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 

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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.

  

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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

  

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Lion Rock

Lion Rock — alternate south facing view

  

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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:

  

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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:

  

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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.

  

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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

  

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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.

  

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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.