Over the last 150 years the Rock of Castle Rock has stood as the majestic symbol of the pioneering spirit of Cowlitz County. As it resides along the Cowlitz River, the Rock was used as a landmark for navigation by both the early settlers and indigenous population as they traveled North or South along the western edge of the Pacific Northwest. (Jackson 14-15)
For my Term Project for Geology 208, I took on the challenge to understand, not the recent history of the Rock, but rather the potentially exceptional geological history of this landmark and mascot of Castle Rock Washington.
Located to the South of the City of Castle Rock at 46° 16’04.27”N 122° 54’25.31”W. The Rock sits along the eastern bank of the Cowlitz River. According to my GPS the elivation at the top of the Rock is 180+ feet.
The Rock also sits along side and almost attached to “Quaternary volcaniclastic rocks, tuffs, lahars, and pyroclastic rocks.” (DNR) Although the map shows the Quaternary volcaniclastic rocks, tuffs, lahars, and pyroclastic rocks to the right and connected to the Rock. This deposit of igneous does not fully connect with the rock and is mostly a plateau along its length as it heads south and north.
On my field trip to the site I was unable to collect samples of the “Quaternary volcaniclastic rocks, tuffs, lahars, and pyroclastic rocks.” (DNR) Much of this area is covered in a thick layer of top soil and when investigating the area along the Cowlitz I found only dredge spoils from the 1980 eruption of Mt. St. Helens.
THE EXPECTATION BASED ON HISTORY
Now for the composition of the Rock itself. When investigating the surface geology of the area I used the Washington State DNR’s Interactive Geological Map on the www.dnr.wa.gov web site. It was the DNR Interactive Map that I used to obtain information on the quaternary volcaniclastic rocks that lie adjacent to the Rock. According to the interactive map the Rock is “Miocene-Oligocene Intrusive Igneous Rocks”. (DNR) This designation also includes a strip of the same Miocene-Oligocene Intrusive Igneous rock. This narrow ridge of similar rock to the Rock it self lies 2.5 miles south of the rock.
Yet the claim of the DNR that the Rock is an intrusive igneous structure was very surprising to me. Being that Castle Rock sits on top of Crescent/Siletz terranes, it was surprising to know that intrusive igneous structures could be found here. The DNR Interactive Map approximately dates the structure to 34 to 5 MA. That means the Rock was forming while this terrane was underwater through the time this entire area was finally buoyant enough to rise above the water. It makes sense that the ocean crust under Castle Rock would have gone through thrusting from the extra ocean crust and sediments being jammed beneath it from the newly sub-ducting Juan De Fuca Plate (Dawes) that could have created fissures and faults. (Roberts 38) Then there was also the Compression of the Juan De Fuca Plate sub-ducting underneath the Crescent and Siletz terrane that formed the Cascade mountains. (Alt 172-173) Also the Rock would have formed in the ocean sediments that have now eroded away. How extraordinary the idea that there was enough ocean sediments here to keep rising magma from erupting at the surface.
Before I left to investigate the Rock I was expecting to find gabbro or perhaps a peridotite deposit at the location. Both best fit the the definition of an intrusive igneous for this area. Or at least that is the conclusion I had come to. Personally I was leaning to towards the idea of the Rock being made out of peridotite. This should be a reasonable expectation because “intrusive igneous rocks such as diorite, gabbro, granite and pegmatite that solidify below Earth’s surface.” (geology.com)
I first began my investigation of the site “Miocene-Oligocene Intrusive Igneous Rocks” (DNR) south of the Rock I was met with a Cowlitz County Utility “No Trespassing” sign. Choosing to adopt ethical specimen collecting, I decided to heed the sign and not trespass for this project.
It was then decided by me, myself, and I that specimen rock collecting should be relegated the the location chosen for the term project. The picture to the right is an exposed outcropping of rock on the flank of the Rock. As can be seen in the picture the outcrop is composed of small blocks of what appear to be igneous in origin. With careful prying I was able to collect a small sample.
This exposure of rock on the Rock was my second collection site at the Rock in Castle Rock. Located on the western flanks of the Rock, this exposure has some larger blocks of rock. I was able to collect a specimen with some effort from this site.
Just 50 feet from the second collection site I was able to scramble onto the edge of this section of rock just left of center of this picture. This outcrop of rock is quite solid compared to the rest of the crumbling rock exposures on the Rock.
