The Geology of the Two Lights and Crescent Beach State Parks Area, Cape Elizabeth, Maine
Geologic History Prior to Glaciation
Now that we have examined the rocks and their structures, let's turn our attention to the history of the geologic events that have affected this area. When were the oldest rocks formed? The youngest? How did the area change over the long span of geologic time? Were there seaways where we now have land? Were there ever volcanoes in the area? Was the region ever mountainous, and if so, how did the mountains originate? Has the present Atlantic Ocean existed throughout the geologic history of the area? These are but a few of the many questions we raise in order to interpret the record of events preserved by the bedrock of the area. It is not possible to reconstruct all these events from geologic relations seen only within the bounds of the two state parks, and evidence from a broad area of Maine must be examined in order to understand what has happened in this local area.
As we move along in this discussion of the geologic history of the area, you will have to become used to thinking in terms of tens or hundreds of millions of years. A million years is an utterly unattainable time span in terms of a human life, yet in our deliberations on the geologic history of this or any region of the earth, a million years is like the passage of merely a week in the life of a human being (Figure 10).
Our geologic record begins approximately 500 million years ago at the end of the Cambrian Period or beginning of the Ordovician Period. An ocean covered all of Maine at that time, and the shoreline of the North American continent was located almost 300 miles to the northwest. In what is now the Portland area a great thickness of mud, sand, and limy mud accumulated on the ocean floor to form the Cape Elizabeth, Scarboro, Diamond Island, Spurwink, and Jewell Formations of the Casco Bay Group. Islands rising from the ocean floor somewhere in the general area were the sites of active volcanoes emitting great volumes of ash and lava flows that accumulated to form the Cushing and Spring Point Formations. We can picture the Portland area as part of a long narrow belt of sedimentation and volcanic activity extending from Newfoundland to Alabama. The geography of that time was probably similar to the present area of the Japanese Islands and the sea to the west between Japan and the Chinese mainland - a string of volcanic islands with a sea between them and the main continent. In the early part of the Ordovician Period, the rocks of the Casco Bay Group were folded during an episode of compression (an orogeny). The set of recumbent folds noted in Figure 8 may have been formed at this time.
During Late Ordovician, Silurian, and Early Devonian time, several thousand feet of sand and mud without volcanic eruptions were deposited in what is now southwestern and central Maine. The formations that comprise this sedimentary sequence are not present in the Two Lights-Crescent Beach State Parks area, but are seen to the west of the Casco Bay Group (for example, the Westbrook-Gorham area). These formations may once have extended over this area but were subsequently eroded away.
At the end of Early Devonian time, about 390-400 million years ago, the area underwent a second and much more extensive compressional upheaval known as the Acadian orogeny. During this orogeny the rocks were again folded, forming the second set of folds noted in Figure 7 and Figure 8 (these have the vertical axial planes). As a result of the compression, the sedimentary and volcanic rocks of the Casco Bay Group were depressed deep into the earth's crust, where high pressure and temperature metamorphosed them into phyllite, quartzite, and other associated rocks that we now see. The clay minerals contained in muds recrystallized to form fine flakes of mica seen with the aid of a microscope. At the same time, the most deeply depressed sediments began to melt and form large quantities of molten rock called magma. This magma, injected into the remaining metamorphic rocks as large masses, gradually cooled and crystallized to form the extensive granite and pegmatite bodies commonly seen to the north and west of Portland, Geochemists have determined that these granites formed at depths of between 10 and 15 miles below the land surface. At that time, southwestern Maine was probably a mountainous area, with peak elevations rivaling those of the Rocky Mountains and the Sierra Nevada. This gives us some idea of how much erosion of the land mass has occurred from Early Devonian time to the present.
Toward the close of the Acadian orogeny, the major faults that cut and displace the rocks in this area underwent their major movement. Additional fault movement occurred during the Mississippian, Pennsylvanian, and possibly Permian Periods (see Table 2). These faults extend from New Brunswick to Connecticut.
