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The Rocks of the Two Lights - Crescent Beach Area
The rocks of the Cape Elizabeth area are divided into seven formations, each with its own name as indicated in Table 1. These seven formations are collectively referred to as the Casco Bay Group. Only two formations of the Casco Bay Group, the Cape Elizabeth Formation and the Scarboro Formation, are exposed within the limits of the two state parks. The Cape Elizabeth Formation forms the bold cliffs at Two Lights State Park and the Scarboro Formation is exposed in Crescent Beach State Park at the western end of the beach and in small road cuts between the gatehouse and the parking area.
The Cape Elizabeth Formation at Two Lights State Park
Phyllite is a rather soft metamorphic rock of such fineness of grain that the individual mineral particles can only be seen under a microscope. If you were to examine a thin slice of this rock with a microscope, you would find that it is composed mostly of fine grains of white and dark colored mica, minerals that readily split into sheets. Partly due to the presence of large quantities of mica, phyllite splits easily in one direction, parallel to the mica flakes. We say it has "cleavage." In this respect, phyllite is somewhat like slate, except that its cleavage is wavy or crinkled rather than smooth like that of slate.
Quartzite is a metamorphic rock composed almost entirely of the mineral quartz and is very hard and resistant. Quartzite of the Cape Elizabeth Formation, when examined with a microscope, has small amounts of mica, feldspar, and two carbonate minerals (calcite and ankerite); thus it is not a pure quartzite.
Originally these rocks accumulated as alternating layers (beds) of muddy sand and mud on the floor of an ancient ocean. The deposition of minerals between the sand grains to cement them together and the compaction of the mud by the weight of sediments that accumulated above them transformed these sediments into hard shaly sandstone and shale. When the sedimentary rocks were deeply depressed into the earth's crust by powerful compressional forces (which will be discussed fully a little later) they were recrystallized, or metamorphosed, the shale becoming the dark phyllite, and the shaly sandstone becoming the tan quartzite.
One of the most conspicuous aspects of the Cape Elizabeth Formation is its striking resemblance to weather-beaten wood (Figure 3). Indeed, one of the most frequently asked questions is how all the petrified wood was formed at the park. Actually, there is no petrified wood here, and the similarity is in superficial appearance only since all the materials of the Cape Elizabeth rocks are of inorganic sedimentary origin. The woody appearance is due to the presence of very closely-spaced microscopic fractures which cause the quartzite, in particular, to weather to a wood-grain appearance.
As you examine the Cape Elizabeth Formation, you will become aware of the sedimentary layering or bedding of these metamorphic rocks. This bedding indicates that the rocks were originally sediments prior to compaction and metamorphism. You may occasionally observe beds like those to the right of the hammerhead in Figure 4. Notice that each smooth light-colored quartzite layer grades progressively into the rough dark-colored phyllite layer above, followed at an abrupt boundary by the quartzite layer next above. We refer to this as graded bedding. At the time of deposition, the quartzite was the coarser-grained part, and the phyllite the finer-grained part of a graded bed. Graded beds are useful in determining the sequence in which beds were deposited, especially in areas like Two Lights State Park where the rocks have been strongly deformed by folding. Of perhaps greater significance is the fact that these graded beds suggest that the Cape Elizabeth Formation accumulated in relatively deep water, possibly at the foot of an ancient oceanic island arc slope. The sediments were deposited by turbidity currents which required relatively steep submarine slopes down which to transport their sediment load. At the base of the slope, the turbid mass of sediment-laden water slowed, and as it did the coarser materials settled out first, followed by the finer materials, thus producing a graded bed such as seen in Figure 4.
In addition to the quartzite and phyllite that comprise the Cape Elizabeth Formation, two other rock types are present - vein quartz and basalt. When the rocks of the Cape Elizabeth Formation were deformed and metamorphosed, vein quartz was formed by the precipitation of quartz in fractures in the rock. Veins of this white milky quartz occur throughout the Cape Elizabeth Formation in a variety of shapes ranging from short, relatively straight veins to very thin crinkled veins and irregular pods. Basalt is an igneous rock formed from molten rock that we call magma. Near the northwest boundary of the State Park, basalt occurs in the form of a three-foot (one meter) thick tabular, sheet-like body called a dike. The rock itself is dark gray on newly broken fresh surfaces, but weathers to a slightly rusty color due to the presence of minute amounts of the iron sulfide mineral pyrite. The individual mineral grains that make up the basalt are generally too small to be identified without the aid of a microscope. This dike originated when molten rock from a great depth below the surface rose through and solidified in vertical tension cracks of the earth's crust. We will refer to this feature later when we discuss the geological history of the area, particularly as it relates to the formation of the Atlantic Ocean.
