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ESS 305
Olympic National Park
2 May 2015
Note: This trip focuses on rocks in Olympic National Park. This raises three important issues:
1. Traffic. The Hurricane Ridge Road is very busy. Please drive carefully, and when
examining roadcuts, cross the road carefully and get completely off the road.
2. Rock Hammers and Collecting. It is against Federal law to hammer on the rocks or collect
specimens. If you wish to examine a loose specimen, find one on the ground, don't break it off
of the outcrop.
3. Meadow Trampling. The park is making a concerted effort to protect and restore meadows
in the Hurricane Ridge area. Please stay on the trails. Most of the rocks can be seen well
enough this way.
THE OLYMPIC MOUNTAINS
The Olympic Mountains represent a topographic and geologic anomaly along the Cascadia
convergent margin. Nowhere else from Vancouver Island to northern California are the mountains so
high or so rugged, and nowhere else are the 'guts' of the subduction zone complex exposed as they are
in the Olympics.
The Olympic Mountains reveal three main groups of rocks: the Crescent Formation, the Peripheral
Rocks, and the Core Rocks (see geologic map attached). The Crescent Formation and Peripheral
Rocks are part of the Coast Range Terrane, which extends from northern California to Vancouver
Island. The Coast Range Terrane consists of pillowed and massive basalt flows (the Crescent here)
mostly of Eocene age, overlain by Eocene and younger marine sedimentary rocks (the Peripheral).
The basalts of the Coast Range Terrane are thought to have been formed either (1) as seamounts or
oceanic crust, on the Kula or Farallon plates, or (2) as subduction-related basalts in a basin adjacent
to the continent. The basalts and associated sediments were accreted to the edge of the continent
during the mid-Eocene.
The Crescent Formation, consisting of basalt with interbedded sedimentary rocks, ranges from a few
hundred meters thick near Cape Flattery to over 3000 m south of Port Angeles, where we will view it
on this trip. The Peripheral Rocks, which overlie the Crescent Formation, are more than 6000 m
thick on the north edge of the peninsula. They are mostly siltstone, sandstone, and conglomerate of
mid-Eocene to mid-Miocene age. We will not see the Peripheral Rocks on this trip.
Together, the Crescent Formation and the Peripheral Rocks comprise a horseshoe-shaped belt around
the north, east, and south sides of the Olympic uplift. The horseshoe represents the eroded remains of
a dome or anticline that would form a cap over the core of the Olympics.
The Core Rocks lie within the arms of the basaltic Crescent Formation horseshoe (see geologic map),
and represent sedimentary rocks that were scraped off the subducting Juan de Fuca plate and attached
to the underside of the Crescent Formation basalts. Thus, the Core Rocks represent the 'guts' of the
subduction complex, the accretionary wedge, and have only been exposed here in the Olympics.
The Core Rocks have been divided into two main groups: the western core and the eastern core.
Eastern core rocks, which we will see today, are mostly metamorphosed shale, siltstone, and
sandstone that have been complexly and thoroughly deformed and display a slaty cleavage. The
sediments were deposited by turbidity currents, which are heavy, sediment-laden water packages that
flow down submarine slopes and canyons and deposit their load on the flatter surfaces below.
Western core rocks, which we will not see today, are less thoroughly deformed but do display
complex folds and faults.
Why are the Olympics so high and rugged?
The question really is: Why have the basalts and sedimentary rocks been pushed up so high in the
Olympics? Elsewhere in the Coast Range Terrane, the basalts and associated sediments merely form
a sloping or slightly folded structure, and the 'guts' of the subduction zone aren't exposed. The
answer appears to lie in the shape of the continental margin. The edge of the continental plate is bent,
with the Olympics as the apex of the bend. South of the Olympics, the edge is north-south; north of
the Olympics, the edge is northwest-southeast (see map). This means that the subduction zone is bent
and that the Juan de Fuca plate must also bend as it encounters the subduction zone. A flat plate must
arch up or down to make such a bend (try this with a sheet of paper). The Juan de Fuca plate does
indeed appear to be arched up under the Olympics; the evidence is from seismic data. That arch in
the plate is thought to have pushed the Coast Range terrane and the underlying subduction zone up,
forming the Olympic uplift.
