Belts.” Parts of the
Melange Belts include the remains of island belts,
but much of it is ocean-floor rock, much of it broken into small fragments. The rocks of Fidalgo Island are from this assemblage. These rocks mantle
much of the western slopes of the Cascades, as far
south as Snoqualmie Pass. They can also be found
in the area around Cle Elum, and some distance to Insular
the south. These rocks were probably added to the Belt
continent about 90-95 million years ago, about 20
million years after the Insular Belt was accreted.
This is mid-Cretaceous time, during the heyday of Olympic
the Dinosaurs.
Coast
The final “package” added to the continental margin was the Olympic Coast Belt, which includes
the Olympic Peninsula and the Willapa Highlands.
This is a large section of oceanic crust, covered
with a thick accumulation of submarine basalt
flows and associated sediments. It was accreted
to the continent about 40 million years ago, in a
unique tectonic setting surrounding the demise of
one of the local oceanic plates. While it remained
a shallow marine setting for the next 35 million
years, it became a part of the continent at this date.
Continental Arcs
Belt
Melange
Belts
Ancestral
North
America
Figure 13 (Above) The major accreted terranes of the Pacific Northwest. . The Intermontane and Insular Belts were originally island
groups in the eastern Pacific Basin. They were accreted to the continental margin as North America moved west, and the underlying oceanic
plates moved east. The Intermontane Belt was added about 180 million
years ago, the Insular Belt was added about 115 million years ago.
These accreted belts of islandic and oceanic “terranes” accumulated as the North American Continent moved west,
and the oceanic plates generally moved to the east. This opposing sense of motion was accommodated as the oceanic
plate was subducted beneath the edge of the continent. Inland, this zone was marked by a chain of volcanoes (a “continental arc”) on the surface, and a zone of granite-type rocks which were intruded at depth.
This process persisted over the entire development of the region, as the various terrane-belts were added to the edge
of the continent. As those terrane belts were added, they often re-located the subduction zone to the west, and the
corresponding volcanic arc similarly migrated in that direction. As a result, a succession of four volcanic chains were
constructed across this evolving landscape. These include the Omenica Arc, the Coast Range Arc, the Challis Arc and
the modern Cascade Arc. For all but the younger rocks of the Cascade Arc, all of the surficial volcanics from these
episodes have long since been eroded away. What remains are the deep granite-type rocks which developed underneath the volcanoes.
12
Figure 14 (Right)
Continental Arc Belts of the Pacific Northwest. In these
regions, plutonic rocks accumulated at depth, while
volcanic rocks erupted onto the surface.
Omenica
Arc
180-145 Ma
The Omenica Arc developed after the (180 Ma) accretion of the Intermontane Belt, and persisted until about
145 million years ago,
The Coast Range Arc developed after the (120 Ma) accretion of the Insular Belt, and persisted until about 60
million years ago.
The Challis Arc evolved during a period of north-directed plate motion, and persisted from 60 -40 Ma.
The Cascade Arc had its inception at about 40 Ma,
after the accretion of the Olympic Coast Belt. It persists
unto today.
In all of these areas except for the southern Cascade
Arc, erosion has long removed the volcanic cover which
accumulated during their active period. What remains
are the deep plutonic rocks (Granodiorite, granite)
which accumulated beneath the volcanoes. In several
areas, 2-3 different arc belts overlap.
Coast Range
Arc
115-60 Ma
Cascade
Arc
40 Ma - 00
Challis
Arc
60-40 Ma
The Strange Tale of Ancestral Fidalgo Island
The rocks which would ultimately make up Fidalgo Island had their origins back in Jurassic time, probably somewhere in the eastern half of the Pacific Basin. They developed on a section of oceanic plate, first as a basaltic shield
volcano, but later as an andesite stratovolcano. We don’t know how big the island originally was, or what chain it
belonged to. We suspect it was scraped off the subducting oceanic plate back in early Cretaceous time (~140 million
years ago), and was added to the edge of the continent in that manner.
Our best guess is that this happened somewhere to the south, likely in what is now southern Oregon or northern
California. Here, as the oceanic plate was being subducted, a thick “wedge” of ocean-floor sediments, fragments, and
small sections of island belts, accumulated beneath the leading edge of the continent. This “accretionary wedge” is
known as a “Melange” unit, because it is comprised almost entirely of sheared fragments, in a largely unrecognizable
mix of rock types. The term “mélange” is French for a “mix.”
While this process usually breaks up island belts into small fragments, the rocks of the Fidalgo Island remained at
least partially intact as they were accreted. They were preserved in this thick “wedge” of accreted material, riding
above the subduction zone.
13
Ancestral Fidalgo Island
Figure 14 (Right)
Accretionary Wedge
Diagrams illustrating
the accretion of the
ancestral Fidalgo Island
to the coast of northern
California. As illustrated, an “accretionary
wedge” of accumulated
oceanic rocks developed
beneath the advancing
edge of the continent.
When the ancestral
Fidalgo Island arrived
at this setting, it was
incorporated into that
“melange” of accumulated material.
Most rock assemblages
are completedly dismembered in this process,
being reduced to sheared
fragments and isolated
blocks. It would appear,
however, that the rocks
derived from the ancestal Fidalgo Island were
preserved as a more
competent unit.
North America
(N. California)
Accretionary Wedge
(Melange Rocks)
Ancestral
Fidalgo
Island
North America
(N. California)
This arrangement persisted until something like 120 million years ago. At that time the eastern Pacific underwent a
major change, with the development of a new oceanic plate. This new oceanic plate (the Kula Plate) developed as
a new mid-ocean ridge developed along an east-west axis in the northern Pacific. This ridge intersected the North
American Continent somewhere in what is now Northern California. As this happened, large sections of the coastline
to the north were sheared off the continental margin – including that section hosting the rocks of the Fidalgo Islands.
Driven by a north-directed sense of plate motion, these fragmented sections of the coastline were transported northward toward the accreted terranes of the Pacific Northwest. When they arrived here, they were once again scrapped
off the subducting oceanic plate. This time however, they were scrapped off on top of the continent. As the underlying
plate was subducted, these rocks were thrust hundreds of kilometers northward along low-angle thrust faults. These
rocks now mantle much of the western slopes of the North Cascades, and are also found on the east side around the
town of Cle Elum, and further south.
