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written post Geology field trip to the Northern Oregon Coast
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Running Head: SEA STACKS: MONOLITHS OF THE SHORELINE
Sea Stacks Monoliths of the Shoreline
Andrea E. Janda
Portland State University
G‐200‐001 FIELD STUDIES OREGON COAST
Michael L. ummings S
Cummer 2010
July 2, 2010
SEA STACKS: MONOLITHS OF THE SHORELINE 1
INTRODUCTION
Aerial View of Arcadia Beach, Seaside, OR
Arcadia Beach is a beach and state park located in Clatsop County, Oregon on the
northwest Oregon Coast two miles south of Cannon Beach. One of Arcadia Beach’s prominent
features is a cliff wall or headland extending down into a small collection of rock formations or
sea stacks known as Lion Rock, most likely named for the lion-like shape and appearance they
cast as a combined group, looking off into the watery distance like a diminutive Sphinx.
SEA STACKS: MONOLITHS OF THE SHORELINE 2
Similarly, Cannon Beach is a scenic location popularized by its landmark, Haystack
Rock, the third-tallest igneous rock and sea stack formation in the world (Wikipedia, 2010). The
term igneous is derived from the Latin igneus meaning of fire, and from ignis meaning fire,
which should offer a striking clue as to the origin of these sea stacks. Haystack Rock and its two
smaller, neighboring rock formations cleverly named the Needles, are some of the most
photographed sea stacks, attracting both tourists and marine wildlife (Wikipedia, 2010). As such,
it has been designated a protected Marine Garden, sanctuary, and refuge for nesting birds
(Haystack Rock, City of Cannon Beach, 2007).
I had expectations (or more, hopes) that the Field Study I participated in would take me
to Cannon Beach to see Haystack Rock since we were so close, but I happily resigned myself to
taking photographs and studying the smaller sea stacks of Arcadia Beach to include Lion Rock.
DATA
Sea stacks at Arcadia Beach, Lion Rock, to the right just before the cliff
SEA STACKS: MONOLITHS OF THE SHORELINE 3
Lion Rock
Lion Rock — alternate south facing view
SEA STACKS: MONOLITHS OF THE SHORELINE 4
Upon entering Arcadia Beach, I tried to contemplate how these huge rocks were “put”
there. I observed several large rock formations just at the foot of the major cliff wall and
wondered if they had broken off or tumbled there, but then—what about the rocks further out?
Had the shoreline been depleted or instead, been built up by varying sand deposits? Had they
been pushed or gradually carried out by the waves or the moving sand? Highly unlikely, but
what did I know of geology except for what I could see and attempt to imagine?
Getting closer views of the rock formations revealed several different structural patterns,
colors and shapes. Some looked almost crystalline in formation, jagged and broken off, like
disjointed columns as in the photo I took below:
SEA STACKS: MONOLITHS OF THE SHORELINE 5
I observed what appeared to me as if several stone varieties had been mashed up together,
like occlusions, with textures ranging from sheer and flat to lumpy and bubbled with colors
ranging from white, beige, red, brown, rust, green and on into black.
There were some stones closer up that looked more porous, like sponges, but still
exhibited that mottled color range as in the photo I took below:
Even more curious were those rock features that looked like melted stone, pooling out
layer upon layer, as if they had frozen like several puddles or chunks of melted brown butter,
with some even appearing grey, black and antique green as in the photos I took below:
SEA STACKS: MONOLITHS OF THE SHORELINE 6
SEA STACKS: MONOLITHS OF THE SHORELINE 7
DISCUSSION
To understand what I was looking at, my first inquiry was to the nature of the large rocks.
During the Field Study I learned that these formations were called sea stacks and so I went about
researching their placement and composition as it applied to local Oregon geology.
In essence, a sea stack is similar to a small island. In fact, some begin their lives as such
but are progressively worn away or eroded. Typically, a sea stack is formed by the natural
erosion of headlands or projections along the coastline (Encyclopædia Britannica, 2010). It is a
process by which the ocean laps at the side of a headland, creating what are called wave-cut
platforms, eventually wearing a hole, and thereby creating an arch which continues to expand
over time as it erodes. This arch ultimately collapses, leaving behind what looks like an isolated
rock or two, the sea stack(s), on one side and the remaining headland on the other side (Tarbuck
& Lutgens, 2006). Once it is separated from the shoreline, the ocean begins to work on the sea
stack as well, wearing it down until it collapses or slowly melts away into the water
(Encyclopædia Britannica, 2010).