In late Permian to early Triassic time, approximately 225 million years ago, magma was again generated and injected into the metamorphic rocks and older granites in the Mt. Agamenticus area in York to the south. In late Triassic time, basaltic magma was injected into numerous tension fractures in the older rocks, forming dikes of fine-grained black basalt like the one that can be seen in the northwest part of Two Lights State Park. Similar dikes are sparse in the Portland area but are very common to the south between Biddeford and Kittery. Finally, during the Cretaceous Period, approximately 120 million years ago, renewed volcanic activity and magmatic intrusions occurred in the York, South Berwick, and Alfred area. No further tectonic events, except sporadic crustal uplift and continued erosion, are recognized in the bedrock record for this area.
Migrating Continents and Spreading Sea Floors
You may now be wondering how the different periods of compression came about. What was the cause of these orogenies that deformed the rocks, metamorphosed them, and caused the granites and pegmatites to form? The answer is best found in our modern geological notions of mobile, spreading sea floors and the movement of large continental plates.
In 1912, a German climatologist, Alfred Wegener, proposed the revolutionary idea that continents had not always been in their present positions relative to one another. He pulled together several lines of evidence which suggested that the continents of the earth had long ago formed a single super landmass which he called Pangea. Wegener hypothesized that Pangea broke up into several separate pieces to form the present continents, and that these then drifted apart. Although the years between 1920 and 1950 were unkind to Wegener and his grand idea, we now realize he planted the seeds of a very cogent global concept. We have now returned to a modified version of "continental drift" in which the continents are considered to have been displaced relative to one another by a process which we call "sea-floor spreading."
Oceanic and continental lithosphere rest upon a shell of material capable of movement when heated from below. This shell, which is referred to as a mantle*, extends down to a depth of about 1900 miles (2900 kilometers). Beneath the mantle is the earth's core, which is composed of iron and nickel. It is in the mantle where the motions originate that cause the sea floors to move laterally. Due to heating, probably from the decay of radioactive elements, convection currents form in the mantle which cause the lithosphere to move (Figure 11).
*Note: The professional geologist will recognize that this is not a precise definition of the layers of the Earth. Technically the lithosphere includes the upper, rigid part of the mantle. For simplicity, we will refer to the mantle as the shell between the lithosphere and the core.
We now realize that the imposing mid-ocean ridges, such as the Mid-Atlantic Ridge which extends down the middle of the Atlantic Ocean, are areas where new oceanic crust constantly forms and spreads to either side. Just as oceanic lithosphere forms at these mid-ocean ridges, it must return to depth somewhere else to complete the cycle. The sites of this return, we believe, are the deep, narrow, and linear to gently curving ocean-bottom trenches such as those so well developed in the Pacific Ocean. At these subduction zones, plates of ocean lithosphere slide diagonally downward into the mantle. It is also along active subduction zones where the largest earthquakes take place. The trenches and the earthquakes are created by the downward drag of the subsiding plates (Figure 12). As the oceanic lithosphere slab descents into the mantle, heat begins to melt it and form magma which works its way toward the surface. This magma pours out through volcanic vents to begin the formation of a chain of volcanic islands (an island arc) on the opposite side of the trench from the moving and descending plate. As the island arc builds to a relatively large size, some of the magma produced from the subsiding slab is injected into the rocks of the arc to form large bodies of coarse-grained igneous rocks.
With this brief introduction to the concept of Plate Tectonics, we can now proceed to outline the broad picture of how the rocks throughout the northern part of the Appalachian Mountain belt may have evolved, and how the rocks of the local area fit into this grand scheme. Figure 13 presents a series of cross-sections that illustrate in generalized form the stages in the development of the area in terms of plate tectonic theory. Our understanding of the details of this evolution is still very sketchy and will be refined as future studies clarify our knowledge of the regional geology.
