Welcome to Alki Point
At Alki Point in Seattle, a small piece of the original Plymouth Rock rests within the Birthplace of Seattle Monument. While it may seem like a simple stone, this rock is a direct connection to Earth’s deep history. Carved from Dedham Granite, a coarse-grained igneous rock, it formed over 600 million years ago deep beneath the Earth’s surface. Through this EarthCache, you’ll explore the geological story recorded in the rock, from its mineral composition to its formation deep underground.
The Story of Plymouth Rock
The stone we now call Plymouth Rock began forming over 600 million years ago during the Ediacaran Period, deep underground when molten magma slowly cooled and crystallized into coarse-grained Dedham Granite. This slow cooling allowed large mineral crystals to form, primarily quartz, feldspar, and mica. The pinkish-gray color comes mainly from feldspar, while the shiny mica flakes and glassy quartz add texture and sparkle to the stone. The rock’s coarse-grained texture and visible crystals are a direct record of the slow crystallization process that took place far beneath the Earth’s surface.
Over hundreds of millions of years, tectonic forces gradually uplifted the Dedham Granite toward the surface. The region experienced repeated episodes of folding, faulting, and erosion, exposing portions of the granite and allowing it to eventually become part of the landscape. During the last ice age, massive glaciers scoured New England, picking up pieces of Dedham Granite and transporting them across the terrain. Plymouth Rock was carried by glacial ice from its original bedrock source and deposited at Plymouth Harbor when the glaciers melted, making it a classic example of a glacial erratic.
Although humans have carved, chipped, and moved the rock over centuries to preserve it and display it for visitors, this Plymouth Rock fragment still preserves its original mineral composition and ancient formation story. The quartz, feldspar, and mica visible today are the same minerals that crystallized hundreds of millions of years ago. By studying the rock’s texture, color, and composition, we gain insight not only into the history of Dedham Granite but also into the deep geological processes that shaped New England long before humans arrived. Plymouth Rock remains a tangible connection to both Earth’s deep past and the historical events associated with it.
Typical Granite Composition
Granite is identified and classified by the minerals it contains, which form as magma cools slowly deep within the Earth’s crust. Typical minerals present in most granite include:
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Quartz – A light gray to clear, glassy mineral with no cleavage that breaks with curved surfaces. Quartz crystallizes late from magma and fills spaces between other minerals, contributing to granite’s hardness and interlocking texture.
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Potassium feldspar – Often pink, salmon, or light red in color and commonly the most visible mineral in granite. It forms blocky crystals with two cleavage directions at right angles and reflects the potassium-rich chemistry of the magma.
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Plagioclase feldspar – Usually white to gray and sometimes slightly cloudy in appearance. This mineral also shows two cleavages at right angles and may display fine parallel striations on crystal surfaces, helping distinguish it from potassium feldspar.
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Biotite – A dark brown to black mica that appears as thin, shiny flakes. Biotite splits easily into sheets and records higher-temperature conditions during the early stages of granite crystallization.
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Amphibole – A dark green to black mineral that forms elongated, prismatic crystals. Amphibole crystallizes early from magma and provides clues about the temperature and water content of the magma that formed the granite.
The size of these mineral crystals provides important clues about how the rock formed. Large, easily visible crystals indicate that the magma cooled slowly, allowing minerals time to grow. If the crystals are smaller or more uneven in size, it suggests changes in cooling rate or magma movement during crystallization. By observing mineral size and distribution, geologists can interpret the cooling history and depth at which the granite formed.
Tasks for This EarthCache
To log this EarthCache, visit the site and complete the following tasks. Send me your answers via Geocaching or email.
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Include "A Piece of Plymouth Rock in Seattle - GCBHX3T" on the first line of your message.
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Describe the overall (a) size, (b) texture, and (c) color of the rock. Which characteristics help you recognize it as granite?
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Examine the mineral grains closely. Which mineral seems most common, and which is least common? How does this support the identification of the rock as Dedham Granite?
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Look at the size of the individual mineral crystals. Are they large, small, or a mix? What does their size tell you about how slowly or quickly the granite cooled?
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Do you notice any signs of weathering on the rock’s surface? Compare it to other parts of the monument. Why do you think this section has weathered more or less than the rest?
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In your log, attach a photo of yourself or a personal item with the piece of Plymouth Rock. (Note: photos predating the publication of this EarthCache are not accepted.)
Works Cited
https://www.britannica.com/topic/Plymouth-Rock-United-States-history
https://en.wikipedia.org/wiki/Dedham_Granite
https://readtheplaque.com/plaque/from-plymouth-rock-to-alki-point#gsc.tab=0
https://www.geologyin.com/2024/06/granite-composition-and-colors.html