This erosion along rocky coasts such as Oregon occurs at different rates, depending both
on the rock type, its varied resistance, and the energy of the waves at any given location
(Encyclopædia Britannica, 2010). The figure below illustrates the concept of wave refraction on
irregular shaped coastlines, how it wears down headlands, and the resulting erosion formations
of sea caves, arches, and the fallen sea stacks.
SEA STACKS: MONOLITHS OF THE SHORELINE 8
Figure 15.13 (Tarbuck, & Lutgens, 2006) Wave refraction along an irregular coastline. Waves touch the shallows off the headlands, are slowed and refracted. Wave energy is then concentrated at the headlands where erosion occurs, forming sea caves, sea arches, and eventually, the fallen sea stacks.
Progression of sea stack formation and erosion
The stacks and seacliffs on Cannon Beach and those observed at Arcadia Beach Wayside
are composed of basalt. Basalt, the most abundant rock in Oregon, covers much of eastern
Oregon and the Columbia River Gorge, providing a foundation for the High Cascades,
supporting coastal headlands, and mingling with sedimentary rock throughout the northern
SEA STACKS: MONOLITHS OF THE SHORELINE 9
Oregon Coast (Bishop, 2004). It is estimated that this Columbia River basalt is the product of at
least twelve large basalt lava flows that traveled more than 300 miles from vents in eastern
Oregon and Washington out to the coast (Bishop, 2004).
Some of the colors and textures appearing in these basalts are due to where they “mixed
with, flowed around and even incorporated sediments” (Bishop, 2004, p. 77). The green color
appearing in the sea stacks and cliffs are labeled an obvious, catch-all term, greenstone, and are
“gently metamorphosed basalt or other igneous rock” that become colored as a result of iron-
bearing minerals in the original rock that are altered during this metamorphism (Bishop, 2004, p.
260). The layers of umber-red basalts are the result of oxidation that occurred when seawater
boiled as the hot lava slid down and was cooled. The broken or what are termed “brecciated”
basalts are less stately than the Romanesque, columnar basalts seen elsewhere in Oregon, but still
exhibit that beautifully mixed and mottled set of patterns and textures (Bishop, 2004).
It is the sheer weight of the Columbia River basalt that drove hot lava down into the
softer sediments where the intrusions and invasions exploited the subtle joints and folds, sending
fractures downward (Bishop, 2004). We can observe these sinuous layers today, exposed on the
headlands, scathed away by wind and water. Closer still, we can explore the shoreline monoliths
called sea stacks, which are destined to weather away over the years as we enjoy their curious
hues and shapes and the garden of marine wildlife that surrounds them.
SEA STACKS: MONOLITHS OF THE SHORELINE 10
REFERENCES
Aerial View of Arcadia Beach, Seaside, OR, 97102. 45°50'50.43" N and 123°57'41.65" W.
Google Earth. June 29, 2005. January 17, 2009. Retrieved June 30, 2010.
Bishop, H.M. (2004). Hiking Oregon’s geology. (2nd ed.). Seattle, WA: The Mountaineer
Books.
coastal landforms. (2010). In Encyclopædia Britannica. Retrieved from Encyclopædia Britannica
Online: http://www.britannica.com/EBchecked/topic/123147/coastal-landform
Haystack Rock. (2007). City of Cannon Beach, website. Retrieved from:
http://www.ci.cannon-beach.or.us/~Natural/hrap.html
Haystack Rock. (2010, May 27). In Wikipedia, The Free Encyclopedia. Retrieved from:
http://en.wikipedia.org/wiki/Haystack_Rock
sea stack. (2010). In Encyclopædia Britannica. Retrieved from Encyclopædia Britannica
Online: http://www.britannica.com/EBchecked/topic/530748/sea-stack
Tarbuck, E.J. & Lutgens, F.K. (2006). Earth science. (11th ed.). UpperSaddle River, NJ:
Pearson Prentice Hall.
U.S. Geological Survey. (1999, September 10). Park Shoreline. Sea Stack. Retrieved from:
http://www.nature.nps.gov/geology/usgsnps/sea/gseastack.html
Wave refraction. Figure 15.13. Diagram. Tarbuck, E.J. & Lutgens, F.K. (2006). Earth science.
(11th ed.). UpperSaddle River, NJ: Pearson Prentice Hall, p.418.