Why are they so rugged? Since the Olympics rise so high, they 'rake' a lot of moisture out of the air
masses coming off the ocean. The voluminous precipitation has supported extensive glacial systems
during the ice ages; the glaciers existing today are mere remnants of this. Between ice ages, raging
rivers follow the same drainages. Together, these forces have carved the topography.
Finally, it should be noted that although the Olympic rocks have been assigned to terranes, these
rocks are not necessarily exotic. Some geochemical evidence suggests that some of the rocks came
from far away, but most of the rocks may well have formed right at the edge of the continent. Not all
terranes are exotic.
Take Interstate 5 North to the Edmonds/Kingston Ferry exit. Follow the signs to the ferry dock, then
take the ferry to Kingston. Follow Highway 104 West out of Kingston, towards Port Gamble and the
Hood Canal Bridge. Continue to the intersection of Highway 104 and Highway 101. Take 101 South
to the Mt. Walker Viewpoint (left turn), and proceed to the top.
STOP #1: MOUNT WALKER VIEWPOINT
Weather permitting, view the mountains to the east and west. How would you characterize and
explain the differences in topography between the Cascades and the Olympics?
Return to 101 North. Follow 101 through Sequim to Port Angeles. In downtown Port Angeles, turn
left at the sign for Olympic National Park - Hurricane Ridge. Follow the Hurricane Ridge Road up
past the Visitors Center and Heart O' the Hills Campground. Park in the lot on the left-hand side of
the road, immediately before the first tunnel.
STOP #2: TUNNELS OVERLOOK
If the weather permits, you can look across the Straits of Juan de Fuca to another exotic terrane,
Vancouver Island. Note that the lowland areas are much smoother than the more rugged topography
where you are standing. Why is this?
Set your trip odometer to zero at the Tunnels Overlook. Continue up the road for 1.9 more miles,
then park in the road turnouts along the sides of the road.
STOP #3: HURRICANE RIDGE ROADCUT
Study the rock in the tall roadcut along the right side of the road (looking uphill). Specifically, what
kind of rock is exposed here?
Did this rock form above or below sea level? How do you know?
Continue uphill 7.1 miles from the Tunnels Overlook, then pull off into the small paved road turnouts.
STOP #4: HURRICANE RIDGE FAULT ZONE
A) Walk about 30 yards uphill, then study the rocks exposed in the roadcut. Are these igneous,
metamorphic, or sedimentary rocks?
Specifically, what kind of rock is exposed here?
If these rocks originally formed in a horizontal orientation, how much have they been tilted?
B) Begin walking downhill. Note the small gullies that mark the locations of a series of faults
referred to as the Hurricane Ridge Fault Zone. Walk about 100 m downhill, and compare the rocks at
stop 4A to the rocks here. At which of these two locations are the rocks more intensely deformed?
Look at the rock types themselves. How do the rock types change on either side of the Hurricane
Fault Zone?
What was this area like when the sediments that form these rocks were deposited?
Continue driving uphill 8.2 miles from the Tunnels Overlook, and park along the side of the road.
STOP #5: SUMMIT ROADCUT
You are approaching the top of Hurricane Ridge. Compare the rocks exposed in the low roadcuts on
the right side, to the rocks from stop #4A. At which of these two locations are the rocks more
intensely deformed and metamorphosed?
Specifically, what kind of rock is found here?
Continue up the road for another mile to the Hurricane Ridge Lodge.
STOP #6: HURRICANE RIDGE LODGE
If the weather permits, you have a great view of the core of the Olympic Mountains. Note the
glaciers on Mt. Olympus and the rugged topography produced by the tilted rock layers and glacial
erosion.
Think about the series of stops made today. If the sequence of rocks from Stops 1 through 5 were
repositioned into their originally horizontal orientation, would you have traveled upward or
downward through the sequence? Briefly explain your answer, citing evidence from the various
stops.