14
(Accreted)
Melange Belt
Terranes
Northward Transport
New
Mid-Ocean
Ridge
Old Subduction
Zone
Figure 16 (Above)
Diagram illusrating how the melange belts accumulated along the coast of Northern California were rifted from that margin,
transported northward on the oceanic plate, and thrust over the edge of the accreted terranes of the Pacific Northwest. The new
oceanic ridge represents the boundary between the Farallon Plate to the south and the Kula Plate to the north. This is thought to
have formed about 120 million years ago.
The rocks that mantle most of the modern San Juan Islands and most of the western slopes of the North Cascades all
share in this origin. They were fragments of oceanic crust, ocean-floor sediments and rare sections of island-arc belts
which accumulated along the coast somewhere south of here, were later rifted off the coastline, transported north, and
were thrust over the top the continent here between 90 -95 million years ago. While some large sections of oceanic
crust are preserved north of the Skagit River, most of these rocks are a disorganized hodge-podge of types, usually
as blocks and smaller fragments. While a few more intact sections are preserved, such cases are an exception to its
general character.
The rocks of Fidalgo Island are one such exception. This is a ~6 km block of largely intact lithologies (rock layers),
lithologies which are typical of a volcanic-island complex. The entire block is tilted on its side, exposing these layers
across the surface of the island. This makes it an exceptional locale for considering the character of these important
geologic features.
15
Figure 17 (Right)
Diagram illustrating how the rocks
of the Melange
Belts were obducted
(thrust across) over
the continent along
low-angle thrust
faults.
North
South
Melange
Belt
Rocks
Older (Insular Belt) Rocks
Low-Angle Thrust Faults
The Later Evolution of the Melange Belts
The rocks of the Melange Belts were likely accreted in early Late Cretaceous time, perhaps 90 – 95 million years ago.
They were accreted as a “stack” of terrane units, separated by low-angle faults. That “stack” was probably quite a bit
thicker than it is now, given some 90 million years of history. This means that the modern exposures on Fidalgo Island
were probably low in that stack, the overlying layers having been lost to erosion.
The Melange Belts were present as the ancestral Canadian Coast Range developed over Late Cretaceous time, a formidable range of mountains constructed between 95 and 65 million years ago. This range extended much of the length
of North America.
Figure 18 (Right)
The paleogeographic setting of
the region in early
Eocene time, about
60 million years
ago. At this date,
most of Washington
State was a low
coastal floodplain,
covered in paratropical vegetation.
A large river flowed
across the landscape, depositing
sediments across
the river basin.
Columbia Embayment
16
Figure 19 (Left)
Fossils from the
Chuckanut Formation, near Bellingham. These rocks
are about 50 million years old, from
a time when this
region was a broad
coastal floodplain.
The fossils seen
here are palm
fronds, indicating
a substantially
warmer climate
than exists today.
This region was a lowland province between 60 and 40 million years ago, a coastal floodplain marked by paratropical
vegetation and changing river courses. Over this period, the Melange Belt rocks were buried under thick accumulations of sandstone and siltstone, brought by those rivers.
Starting about 40 million years ago, the Cascade Volcanic chain started erupting along the coastal margin here. These
were the first of a long history of Cascade Volcanoes, the modern peaks of which are only the most recent members.
Those volcanoes did not really form a mountain chain, but rose as isolated summits above a modestly elevated province.
The modern Cascade Range is a much more recent feature on the landscape. The Cascade and Olympic Mountains
probably didn’t start rising until about 5 million years ago. They are a set of north-south trending folds which developed as the Juan De Fuca plate to the east started to deform the edge of the continent.
North of modern-day Everett, the uplift of the North Cascades widens to both the east and west. To the west, it
extends into the San Juan Islands, to the east it merges into the Methow region. On the west, that extension follows
along a prominent fault line, one of the low-angle thrust faults originally formed in the accretion of the Melange Belts.
Known as the Darrington-Devils Mountain Fault, it cuts the landscape just south of Fidalgo Island. To the north of this
line, there has been significant uplift over the last five million years. Over the same period, the Olympic Peninsula has
emerged along the coast, and the Olympic and Cascades Mountains have been formed.
Sculpting the Modern Landscape
Two million years ago, the modern summits of the San Juan Islands may have been rocky hills rising above a lowland basin. Since that time, dozens of episodes of continental-scale glaciation have sculpted the modern landscape. At
times, ice over a mile thick cloaked this region. That ice advanced south out of Canada, carving the rocky hills into
streamlined forms, and excavating the softer sediments between them. Eventually, they were reduced to isolated rocky
knobs, rising above the landscape.
17
Figure 20 (Right)
Illustration showing the extent of
the glacial advance
during the last
major episode
of glaciation.
In the Fidalgo
Island area, the
ice was well over
a mile thick. At its
greatest extent, it
completely filled
the Puget Basin, as
well as the Strait of
Juan De Fuca. .
Just when these knobs became islands is uncertain, as it depends largely on the prevailing sea level at any one time.
The islands of the Puget Sound basin were not formed until the last ice age, which ended ~12,000 years ago. The San
Juan Islands are probably older, but may have first formed over the last half-million years. Over the glacial episodes
they alternated between island and coastal settings, depending on the sea level at the time. They all show evidence for
repeated glacial episodes, yet they remain prominent features on the landscape.
Summary
The rocks of Fidalgo Island date from perhaps 200 million years ago, as the great supercontinent of Pangea started
to break up, and North America started to move west with the opening of the Atlantic Ocean. Somewhere out in the
Pacific Basin, two sections of oceanic plate were forced against each other, and in the process, one was forced beneath
the other. As it was forced down to depth, that plate partially melted, giving rise to a chain of volcanic islands on the
surface above. Part of one of those islands would eventually go on to make up modern-day Fidalgo Island.
Over the eons, the combined motion of the oceanic and continental plates brought that island group to the edge of
North America, probably along what is now southern Oregon or Northern California. There, these islandic rocks were
added (accreted) to the continental margin as the oceanic plate slid beneath it.
