The Geology of Two Lights State Park, Cape Elizabeth

The rock ledges exposed at Two Lights State Park (Figure 1) represent the current stage of a very long and dynamic geologic history. The rock that makes up the ledges is classified as metamorphic rock, which means that at one time it was affected by heat and pressure inside the earth which has significantly changed its form. Before metamorphism it was sedimentary rock, which had formed originally by hardening of deep-sea sediment (sand and mud) into rock. After metamorphism, continued geologic stresses on Maine's bedrock crust have produced fractures (faults and joints) cutting through the rock mass in various directions. The modern coastline is still being shaped by the ocean which has washed away all of the unconsolidated glacial deposits that used to cover the bedrock along the shore so that the rock ledges are exposed. Occasionally, even large blocks of rock are dislodged.

Here is a brief outline of the geologic events that affected this part of Maine, with a description of specific features to look for at Two Lights. Note that geologic processes occur very slowly so that this history covers an immense span of geologic time. The estimated ages are given in millions of years ago.

420 million years ago, Silurian Period: Sediment accumulated in a deep ocean basin. This basin lay between the edge of North America to the west, which at the time ran approximately from Flagstaff Lake to northern Moosehead Lake, and an ancient small continent to the east, the microcontinent called Avalon. The layers of sediment that accumulated in that Silurian ocean are preserved in the rocks at Two Lights as light gray, massive quartzite and dark gray phyllite layers (Figure 2). The quartzite layers were originally sand or silt beds, and the phyllite layers were originally clay or mud beds. As the layers accumulated one on the other, some were deposited individually with sharp boundaries between them, and others were deposited as mixed masses in graded beds (Figure 3) that change gradually from the bottom to the top of the layer. All the rocks at Two Lights State Park belong to a single geologic formation. Geologists in the early 1900's referred to it as the Cape Elizabeth Formation, but geologists now think that these rocks are part of the Kittery Formation (Figure 4), which is found along the southern Maine coast and into New Hampshire.

Figure 2

Figure 3

Figure 4

405 - 380 million years ago, Devonian Period: As the Avalon microcontinent moved slowly toward the North American continent by plate tectonic motion, the Silurian sediments of the intervening ocean became severely compressed and deformed. The earth's crust in this collision zone became thicker as it was squeezed, producing a great mountain range. The rocks we see at Two Lights were subjected to heat and pressure during this mountain-building process. The layers were contorted into folds (Figure 5). Microscopic mineral grains became aligned to produce a rock cleavage, a wood-like "grain" along which the rock splits into sheets and splinters (Figure 6). Many visitors to Two Lights State Park mistakenly think that the wood grain appearance of the rocks is due to their being made of petrified wood. There is no petrified wood here. Minerals in the rock recrystallized under the prolonged heating to grow slowly into metamorphic minerals such as mica and chlorite, turning mudstones into phyllite and turning sandstones into quartzite. Many of the white quartz veins (Figure 7) here were produced during metamorphism as hot fluids containing dissolved silica circulated through small open spaces in the rock, leaving small quartz deposits.

Figure 5

intersection of bedding and cleavage in quartzite

Figure 6

Figure 7

290 million years ago, Late Carboniferous Period: Continental-scale motions in Maine at this time were no longer compressional, but were mainly sideways, with eastern Maine moving toward the southwest with respect to central Maine. The major mountain-building event was occurring farther to the south, in Pennsylvania, West Virginia, and Kentucky. The rocks at Two Lights are thought to have cooled since the Devonian, because the geologic features of this age are typical of brittle deformation in which the rocks broke rather than folded. One small fault near the southwest end of the park can be traced for a long distance across the ledges (Figure 8, Figure 9, Figure 10, Figure 11). Along this fault, the rock was obviously broken as the south side of the fault moved toward the west in relation to the north side. At the same time, some straight, vertical fractures opened in the rock and were filled with quartz to produce long, white veins.

200 million years ago, Mesozoic Era: The Mesozoic Era (the "Age of the Dinosaurs") saw the beginning of the modern Atlantic Ocean. At this time, Europe and Africa began to rift eastward away from the Americas. The Atlantic did not open along a clean break. Rather, the whole neighboring region of crust contains numerous extensional fractures. Magma (molten rock) from the base of the earth's crust erupted through these fractures, and solidified into thin sheets of rock called dikes. Dark-colored Mesozoic dikes, typically a foot or two thick, are common in southern Maine. At Two Lights one such dike is located in the extreme southwestern section of the shoreline exposure (Figure 12, Figure 13). Similar rock continues to erupt along the axis of the Atlantic Ocean, now located in Iceland.

Figure 12

Figure 13

Up to the present: Since late Mesozoic time New England has been relatively quiet, geologically. It has been a time of fundamental stability even as surface processes and gradual erosion have continued. The most recent increment of erosion was caused by a continental glaciation that waned ca. 14,000 years ago. This erosion has removed the upper few thousand feet of the earth's crust allowing us to look at rocks that were affected by geologic processes deep in the earth and over a long span of time.

References

Berry, H. N., IV, and Hussey, A. M., II, 1998, Bedrock geology of the Portland 1:100,000 quadrangle, Maine and New Hampshire: Maine Geological Survey, Open-File Map 98-1.

Swanson, M. T., 1995, Detailed structure of brittle strike-slip faults in coastal Maine exposures, in Hussey, A. M., II and Johnston, R. A. (eds.), Guidebook to fieldtrips in southern Maine and adjacent New Hampshire: New England Intercollegiate Geological Conference, p. 291-301.

Text by Henry N. Berry IV and Robert G. Marvinney

Photographs by Robert G. Marvinney

Originally published on the web as the June 2002 Site of the Month.

Last updated on October 6, 2005