Marshall Point Lighthouse and Geology, Port Clyde, Maine
Spectacular scenery and geology combine to make a trip to Marshall Point
Lighthouse near Port Clyde a worthwhile exercise for geologists of all
ages and experience. Broad expanses of exposed bedrock greet the visitor
along the raised walkway to the lighthouse. Most of what is exposed here
is metamorphic rock of the Benner Hill sequence, a group of formerly sedimentary
layers of probable Ordovician age (440-500 million years ago) that have
been highly deformed and metamorphosed through a number of mountain building
events that have shaped the geology of coastal Maine. Dikes of younger igneous rocks crosscut the metamorphic rocks.
|Figure 1. Marshall Point Lighthouse, built in 1832 and rebuilt in 1858,
guards the eastern side of Port Clyde Harbor. Much of the geology described
here can be viewed directly from the walkway in this photograph. For the
more adventurous, a scramble across the rocks will reveal many interesting
|Figure 2. Much of the rock exposed at Marshall Point is interlayered
quartzite (the light bands) and gray mica schist (the dark bands), here
wildly contorted. The complicated layering and folding is the result of
several events. Probably some of the layers of sand and mud, originally
deposited in an ocean basin, slid down the slope and became contorted before
they hardened into rock. Some of the folding resulted from the heat and
pressure of later mountain building events and the intrusion of nearby
igneous rocks between 440 and 360 million years ago.
|Figure 3. This photograph shows that other processes have affected these rocks since they were deposited. Much of the layered material is broken up into fragments of different size. Surrounding these blocks is migmatite, a granular quartz and feldspar rock that results from the melting in place of metamorphic rocks like these. The migmatite is additional evidence of the intense heat that affected these rocks which may be related to younger intrusions in the area.
|Figure 4. Cutting across the metamorphic rocks from bottom center to upper right is an igneous dike of younger age than the metamorphic rocks. It is probably of Silurian age (about 420 million years old), and related to nearby intrusions of this age. The gray granitic dike includes dark blobs or enclaves of a darker igneous rock called basalt. The dike is clearly younger than the quartzite and schist metamorphic rocks and the migmatite because it cuts right across both of these units.
|Figure 5. Close-up of the dike shown in Figure 4. Blobs or enclaves
of dark basalt are surrounded by lighter colored granitic rock. The scalloping or cuspate nature of the edges of the basalt indicates that the separate magmas (molten rock) that became the basalt and the granitic rock were both molten at the same time and interacted but did not mix very well. Some fantastic shapes form in the process. Because basalt solidifies at a higher temperature than granite, it solidified quickly and the granite solidified more slowly.
|Figure 6. This is another example of a granite and basalt dike. The light gray granite surrounds dark masses of basalt.
Marshall Point can be easily accessed from Rockland by taking Route
131 south to Port Clyde. Follow signs to the lighthouse. To learn more, see Marshall
Point Lighthouse and Museum.
Text by Robert Marvinney, Maine Geological Survey. Photographs 1, 4, 5, and 6 by Arthur
Hussey, Bowdoin College. Photographs 2 and 3 by Henry Berry.
Originally published on the web as the March 2000 Site of the Month.
Last updated on June 7, 2006