At a later date, a change in ocean-plate configuration resulted in large sections of this coastline being sheared off the
continental margin. These rocks were transported north, and were essentially scrapped off the top of the oceanic plate
as it was subducted in our region. In this process these rocks were thrust over the top of the continent, along a series of
low-angle thrust faults.
That was 95 million years ago, during the heyday of the dinosaurs. Since that time, the landscape has changed considerably. The biggest changes, however, have occurred over the last five million years. Over this period, the Olympic and Cascades Mountains have been uplifted – the latter including rocks of the San Juan Islands region. Over the
last two million years, the region has hosted dozens of episodes of continental-scale glaciation, where ice advancing
from the north has sculpted the region on a recurring basis. Over that period, the changing sea levels have alternately
flooded and retreated from this area. In the modern interglacial setting, the sea level is high enough that the San Juans
stand as islands in the Georgia Strait.
18
The Geology of Fidalgo Island
Fidalgo Island is underlain by a south-dipping low-angle thrust fault, one of several which divide the mélange terranes
across the northern end of the state. Within that fault zone is preserved a section of ultramafic (mantle) rock, as is typical of these thrust systems. That ultramafic rock (originally dunite, now serpentinite) was presumably exhumed from
the base of the subduction zone along the northern California coast. A very large section of this rock (the largest in the
western Hemisphere) lies about 50 km northeast of here, in the Twin Sisters Mountain to the east of Mt. Baker. These
rocks often occupy fault zones, where they are an effective “lubricant” to tectonic processes.
Above this basal fault zone, the geology of Fidalgo Island appears to dip to the south, exposing the deepest rocks on
the northern end of the island. The deepest rock here is a gabbro pluton, cut by dikes of younger intermediate igneous
rocks. This may represent the magma chamber of the original basaltic shield volcano, the remnants of which appear
preserved above. At its lowest levels, along Alexander Beach, it displays a layering of olivine crystals in the bottom of
the chamber.
The dikes of intermediate (dioritic) igneous rocks which cut the gabbro feed into a section of dioritic to granodioritic stocks (small plutonic bodies) and plutons (larger plutonic bodies) above. These were generated as the volcano
matured with increasing thickness. These stocks and plutons accumulated beneath the basaltic “shield” of the early
volcano, erupting on its surface as andesite volcanics. These intermediate volcanics presumably accumulated a large
stratovolcano on top of the older basalt edifice. The presence of igneous breccias (a rock containing angular fragments
of older rocks) containing both basalt and diorite clasts suggests that some of these were violent eruptions.
Late in this process, a dioritic pluton intruded through the basalt, and presumably into the andesite volcanics above.
Alternately, it may be part of the central magmatic conduit for that volcano. The volcanic rocks appear to have been
completely removed over time. This pluton makes up modern-day Mount Erie.
Above the intermediate stocks and plutons lies the basalt of the original shield volcano. These are likely derived from
the gabbro seen deeper in the pile. Most of these appear to be submarine eruptions, displaying the classic “pillow”
form characteristic of that setting.
Early Island Development
Submarine
volcanics
(Pillow
Basalts)
Subarial volcanics
(Basalt)
Shield Volcano (Basalt)
Basalt
Gabbro
Gabbro magma chamber
Magma from subduction zone
Figure 21 (Above) Diagram illustrating the early development of the ancestral Fidalgo Island. In this stage, an broad basalt
shield volcano was formed.
19
Mature Island Development
Mature Composite Stratovolcano
(Andesite)
Graywacke Sandstone
Mt. Erie Pluton (Diorite)
Tuffaceous
Argillites
(Siltstone)
Original Shield Volcano
Dioritic Plutons
Sedimentary
Breccias
Old Gabbro Pluton
Figure 22 (Above) Diagram illustrating the mature phase of development of ancestral Fidalgo Island. In this stage, a large composite (andesite) stratovolcano was constructed over the older basalt shield volcano.
On the southern end of the island, the various sedimentary packages which accumulated around the island are preserved. Directly over the basalt section is a layer of volcanically-derived sediments, much of it a breccia of basalt and
intermediate volcanic fragments. Some of these may be lahar (volcanic mudflow) deposits. Above this mixed layer is
a layer of fine-grained sediments, These are derived from a volcanic source, but are typical of the mud that accumulates on the ocean floor around island belts.
Above this layer is another bed of fine-grained sediments, this one containing a considerable amount of volcanic ash.
This suggests that volcanism on the island continued to at least this date. Fossils in this layer date from 135 – 150 million years ago. Finally, this whole stack is topped by a layer of volcanically-derived sandstones and siltstones (“greywacke”) which evidence accumulation along the submarine slopes of the island.
All of these rocks have been metamorphosed (altered by heat and pressure) to a minor degree. This probably happened as they were scrapped of the oceanic plate, and were preserved in a “wedge” of accumulated material beneath
the continental margin. The various sedimentary rocks are now a low-grade metamorphic variety called “argillites”.
Moreover, this process certainly deformed the rocks to a significant degree.
More deformation of these rocks occurred when they were finally accreted to the coastline in our area. This may have
happened something like 90 -95 million years ago. In this process, they were thrust northward over the continent
along a series of low-angle thrust faults. As a result, all of these rocks are marked by low-angle faults and shears, and
some sedimentary rocks have acquired a strong tectonic fabric. Plutonic sections show considerable internal displacement, which presents a complicated picture in the field. It is likely that all of these “layers” are fault-bound slices
which have been displaced to a certain degree.
In the end, modern-day Fidalgo Island is a collection of rocks which have been through a tortuous history. An ancient
island belt, smeared into the northern California coastline, then rifted off the edge of the continent, transported northward and thrust over the top of continent here, at a date still some 90 million years ago. Since that date they have been
buried in deep sediments, folded several times, displaced to the north, compressed from the south, uplifted, eroded,
and sculpted dozens of times by great sheets of ice flowing over them. Finally, just 10,000 years ago, did they become
the modern island that is Fidalgo.
