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Folded Metamorphic Rocks Near Willard Beach, South Portland
Willard Beach, south of Spring Point in South Portland, is a crescent-shaped pocket beach nestled between rocky headlands (see Figure 1). Figure 2 shows the view looking north across the beach to the cliffs of Spring Point with the Spring Point Ledge Lighthouse in the distance. South of the beach is a small rocky promontory that is this month's geologic site. The bedrock exposed here belongs to the Cape Elizabeth Formation. The Cape Elizabeth Formation is characterized by layers of a massive, gray rock alternating with thinner layers of white quartzite, shown in Figure 3. These layers represent what originally were layers of gray, silty mud and white quartz sand, respectively, that accumulated gradually on the floor of an ocean in the Ordovician Period of geologic time. The Ordovician age of the Cape Elizabeth Formation has not been determined directly, but laboratory tests have shown that the the Cushing Formation, the next older unit to the Cape Elizabeth, is 471,000,000 years old (with a small experimental uncertainty of a few million years), which makes this group of rocks about twice as old as the earliest dinosaurs. The Cape Elizabeth Formation can be traced to the northeast across several islands in Casco Bay, and is also found in a complex pattern on the ground through South Portland and Cape Elizabeth. The rocks just north of Willard Beach, at Spring Point, do not have the same type of layering and belong to different formations.
In Figure 3 and Figure 4 notice that the rock layers at this site are not "flat" (horizontal), the way sediment would normally be deposited, but are tilted and even folded into complicated shapes. The contorted shapes of the layers indicate that the rock has been deformed at some time since it formed. These are therefore metamorphic or changed rocks. Common sense says that solid rock can't be deformed in this way under normal earth-surface conditions, and laboratory experiments confirm that such changes require significant pressures and high temperatures to occur. We are looking at rocks that at one time were heated to several hundred degrees and were well below the earth's surface. The vise-like horizontal pressures were produced by convergent continental motions in the Devonian Period (360 to 420 million years ago) which produced the Appalachian-Caledonian Mountain system of eastern North America and northwestern Europe.
The photo with the rock hammer (Figure 4) shows further evidence for metamorphic changes that took place in the rock. One prominent feature is that the rock is cut by a vertical cleavage. It is common in metamorphic rocks to find that the same horizontal compression that caused layers to crumple also caused a vertical cleavage to develop. As a result, the rock tends to split or cleave more easily in this direction than in other directions due to a microscopic alignment of mineral grains. While the cleavage has certainly been accentuated on the rock surface by exposure to the elements, it is an internal characteristic of the rock itself. Another noticeable metamorphic effect is the development of small, milky white lumps of quartz that were deposited from hot fluids circulating in small veins through the rock.
Originally published on the web as the June 1998 Site of the Month.
Last updated on October 6, 2005
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