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An Underwater View of the Gulf of Maine Sea Floor

Geography of the Maine Coast and Submerged Lands

map of coast of Maine
Figure 1		 The coast of Maine has 5600 kilometers (3480 miles) of tidally influenced shoreline and is the fourth longest in the United States (Figure 1). There are about 3500 islands included in the shoreline length. State submerged lands extend from the low-tide line a distance 5.56 kilometers (three nautical miles) offshore. The sea floor below the Territorial Sea is some 1080 square kilometers (2800 square miles) or about 9% of the land area of the State of Maine.

Sea-floor Relief and Geology

The ocean floor along the Maine coast is as rugged as the shoreline itself. The hills and valleys seen on land are influenced by the bedrock structure underlying the State of Maine. In a similar manner, the sea-floor relief - or bathymetry - mimics the bedrock structure (Figure 2). Relief along the inner continental shelf can easily reach 100 meters (over 300 feet) and often this relief is not far from the shoreline (Figure 3).

coastal relief map of Penobscot Bay, Maine
Figure 2		 graph of depth of sea floor
Figure 3

The sea-bed geology of Maine's inner continental shelf is primarily a complex mosaic of bedrock exposures (Figure 4) and muddy basins (Figure 5, Figure 6, and Figure 7). Rocky sea floor is dominant in water depths less than 50 meters. Muddy sea floor is dominant below 50 meters. Together, these two geologic categories account for 4/5ths of the sea floor.

rocky sea floor
Figure 4		 muddy sea floor
Figure 5		 sea anemones on muddy bottom
Figure 6		 lobster on muddy bottom
Figure 7

Relative abundance of sea-floor types on the Maine inner continental shelf
Rock41%
Mud39%
Gravel12%
Sand8%

Gravel plains and sandy sea floor make up about 1/5th of the ocean floor of state submerged lands. Gravel (including boulders) is a minor bottom type at all depths, but is most common in the 10 to 30 meter (30 to 100 feet) depth range (Figure 8, Figure 9, and Figure 10). Sandy sea floor is rare but present at all depths to 100 meters; sand is only slightly more abundant in water less than 30 meters deep (Figure 11).

lobster on gravel bottom
Figure 8		 sea anemones on boulder
Figure 9		 coraline algae on gravel
Figure 10		 crab on sandy sea bed
Figure 11

The spatial complexity and details of sea floor geology are shown on MGS Open-File maps in the series entitled Surficial Geology of the Maine Inner Continental Shelf. A simplified illustration of the geology of the sea floor in a central location along the Maine coast from Cape Small to Schoodic Point is shown in Figure 12. In this illustration, sandy sea floor is most abundant offshore of the Kennebec River mouth. The outer portions of Penobscot Bay have abundant gravel areas and habitat suitable for juvenile lobsters. This region of Penobscot Bay sustains the most abundant lobster harvests along the Maine coast (Figure 13).

surficial geology map of Maine intercontinental shelf
Figure 12		 ghost lobster trap
Figure 13

References and Related Web Sites

Barnhardt, W. A., Belknap, D. F., Kelley, A. R., Kelley, J. T., and Dickson, S. M., 1996, Surficial Geology of the Maine Inner Continental Shelf, a series of 7 maps covering coastal Maine, scale 1:100,000: Maine Geological Survey, Augusta, Maine.

Belknap, D. F., Gontz, A. M., Wahle, R., and Hovel, K., 2004, Mapping Lobster Habitat with Sidescan Sonar and ROV - A geologic and benthic oceanographic collaboration: Geological Society of America Abstracts with Programs, Vol. 36, No. 2, p. 137.

Gulf of Maine Ocean Observing System

Kelley, J. T., 1998, Final Report on Surficial Mapping for the Penobscot Bay Project: National Oceanic and Atmospheric Administration Coastal Services Center.

Kelley, J. T., Barnhardt, W. A., Belknap, D. F., Dickson, S. M. and Kelley, A. R., 1998, The Seafloor Revealed - The Geology of the Northwestern Gulf of Maine Inner Continental Shelf: Maine Geological Survey, Open-File Report 96-6, 55 p.

MGS Field Locality Penobscot Bay 10,000 years ago

Penobscot Bay Marine Resources Collaborative, 1998, The Island Institute, Seafloor Geology report and geographic information system coverage (as in Figure 12).

Taylor, P. H. (editor), 2003, Mapping the Undersea Landscape (455KB pdf): Gulf of Maine Council on the Marine Environment, 4 p.

The Gulf of Maine Council on the Marine Environment

The Sea Around Us: Maine Coastal Program, State Planning Office

Acknowledgments

Funding to support some of the field work and mapping of sea-floor habitats in Penobscot Bay was made possible with funding from the National Oceanic and Atmospheric Administration, National Environmental Satellite, Data, and Information Service as part of the Penobscot Bay Marine Resources Collaborative through the Island Institute. Underwater images were taken by the M-Rover operated by Guy Meadows and Hans van Sumeren of the University of Michigan for a joint Maine Geological Survey - University of Maine effort to ground-truth sea-floor mapping with side-scan sonar. Additional funding to map the inner continental shelf was provided by the Department of Interior, Minerals Management Service to the Maine Geological Survey.

Text by Stephen M. Dickson

Originally published on the web as the May 2004 Site of the Month.

Last updated on October 6, 2005