20
1
Anacortes
Fidalgo Bay
2
3
7
Burrows Bay
8
4
Graywacke Sandstone
Tuffaceous Sediments
Diorite
5
Basalt
Gabbro
6
Figure 23 (Above) Simplified geologic map of Fidalgo Island. Numbers refer to field stops
21
Serpentinite
Modern-Day Fidalgo Island
Fidalgo Island was named for Salvadore Fidalgo, the cartographer on Francisco De Eliza’s explorations of 1790. For
thousands of years prior, it had been home to indigenous people, dating from shortly after the last ice age. On the arrival of European settlers, the island was inhabited primarily by the Samish and Swinomish tribal groups, occupying
the northern and southern ends of the island respectively. A large fern-covered prairie at March Point was an important
native gathering spot.
Fidalgo was first settled by European peoples during the Fraser Gold Rush of 1858 – 59. Amos Bowman and his wife
Anna-Curtis were among the first settlers, and the founders of the city of Anacortes. The name of the city is a mangled
contraction of her name. It was long thought that the island would be the western terminus for the railroad, enough
that Governor Issac Stevens purchased large parcels of land in anticipation. When the railroad didn’t come, he sold off
his land to the Bowmans.
The population Anacortes and the surrounding island has long been tied
to its position as a maritime port. In its early days, it was a shipbuilding center. It is home to the largest fishing fleet in the state, much of it
dedicated to Alaskan waters. In addition to the fishing fleet, Anacortes is
the port for the Washington State Ferries to the San Juan Islands, and to
British Columbia.
Figure 24 (Above) The City of Anacortes, looking to the north. In the distance
is Guemes Island, and the San Juan Islands beyond. The city has a population
of about 14,500 people.
Figure 25 (Right) View of Campbell Lake and the Puget Sound. From the summit of Mt. Erie. The peak is a city park, and is popular with sight-seers, rockclimbers and hang-gliders.
22
The island had a big boost in 1935, when the Deception Pass
Bridge was completed to Whidbey Island, opening a popular
tourist route. The establishment of the Whidbey Island Naval Air
Station in the 1940’s attracted additional people, to serve on its
workforce. A major development took place in the 1950’s, when
Shell and Texaco built the refineries on what was once the ferncovered prairie of March Point. Oil tankers (e.g. from Alaska)
dock here regularly. It is Washington’s only major oil port.
Over the 1960’s, numerous housing developments added to the
population, which was becoming popular as a retirement community. Over the last twenty years, people have been increasingly attracted to its unique island-maritime setting. Recently, it has come
to hosting increasing numbers of high-end “estate” homes and
Figure 26 (Above) The rocky shorelines of the island are increasingly
beeing covered with residential developments.
Figure 27 (Right) Biz Point, along the southwestern shoreline of the
island.
23
communities, many built as retirement residences. Increasingly, the
island’s greenery is being replaced
by residential development. It is
particularly attractive because
it sits in the rain shadow of the
Olympic Mountains, and receives
considerably less rain than the surrounding areas. At the last census,
the town had a population of about
14,500 people, with another 6,000
people living in the surrounding
rural setting. The island covers
about 41 square miles, with the
city covering about a quarter of
that.
Figure 28 (Right) A herron surveys
the inlet of Burrows Bay
Figure 29 (Below) A narrow channel
along the south end of the island.
Tidal currents make these dangerous
waters during certain times of the
day.
24
25
Fidalgo Island Field Trip
(Driving directions are printed in italic type)
From Seattle, drive north on Interstate 5 to the town of Burlington (just north of Mt. Vernon), at the intersection with
State Route 20 (about 60 miles). Take the exit from the freeway, and turn right onto W. Rio Vista Road (SR 20). Take
the next right turn, leading to the parking lot at Haggens Market.
Public restrooms and general provisions here
Return to SR 20, and turn left, heading west. This passes under the freeway, and becomes the Avon Cutoff Road. Follow this road across the Skagit flats, and over the Swinomish Channel to Fidalgo Island. Follow the road into Anacortes.
At Commercial Street, SR 20 turns right, and continues through downtown until it reaches 12th street. Here, turn left
on 12th, which eventually becomes Oakes Avenue. Highway 20 continues to the Ferry Terminal. Just before it heads
to the water, the road forks at a “Y”. Here, take the left form, which is Sunset Avenue. Follow Sunset Avenue to the
parking areas at Washington Park.
(Summary): Take Interstate 5 north to Burlington. .Head west on State Route 20, to the Anacortes Ferry Terminal.
Just before the terminal, turn left onto Sunset Drive, and continue to Washington Park.
Figure 30 (Left) Map of Fidalgo Island, showing field trip stops
Figure 31 (Above) Rosario Beach
26
Stop 1: Washington Park
Mantle Rocks from the basal fault zone
Walk down to the beach to the west, where the rocks outcrop.
These are rocks of the Earth’s mantle, exhumed from depths of 10-12 km. We think that these rocks were brought to
the surface ~120 million years ago, as a new oceanic spreading center intersected the coast of ancestral northern California. That new spreading center rifted thick sections of the continental margin off the coast, and transported them
northward. These rocks were from the very bottom of that rifted section.
As they were thrust across the continent here, these particular rocks became concentrated along the low-angle thrust
faults which separate the different “thrust sheets” of the Melange Belts. These rocks occupy the low-angle thrust fault
which lies below modern-day Fidalgo Island. In the outcrop, you can see what appear to be low-angle shear faults in
the rock.
The rocks of the Earth’s upper mantle are a variety called dunite. It is comprised largely of the mineral olivine. The
rocks here have been somewhat altered from that original composition. Stretching and shearing of these rocks has altered the olivine to a mineral called serpentine. The rocks are now known as “serpentinite”. These are iron-rich rocks,
so the surface quickly weathers to a reddish color.
Exit the park via Sunset Drive, and continue until just before reaching SR 20. Here, turn right onto Anaco Beach
Road. At it reaches the water, this road turns left and becomes Marine View Drive. Just past two prominent bends in
the road is the intersection with Marine Heights Way (on the left). Park here, on the right side of the road. This is a
construction site.
27
Figure 32 (Left) Outcrop at Washington
Park beach. There is a prominent fault
which runs up the center of this image,
forming the gap in the distance.