This solid outcrop is so stable this anchor point has been drilled into one side of this mass of rock. There was an attempt to collect a specimen from this site yet the rocks here proved to be so stable I was unable to remove a sample.
I attempted to scale the Rock to find more specimens to collect. On my way up I noticed that the Rock is covered with a thick layer of topsoil and what rock that was available at the surface to collect was most likely transported there to build up the trails on the Rock. Not wanting to disturb this historic monument of the proud peoples of the City of Castle Rock, I left the soil undisturbed.
Using newspaper as a scale reference I began taking pictures of the top rock (specimen #1) in this picture. This specimen measures 15 to 18 millimeters thick and is approximately 10 centimeters wide. My initial impressions about the rocks I was collecting not exhibiting characteristics of intrusive igneous rocks was made very obvious when examining this sample.
It was all to clear by the time I made an even closer examination of specimen two. Specimen two is a 5 x 10 mm chunk of basalt. There is nothing peridotiteor or granadiorite about either of the two specimens I collected. Rather both specimens are very black and are fine grained. Even with a jewlers loupe I was unable to distinguish the specimens I collected from the Rock from other examples of basalt from the local area.
After determining my samples as basalt I contacted my sister in-law whom possesses a degree in geology. I asked her to examine my specimens independently. I made sure to not let her know what my conclusions were from my observations of the specimens. When my sister-in-law was able to examine the rock specimens from the Rock she took but a moment pause and said “It’s basalt. So what do you wanna know?” I explained to her my findings with the Washington DNR Interactive Maps. My sister-in-law then immediately asked to use my computer to search the USGS website for corroborating information. After taking over 30 minutes to search or more information she concluded she was unable to find any information about the Rock online and was amazed I found as much as I did. After doing my own research the USGS mostly had water data about the Cowlitz River in Castle Rock. Ever since the day I picked the Rock in Castle Rock as my field site I have been trying to access the USGS geologic map of Castle Rock with little success. This has made my search for corroborating information from a reputable scientific organization far more difficult.
Undaunted by the lack of information, I turned to the source of my apparent bogus information about the Rock being intrusive igneous. I first fired off an email to the Washington State DNR to get some clarification on the classification of the Rock. I did not receive any response. I then found a general information e-mail address and received a response stating that my email had been forwarded the Geology & Earth Resources Division. As of the writing of this term project I have yet to receive a response to my e-mail.
This image contains the number two sample from the Rock and a specimen of granite from the Delameter Creek in Castle Rock. The Delameter Creek granite sample is what I would have expected to find when collecting samples from the Rock or perhaps peridotite would have been a better and more reasonable find at the Rock
The map above depicts the location of the samples from both the Rock and Delameter Creek locations. Its a personal curiosity why someone is able to find granite in a creek that cuts through “Quaternary alluvium, dune sand, loess, and artificial fill” (DNR) and not apart of a rocky outcrop that is supposedly intrusive igneous. The picture of the fossils that were collected from Delameter Creek.
The Hypothesis Based On My Findings.
The samples I have collected are just simply not what I have learned to be the mineral types expected from intrusive igneous rocks. Given the information I have found about the the Rock being a plutonic structure and my finding when analyzing the samples collected, I have generated a hypothesis.
Approximately 25 to 20 Ma the Juan de Fuca Plate began to sink into the earth and apply pressure against the the oceanic slab of the Crescent/Siletz terranes. This compressive force began to slightly deform the crust of the Crescent and Siletz terrane. At this point of Washington’s geologic history the Juan de Fuca plate had not reached far enough beneath Crescent terrane to infuse any rising magma with large amounts of dissolved gasses. The oceanic slab that western Washington now site’s on was still very moist in spots and this lowered the slabs melting point on those areas. The slab beneath Castle Rock started to melt. In combination of compressive forces of subduction and melting of the slab under Castle Rock the magma slowly made its way through the Crescent oceanic slab. As the magma came in contact with the ocean sediments above the slab the magma began to cool rapidly. This fast cooling close to the surface would be considered a hypabyssal igneous intrusion. Being that the intrusion was close to the surface, in moist ocean sediments, and being a small volume of intrusive magma; the cooling of the magma was rapid in terms of intrusive magmatic events. Such rapid cooling would prevent course crystallization of minerals found in a typical intrusive igneous structures. The Rock was exposed over time from erosion caused by the Cowlitz River as it cut out the valley it now flows through revealing a plutonic structure. Given the crumbling condition of much of the exposed rock, it is possible as much as 1/3rd or more of the original plutonic structure has been eroded away.
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