Structural Features of the Cape Elizabeth Formation
The rocks of the Cape Elizabeth Formation are broken into large rectangular blocks by fractures called joints. Joints are planar rock fractures along which no perceptible movement has taken place. Joints do not form individually but occur in sets of parallel fractures; commonly two or more sets having different orientations are developed. In Figure 3, two sets of joints are visible; still other sets occur elsewhere in the formation. These joints may have been formed during the late stages of a period of uplift and erosion which brought the deeply-buried rocks of the Cape Elizabeth Formation to the surface and relieved the internal stresses in the rock.
The inclined part of a fold between adjacent axial planes is the limb of a fold, and the region where the axes are is the hinge (Figure 5). Note that the limb of a syncline is also the limb of an adjacent anticline.
Let's take a look at some actual examples of folds that you are likely to see as you walk over the ledges at Two Lights State Park. Figure 6 shows beds of Cape Elizabeth Formation bent into a gentle syncline. We say "gentle" because the inclination of the limbs toward the axis is very slight. The axial plane of this syncline is nearly vertical. Figure 7 shows several small anticlines and synclines with vertical axial planes. A close look at the area with the box in Figure 7 (Figure 8a) reveals the complexity of folding that has affected the Cape Elizabeth Formation. Figure 8b is an interpretative sketch of the two different sets of folds that are present. To the left of center in this close-up one can see some small-scale folds whose axial planes are essentially horizontal; these folds lie on their sides, and we say they are recumbent. Note in Figure 8a the closely-spaced fractures that are roughly horizontal and parallel to the axial planes of the recumbent folds. These fractures form what we call an axial plane cleavage. They were formed at the same time and by the same forces as the recumbent folds. Another very important relationship shown in Figure 8a is the warping of the axial plane cleavage; it is deformed very gently by the same set of folds with the vertical axial planes that we noted in Figure 6 and Figure 7. It is evident that the rocks of the Cape Elizabeth Formation have been subjected to at least two episodes of folding. Figure 9 will, perhaps, help you visualize and understand the relations of these folds to each other - tight recumbent folds formed during an early period of compression, gently deformed by open, upright folds of a later compressional episode.
You can now see that the rocks of the Cape Elizabeth Formation preserve within them quite a bit of history: deep-water sedimentation by turbidity currents at the foot of an ancient island arc slope; burial to great depths in the earth's crust where metamorphism took place; and multiple episodes of compressional deformation during each of which folding occurred. During a long period of uplift and erosion, these rocks were brought from the depths of the earth's crust to the surface where they originate, but with a totally changed appearance.
The Scarboro Formation at Crescent Beach State Park
The ledges located at the southwest end of Crescent Beach expose three different rock types that make up part of the Scarboro Formation. As you walk southward and come to Jordan Point at the end of the sandy beach, the first rocks you encounter are gray and slightly greenish phyllite. Tiny red garnet crystals can be seen in some beds. Upon rounding the point where the 10-foot (3 meter) high seacliff begins, you walk across about 30 feet (9 meters) of medium gray limestone intermixed with thin beds of dark gray phyllite. The remainder of the ledge to the south consists of rusty-stained and contorted phyllite and gray quartzite beds up to two feet thick. The rust staining is the result of the weathering and decomposition of a small amount of pyrite. Before being metamorphosed, the rocks of the Scarboro Formation were marine shales with a thin unit of limestone. As you walk along these ledges, be on the lookout for folds, joints, and quartz veins like those described for the Cape Elizabeth Formation. Note that the beds of the Scarboro Formation are more steeply inclined than those of the Cape Elizabeth Formation at Two Lights.
Two faults have been mapped in the general vicinity of the two state parks (Figure 2). A fault is a surface along which rocks have been broken and then have moved past one another. When fracture and movement occur, earthquake tremors are usually generated.
One fault, extending from west of Peabbles Point southwesterly to Ram Island Farm in the far western part of Cape Elizabeth (just to the west of the edge of Figure 2), is recognized on the basis of the abrupt termination of the Cushing and Spring Point Formations as shown on Figure 2. In addition, a zone of massive white quartz (which geologists refer to as a "silicified zone") has been formed along the fault and can be seen on the road leading to Ram Island Farm. This vein quartz, in places up to 20 feet (7 meters) wide, can be followed for approximately a mile and a half (2 km) along the fault. The other fault is covered by surficial materials and its location is inferred from evidence visible in the surrounding rock outcrops.
Last updated on January 16, 2008
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