Figure 33 (Right) Detail of the rock
exposed here. This is serpentinite, an
altered ultramafic rock derived from
dunite - a rock rich in the mineral olivine. These are rocks of the earth’s upper
mantle region. Note the thick weathering front in the rock, where iron-rich
minerals have oxidized to a distinctive
red color.
Figure 34 (Below) A horizontal shearfault (just above the hammer) in the
rock wall along the beach. The upper
block has likely been displaced to the left
(north).
28
Stop 2: Marine View Heights
Diorite of the Mature Volcano
The rock here is an igneous variety called “diorite.” It is a roughly equal mixture of light and dark minerals. These
rocks were intruded below the volcano, cooling slowly to form a rock made up of large crystals (a plutonic rock).
This happened during the later, mature stage of island development. Above these rocks, a large stratovolcano accumulated from the andesite volcanics which erupted in this stage. Those upper rocks have long since been lost to the
forces of time. What are preserved here are the deep “roots” of that volcano.
The outcrop here provides good evidence for how sheared and deformed these rocks are. From the right angle, you
can see how they are stacked sheets of rock, juxtaposed along low-angle shear faults. While the “sheets” appear to be
different colors, they are most importantly sections of different grain size – reflecting cooling at different levels. There
is considerable internal displacement in this unit. The rocks are cut by numerous dikes, including a prominent lightcolored one. Note that these dikes do not cut across the faults. This means that they were faulted after the dikes were
intruded.
Cross the road to the entrance to the Marine Heights development. The rocks here show distinct evidence of having
been smoothed by glacial ice. This has happened repeatedly over the last 2 million years. Also seen here (in the retaining wall) are igneous rocks comprised of fragments of other (older) igneous rocks. Note that some of those fragments
are dark varieties from the original basaltic shield volcano. This mix is called a “breccia”, and it results from explosive eruptions. Andesite frequently erupts in an explosive manner.
Continue south on Marine Drive. Just beyond the intersection with Del Mar Drive (on the right) is a prominent cliff
face along the left side of the road. Stop here.
29
Figure 35 (Left) Outcrop exposed by current construction
work. Note that these rocks appear in distinct “layers,”
separated by faults. These rocks are all diorite. The darker
(brownish) section is comprised of finer-grained rock than
the light-colored section. Note light-colored pegmatite
dike.
Figure 36 (Above) Detail of dioritic rocks, showing a
roughly equal percentage of light and dark minerals.
Figure 37 (Right) A block of volcanic breccia, comprised
of angular fragments of gabbro, diorite and volcanic rocks,
welded together in a volcanic matrix.
Figure 38 (Below) Glacial striations in the rock, at the entrance to the housing development. These show movement
of the ice from right (north) to left (south)
30
Stop 3: Del Mar Roadcut
Gabbro of the EArly Volcano
The rocks here are igneous (plutonic) varieties, and some of them look much like the rocks seen at the last stop. Much
of the rock, however, is a much darker variety. This rock is gabbro, and it is older than the diorite sections which
intrude it. This was likely the magma chamber of the original basaltic shield volcano, from the earliest eruptions of the
island volcano. These are the oldest rocks of the group here. They may be something on the order of 190-200 million
years old.
In numerous places, one can see dikes and sill of the younger dioritic rocks intruding the gabbro. Fragments of gabbro in the lighter diorite suggest that the gabbro was solid when the diorite intruded. In other places, more gradational
contacts suggest otherwise. On close inspection, several different episodes of intrusion are in evidence.
Stepping back from the rock, several prominent faults can be seen in the face. These are marked by sections of tectonic “gouge” – a powdery mixture from the grating of rock faces. Over the length of the outcrop, both high and lowangle faults can be seen. The low-angle faults generally dip to the south, and were likely formed as these rocks were
thrust across the continent.
Continue south on Marine Drive to Havekorst Road, Turn right and follow this to the “Y” intersection with Rosario
Road. Take the right fork on Rosario Road and follow it until the intersection with Sharpe Road. Turn left here. Follow Sharpe Road for a mile to the intersection with the Ginnett Road. Follow the Ginnett Road to a point about 200
feet before it ends. Park off the road on the left, beneath the rock cliffs.
31
Figure 39 (Left) Roadcut outcrop
along Marine Drive. Most of this
outcrop is gabbro, cut by numerous dikes of diorite. A number
of prominent faults can be seen
cutting these rocks, including low
and high-angle varieties.
Figure 40 (Above) Local intrusion
of dioritic rock, preserving the
older gabbro as fragments within
it. These relations establish that
the diorite is younger than the
gabbro.
Figure 41 (Right) A hand-sample
of gabbro. This is a mafic plutonic rock, comprised primarily
of the minerals olivine, pyroxene,
amphibole and calcium feldspar.
It is distinctly dark in appearance,
typically with a slight green tint.
32
Stop 4 Red Rock Quarry
Marine Sediments From the shallows around the island.
The former quarry site is now occupied by a house just before the end of the road. The driveway is constructed of
quarry debris, and offers quite suitable samples.
This is a sedimentary rock, formed from sand, silt and volcanic ash which accumulated in the shallow waters around
the ancestral Fidalgo Island. The different (red, black, green) colors result from chemicals produced in the alteration
of the original sediments. These rocks also contain the fossil remnants of microscopic sea life, silica-shelled animals
called “foraminifera”. The species seen here suggest that these sediments were deposited about 150 million years ago.
Interbeds of these rocks can be found in the upper parts of the volcanic rocks of the island. Some of these interbeds
are up to 165 ft (50 m) in thickness. Much of this material is from pyroclastic flows, glowing avalanches of debris
which swept down the slopes of the volcano. Some portions are of much coarser material, forming sedimentary breccias. The coarse nature of the material suggests a close proximity to the volcanic center.
These rocks reflect continuing volcanic activity, as the island grew and matured over time. At times, that growth was a
violent affair.
Return to Rosario Road and continue south. After a few miles, take the turnoff on the right to the Rosario Bay Road.
