A Geological Tour of Bartlett Narrows and Vicinity

Many geological wonders are exposed on Mount Desert Island, providing numerous opportunities for exploration either by land or sea by interested visitors who know where to look. This feature focuses on the fascinating geology around Bartlett Narrows, a relatively sheltered body of water on the western side of the island.

Most of the bedrock or ledge exposed on Mount Desert Island is granite or related rocks that solidified from molten magma within the crust of the earth and the older rocks into which the molten magma was intruded. After many millions of years of erosion, these rocks are now exposed at the surface.

Figure 1

Figure 1 is part of the bedrock map for Mount Desert Island showing the major units that are exposed around Bartlett Narrows. From youngest to oldest, these are:

Igneous rocks

Dg3 - this is a young granite exposed in several small bodies around Mount Desert Island. It is pink and has crystals of medium size. It often occurs as thin dikes within the older rocks.

Dsg, Dsg1 - this is the granite of Somesville, so-named because of its excellent exposure there. It has pink and gray feldspars, and black biotite in crystals of medium size.

Dcg - this is the Cadillac granite which is has very large crystals of pink and gray feldspar, glassy quartz, and black hornblende.

Dgd - this is a mix of dark gray, coarse-grained gabbro where the primary minerals are pyroxene and hornblende, and diorite which contains more gray feldspar and is lighter gray.

Metamorphic rocks

COe - these are older metamorphic rocks of the Ellsworth Schist. It is a dark green or gray schist with distinctive thin, lighter colored layers of quartz and feldspar. It was sand and mud on the ocean floor many millions of years before being squeezed and heated, and then intruded by the various molten magmas.

While this map shows sharp boundaries or contacts between the various types of rock, in reality, the contacts are often gradational in one way or another. Often large blocks or rafts of the Ellsworth Schist may be caught up completely within the younger granites and gabbros, even at considerable distance from the mapped contacts. This occurs at Dogfish Cove and on Folly Island. Sometimes the magmas of two different igneous rock types occurred at the same place at the same time but, like oil and water, did not mix. There are examples of this along the mainland shore east and south of Folly Island.

The following images provide examples of some of the bedrock units and their contact relationships. While these images are from specific locations, they are common throughout the exposures along Bartlett Narrows, where the observant visitor can identify them.

Figure 2	Figure 3	Figure 4	Figure 5	Figure 6
Figure 7	Figure 8	Figure 9	Figure 10	Figure 11

Access

Please note that most of the shoreline around Bartlett Narrows is private land, except for Acadia National Park's picnic area at Pretty Marsh. Please respect private property. The boat ramp at Bartlett Landing is a popular access point for kayaking and boating the Narrows. Access has generally been permitted on Johns Island, Folly Island, and at Dogfish Cove on the south end of Bartlett Island for day users who follow appropriate rules of etiquette.

Text and photos by R. Marvinney

Originally published on the web as the January 2011 Site of the Month.

Last updated on February 1, 2011