Our narrative begins back in Middle Cambrian time, about 530 million years ago. At that time an ocean much like the Atlantic Ocean covered our area (Figure 13a). On the western margin of this ocean was a continental landmass, ancient North America, and on the eastern margin, a landmass geologists refer to as Avalonia (after the Avalon Peninsula in Newfoundland which is believed to be part of that ancient landmass). The ocean between the two has been given the name Iapetus. Although it existed in much the same area as the present Atlantic Ocean, Iapetus closed (as I note below), and much later the present Atlantic Ocean opened.
In Late Cambrian to Early Ordovician time, volcanic activity commenced in our area with the deposition of volcanic ash and related material that make up the Cushing Formation. Similar eruptions began about the same time in western New Hampshire. Geologists believe this volcanic activity resulted when Iapetus started to close. Closure took place when slabs of oceanic lithosphere began to descend back into the mantle along subduction zones (Figure 13b). Upon reaching sufficient depth, these slabs, along with the oceanic sediments dragged down with them, began to melt and form magma that worked its way to the surface and erupted, initiating an island arc. The Cushing and Spring Point Formations are evidence of this island arc activity. As the exposed parts of the island arc eroded, the rocks of the Casco Bay Group were deposited as an apron of sediments around the edge of the arc. Eventually the oceanic slab was consumed beneath the island arc, and the arc collided with the Avalonian landmass (Figure 13c), resulting in the first deformation of the rocks of the area. This probably happened in Early Ordovician time and is the event during which the recumbent folds were likely developed.
A similar island arc that formed in what is now western New Hampshire collided with ancient North America in Late Ordovician time as the intervening ocean slab was consumed by subduction beneath the arc. This collision resulted in the deformation of the rocks of that arc, an event that we refer to as the Taconic orogeny (Figure 13d). Following this deformation, sedimentary rocks of Silurian and Early Devonian age accumulated in the remaining part of Iapetus (Figure 13d). Continued shrinkage of Iapetus by subduction at its eastern edge produced volcanic activity on the edge of Avalonia, presumably just east of the Portland area (Figure 13e). Finally, in Early Devonian time, Avalonia collided with North America, completely closing Iapetus and causing the Acadian orogeny (Figure 13f). This took place about 390 million years ago and completed the consolidation of New England as part of the North American continent. Along with this deformation, during which the upright folds of the Casco Bay Group were formed, granitic magma was generated and injected into the deformed rocks, forming the large granite masses and numerous small pegmatitic bodies that are common in the general area (Figure 13f). The deformed rocks were also metamorphosed at this time.
Parts of southwestern New England, New Brunswick, and Nova Scotia record still another deformational event, the Alleghenian orogeny, during Pennsylvanian to Permian time. Geologists believe this may have been a result of the collision of Africa, South America, and Eurasia with North America. It is possible that some of the faults of the Portland area may have moved during this time; however, most of the fault movements appear to be related to the Acadian orogeny.
About 190 million years ago, in Late Triassic time, convection currents of hot mantle material began rising and spreading apart below the newly consolidated supercontinent called Pangea. Under this tension the lithosphere cracked and formed rift basins such as the Connecticut River Valley and Fundy Basin. These basins rapidly filled with sediment and served as ideal habitats for dinosaurs which roamed the shallow marshes and mudflats of the time. By the end of the Jurassic Period, rift basins east of the present coastline began to split open as new ocean lithosphere formed along the rift. Continued upwelling of oceanic lithosphere caused the rift margins to move farther apart, eventually allowing inundation by the sea to form the Atlantic Ocean (Figure 13g). This activity, the upwelling and emplacement of oceanic lithosphere along the rift axis, the Mid Atlantic Ridge, is still going on today, resulting in an ever-widening Atlantic Ocean. The basalt dikes described above, one of which can be seen in the northwestern part of Two Lights State Park, are believed to have formed during the Late Triassic to Late Jurassic episode of lithosphere tension and fracturing. Table 2 summarizes the geological events of the area surrounding the two state parks.
Last updated on January 16, 2008