33
Sedimentary
Rocks
Igneous
Rocks
Figure 42 (Left) Various rocks from the Red Rock Quarry. Visable are red, gray, green and black varieties. These
are mildly-altered siltstones and mudstones, generically
known as “argillites.” These are “tuffaceous” rocks,
containing a significant proportion of volcanic ash.
Figure 43 (Above) A fault marking the contact between
the fine-grained argillites (above) and diorite (below).
This is a good illustration of the degree to which all of
these rocks are displaced along local faults.
Figure 44 (Right) An outcrop of sedimentary rocks along
the side of the road. These are mildly-altered siltstones
and mudstones, generically known as “argillites.” Alteration is extensive enough that it is difficult to pick out
bedding (layering) in the rocks, and most appear massive
in aspect.
34
Follow the road down to the parking area.
Stop 5: Rosario Bay
Rocks of a Different Thrust Sheet
Public Restrooms, Lunch Stop
Walk the trail to the top of the “island” adjacent to the beach.
These are not rocks from the Fidalgo Complex. These are from the “thrust sheet” which lies above Fidalgo Island. The
low-angle thrust fault which separates these two “sheets” lies beneath the narrow isthmus which connects to the rocky
outcrop. That fault dips to the south.
These rocks are chert and siltstone, derived from ocean-floor sediments. Chert is the remains of single-celled silicashelled marine creatures, who’s remains accumulate on the ocean floor. It is largely silica, and is therefore a very
hard rock. The siltstone is ocean-floor mud, accumulated between sections of chert. Chert is not found in the Fidalgo
Complex.
This locale provides a spectacular view along the shoreline, and out onto the Strait of Juan De Fuca.
Return to the Rosario Road and continue on it to the south. At the intersection with SR 20, turn right on the road (SR
20) to Deception Pass. Just before the Deception Pass Bridge, park along the highway on the right-hand side of the
road.
35
Figure 45 (Left) View across Rosario Bay,
looking south. The rock (not quite an island)
is separat4ed from the mainland by a lowangle thrust fault. Structurally, it lies above
the rocks of Fidalgo Island. Although part of
the Melange Belts, this rock is from a different setting than the Fidalgo rocks. .
Figure 46 (Right) Deformed, verticallyinclined beds of chert and siltstone, on the
north end of the outcrop. These are oceanfloor deposits, likely from a deep water setting. Note the minor shear fault at the base
of the image. Knife provides scale.
Figure 47 (Below) Along the west side of
the outcrop. This vantage offers spectacular
views from a truly idyllic setting. If you can
get a view over the cliffs to the rocks underneath, you can see that the chert section
overthrusts a lower section of shale.
36
Stop 6: Deception Pass
Sediments accumulated on the submarine slope of the island.
These are sedimentary rocks, but coarser-grained varieties than the ones seen at the Red Rock Quarry. These are
volcanically-derived sandstones and siltstone, from sediments eroded off the slopes of the growing volcano. These
rocks are known as a “graywacke” sandstone. Elsewhere, these include beds of volcanic flows, debris flows, and ash
deposits. This layer is at least 500m (1650 feet) thick, and is the uppermost rock unit of the complex.
The depositional features revealed in the layers of this rock are typical of sediments accumulated along the submarine slope of the island. Many of these are submarine landslide flows, which periodically avalanched down the island
slope. Some of the beds here display “graded” bedding, with coarser material on the bottom, grading to finer material
on the top. This is a typical pattern seen in submarine debris-avalanche flows.
Unlike most of the rocks on the island, these beds clearly dip to the north. Numerous shears and faults can be seen
cutting the outcrop. In places, rock bolts have been installed to keep pieces in place. At the south end of the outcrop,
you can see some sections of rock which have been strongly folded.
Take the opportunity to visit the Deception Pass Bridge, built in 1935. It connects to Whidbey Island, and is several
hundred feet high. Restrooms are available on the far side.
Return on SR 20 to the Rosario Road, and return on the Rosario Road to the “Y” intersection with the Havekorst
Road. Here, stay on Rosario Road as it turns to the right. Just past Heart Lake, turn left onto the Heart Lake Road.
Several miles up this road, turn right onto the Mount Erie Road. Several miles up this road is a prominent outcrop of
dark-colored rock on the left side. Stop here.
37
Figure 48 (Left) The rock outcrop
along the road. On a large scale,
one can easily pick out distinctive
layers in the rock, which is typical
of sedimentary varieties. These are
fine-grained sandstone and siltsones, deposited along the submarine slope of the island.
Figure 49 (Above) Detail of a
well-bedded portion of the outcrop.
Some of these are “graded” beds,
grading from coarser material on
the bottom to finer material on the
top. This is typical of deposition by
submarine debris-avalanche flows.
Figure 50 (Right) The Deception
Pass Bridge, cloaked in a local fogbank. The bridge connects Fidalgo
Island with Whidbey Island to the
south.
38
Stop 7: Lower Mt. Erie
Volcanic Rocks of the Early Volcano
These deformed black rocks are basalt, from the original volcanic edifice of the early volcano. These are the remains
of a shield volcano which developed early in the history of the island. In less-deformed sections, it is possible to see
distinct “pillow” structures in the rock, indicative of submarine eruptions. Although the intervening dioritic plutons
limit a direct correllation, it can be assumed that the gabbro seen at stop 3 fed this surface volcano.
These rocks have been intensely sheared and folded, to a degree not commonly seen in igneous rocks. This may reflect either the original process of terraine accretion, or when these terranes were thrust across the continent here. By
whatever circumstance, they are a seriously beat-up section of rocks.
Just above the basalt, one can see a prominent outrop of diorite. This is part of the dioritic pluton which makes up the
summit of Mt. Erie. This pluton intruded through this basalt “shield”, at a (later) date when a large andesite stratovolcano capped the island.
Continue up the Mt. Erie Road, to the top of the mountain.
39
Figure 51 (Left) A view of the outctrop. The degree of deformation is apparent in this image. The white arrow above points to
intrusive diorite, which continues up to the summit of Mt. Erie.
Figure 52 (Above) Detail view of the outcrop. While deformation has completely obscured any original features, these probably
started out as “pillow basalts,” typical of submarine eruption. Since that time, they have been sheared and folded to an acute
degree.
40
Stop 8: Mt. Erie
A Pluton of the Mature Volcano
From the road end, hike over a smooth rock outcrop and descend to the rock benches about 50 feet below.
The rock here is diorite, part of a pluton intruded during the mature stage of the volcanic island. Becasue it appears to
intrude through the old basalt shield volcano (but still, at some depth), it must date from a time when a large stratovolcano had developed here. These are the same dioritic rocks we saw at stop 2, and it is a reasonable assumption that
some of those later-phase dikes intruded to supply this pluton above. Because this is an igneous rock, we can obtain a
good estimate of the date at which it crystallized. It would appear that this crystallized in Mid-Jurassic time, perhaps
160-180 million years ago.
Note the rounded form of the rock outcrops here, a reflection of intense glacial-polishing. In places, you can see distinct glacial striations. The mountain provides a spectacular view on the Puget Sound region to the south, and from the
right vantage, on the San Juan Islands to the north The summit is a popular vista with both local residents and tourists
from afar. Popular activities include bird-watching, rock-climbing and hang-gliding.
Descend the Mt. Erie Road to the Heart Lake Road. Turn right onto the Heart Lake Road, and continue to its intersection with the Rosario Road. Turn left on Rosario Road, and follow it to SR 20. Follow SR 20 back to Burlington, and
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Figure 53 (Left) Looking south from the summit of Mt. Erie.
In the foreground is Campbell Lake. To the rear is the Sarasota
Passage, between Camano Island (far left) and Whidbey Island
(right). The small islands are state parks.
Figure 54 (Above) A fresh exposure of the rock on the summit
of Mount Erie. This rock is a diorite, as evidenced by the equal
proportion of light and dark-colored minerals. These are the
same rocks seen at stop 2.
The cliffs along the southwest side of the mountain are popular with rock climbers, where it provides good early-season
practice climbing. It is also used by climbing classes. Another
popular avocation here is hang-gliding, where pilots land in the
fields around Campbell Lake, some 1200 feet below. .
Figure 55 (Right) The view south from Mt. Erie.
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Summary:
The rocks of Fidalgo Island bear witness to a long, complex, and tortuous course of geologic evolution. In the end, the
most remarkable aspect about them is that they still retain enough of their original structure to tell the story of their
past. Unlike most of the rocks of the Western and Eastern Melange Belts, this collection of lithologies has managed to
endure as a coherent unit, despite the multitude of abuses it has suffered over time. This faulted, sheared and generally disheveled collection of rocks tells a quite remarkable tale of island evolution, accretion, rift tectonics and terrane
obduction, as well as more recent events which have moulded their modern appearance. These rocks are crystal balls
on an exotic and remarkable past.
This story may have its origins in the breakup of the supercontinent of Pangea, which persisted until about 200 million
years ago. As the supercontinent started to break up with the bithof the Atlantic Basin, North America started to move
west. As this happened, it may have applied additional pressure on the oceanic plates of the Pacific Basin, causing one
to buckle and initiate the process of subduction. By whatever means, probably sometime between 180 and 200 million
years ago, a new subduction zone developed out in the Pacific Basin.
As one section of oceanic plate was forced beneath another, portions of the descending plate started to melt, generating magma beneath the over-riding plate. This magma had the chemical composition of gabbro and basalt. As it rose
through the over-riding oceanic plate, it erupted on the surface as a submarine volcano. There, it erupted a thick pile
of pillow-basalts, before breaching the surface to accumulate as a broad shield volcano. We were able to visit what
was probably the magma chamber of that early volcano, as seen at stop 3. We were able to see the remains of the basalt volcano which developed above at stop 7. Together, they tell the story of the early development of the ancestral
Fidalgo Island.
As ancestral Fidalgo Island grew in thickness, the magma acquired a more intermediate chemistry. This happens as
some of the darker, high-temperature minerals crystallize out, and are left behind as the magma rises. The resulting
magma has the chemical composition of diorite and andesite. As this magma rose into the crust, it rose through the
gabbro at the base of the early volcano, and accumulated below the basalt above. There, some of the magma crystallized out at diorite, in numerous stocks cut by progressively younger dikes of similar composition. We saw this section
at stop 2. In places, volcanic breccias are preserved, reflecting violent eruptions as this younger magma erupted to
form a new volcano on the surface.
As the island matured, a new andesite stratovolcano grew atop the older basalt shield volcano. In contrast to the thin,
runny basaltic magma which had erupted earlier, the andesite was much thicker and erupted more violently. Eventually, those thick, chunky and explosive lavas constructed a massive stratovolcano. While this part of the rock record has
largely been lost to the forces of erosion, we do know that magma continued to intrude at depth. Late in this history,
a dioritic pluton intruded well into the basaltic “shield” of the older volcano, if not all the way through it. This pluton
now makes up the top of Mount Erie, as seen at stop 8.
Meanwhile, sediments had started to accumulate around the growing island. The earliest of these accumulated on top
of the original shield volcano, a section of volcanically-derived sedimentary breccias which probably accumulated
from pyroclastic flows and mudflows. While these are marine sediments, they apparently did not start accumulating
until the island grew above the ocean surface. We didn’t visit an outcrop of these rocks, but we did see a representative sample at stop 4.
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Above that bottom layer of sedimentary breccias, a layer of finer-grained marine sediments accumulated in the shallows around the island. These rocks contain a significant proportion of volcanic ash (tuff), a reflection of continuing
volcanism. They are probably accumulations from river-borne sediments, pyroclastic flows and ashfall events, and
locally contain larger rock fragments. Alteration of the original minerals has produced rocks with distinctive green,
red and gray colors, as we saw at stop 4. The uppermost layer of sediments are marine graywacke sandstones, largely
from volcanically-derived sand and silt washed off the eroding island above. The rocks preserved here accumulated
along the marine slope of the island, deposited largely by submarine avalanches which swept down from above. We
saw these rocks at stop 6. In some locales, deposits of pyroclastic flows and debris avalanches can be found in this
unit. This confirms that magmatism continued through this phase of the island’s development.
At some time well-into the mature phase of island development, ancestral Fidalgo Island collided with North America
as it’s oceanic plate was subducted. This likely happened along the northern coast of modern-day California. Here,
these rocks were incorporated into a thick “accretionary wedge” of material which was essentially scraped off the
subducting oceanic plate. This thick “wedge” of material is typically a highly sheared and dismembered collection of
ocean-floor fragments, known as a “melange”. Despite the trauma of the event, it appears that the rocks of ancestral
Fidalgo Island were preserved largely intact through this process.
About 120 million years ago, a new spreading center developed in the eastern Pacific Basin, extending along an
east-west axis. Where this new mid-ocean ridge intersected with the continent in what is now Northern California,
it sheared off a thick section of the continental margin to the north, including the “melange” rocks containing the
ancestral Fidalgo Island. These displaced rocks were transported north on the oceanic plate, and probably reached the
southern end of the Insular Belt about 90-95 million years ago. As they reached the subduction zone here, they were
essentially sheared off the top of the descending plate, and obducted across the top of the continent for hundreds of
kilometers. This happened along low-angle thrust faults which floored the obducting blocks, and which sheared them
intermally. The major faults of this system accumulated sections of ultramafic (mantle) rocks in this process, deep
rocks originally exhumed when the continental margin was rifted off Northern California. We saw these rocks in the
thrust fault at the base of the Fidalgo Complex, at stop 1.
These rocks likely laid at the base of an impressive mountain range from 90 - 60 million years ago, and were buried under a thick pile of sediments between 60 and 40 million years ago. They have been exhumed over the last 20
million years, particularly over the last five million, with the uplift of the modern Cascade and Olympic Mountains.
Over the past two million years, the region has been repeatedly sculpted by glacial ice, as continental-scale icecaps
advanced south out of Canada. We saw evidence for this at stop 2, and at stop 8. Modern Fidalgo Island may not have
been an island until the end of the last ice age, about 10,000 years ago.
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Figure 56 (Above) A classic view
along the western shores of Fidalgo
Island. Increasingly, green spaces
like this are giving way to residential development (note houses to the
right)
Figure 57 (Left) A small pool of
water astride an island stream. The
island lies in the rain-shadow of the
Olympic Mountains, and thus does
not see the typical rainfall of the
Puget Sound region. Fresh water is
at a premium here, and surface-water is a rare occurance. .
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Figure 58 (Right) Outcrops at Bix Point, along the
southwestern end of the island. The northern end of
the Olympic Peninsula lies beyond.
Figure 59 (Below) The shoreline at Rosario Beach,
along the southwestern end of the island. This is a
particularly idyllic setting, one which this trip adopts
for a lunch stop.
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Image Credits:
Figure 2: Jonathon Lawson*
Figures 3, 31: Oldwirld*
Figures 4,5,20: Base images from Google Earth
Figures 25, 55: Texas Valerie*
Figure 27: Sabel*
Figures 28, 29: NW Wildman*
Figure 46: Ralph Dawes, Wenatchee Valley College
Figure 50: Elite 3*
Figures 58, 59: Aphex Twin*
Figure 61: Lei Zhichen
* These images are from the Flicker.com website, and are used with permission.
Figure 60
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References and
Further Information
General information and basic instruction in the earth sciences can be found in any current textbook of physical geology. Most students will find (to their amazement?) that they are capable of understanding such textbooks without the
direction of an instructor. They are in fact designed for this purpose.
A much more comprehensive appreciation for the subject can be had through college-level courses offered at NSCC
and the other colleges and universities in our area. North Seattle Community College offers classroom and fieldbased instruction in the following courses:
Geology 101
Geology 103
Geology 110
Geology 111
Physical Geology
Historical Geology
Environmental Geology
Geology of Washington
5 Credits
5 Credits
5 credits
1 credit
Fall, Spring and some Summer Quarters
Spring Quarter
Winter Quarter
Fall and Winter Quarters
Science 111
Science 118
Science 119
Science 121
Grology of the Northwest
Volcanoes of Washington
Natural History of Washington
Natural Disasters
1 credit
1 credit
3 credits
5 credits
Fall and Winter Quarter
Spring Quarter
Spring and/or Summer Quarters
Fall Quarter
Good general-information books on the geology of Washington are difficult to find.. A few popular titles include
Marge and Ted Mueller: Fire, Floods and Faults University of Idaho Press 2005 A non-technical guide to the
field geology of the Columbia Plateau
Scott Babcock and Bob Carson: Hiking Washington’s Geology The Mountaineers, Seattle 2000 A non-technical
guide to the geology on popular hiking routes. Not much general information beyond this.
Tabor and Haugerud: Geology of the North Cascades The Mountaineers, Seattle 1999 A very good guide to the
geology of the North Cascades
Additionally, there are a number of on-line sites which provide some information in this area. A leading reference is
the website on the geology of Washington at the Burke Museum:
Townsend and Figge: 2001 Northwest Origins:
www.washington.edu/burkemuseum/geology (or, you can just google “Pacific Northwest Geology” to get there).
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A Last Word From Your Instructor
The Pacific Northwest is home to some of the most remarkable geology to be found anywhere on the planet.
No region can claim to a greater variety of rock types, or features them in more spectacular settings. More
significantly, no region affords such a remarkable venue on the truly collosal forces which drive the dynamics of our planet, or such graphic illustrations on the variety of geologic processes which they support. There
is simply no better place on the planet to see how the Earth works. There is certainly no better place on
Earth to learn and experience geology.
Most students who are taking this course are not planning to major in the sciences, and have other plans for
their immediate future. My intention is not to dissuade anyone from following their passions. My only point
is that, if you are planning on living in this area (and what rational person wouldn’t?), you should know that
you are living in the midst of some of the most incredible geology in the world. You should know that the
modern landscape that surrounds you is the product of a truly amazing course of geologic history, one that
stretches back hundreds of millions of years. You should recognize that you occupy a unique point in time
and space in that course of history, and in the ongoing geologic processes which will continue to shape this
region into the future.
John
Figure 61(Above) Photo by Lei Zhichen
Figure 62(Right)
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