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Boston Harbor Islands

National Recreation Area


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park geology subheading
Photo of Boston Harbor Islands
Boston Harbor Islands National Recreation Area, Massachusetts

Boston Harbor is a more than 50-square mile estuary that is part of the Massachusetts Bay system. Upland inflow into the harbor is derived from eight primary watersheds including the Quincy Bay, Inner Harbor, Winthrop Bay, Mystic River, Charles River, Neponset River, Weymouth River and Weir River watersheds (Leo et al., 1995). This extensive area includes the City of Boston and the large surrounding metropolitan area. The Boston Harbor Islands national park area contains 34 islands, former islands, and peninsulas lying within or adjacent to Boston Harbor (National Park Service, 2000). They range in size from less than 1 acre to 251 acres and together include more than 1500 acres of land (Figure 1). Ownership and management responsibility for these areas includes the Massachusetts Department of Environmental Management (DEM), the Metropolitan District Commission (MDC), the Massachusetts Water Resources Authority (MWRA) , the City of Boston, the US Coast Guard, Thompson Island Outward Bound Education Center, The Trustees of Reservations, and the Towns of Hingham and Winthrop (Table 1). Unlike many of the islands typical of the New England coast, several of the Boston Harbor islands are coastal extensions of drumlin features, which are glacially-formed, asymmetrical, elongate masses of till formed into smooth-sloped hills on the Boston Basin lowlands. In addition, several of the "outer" islands are the more typical bedrock outcrops more commonly found along the coast of New England. For many years Boston Harbor was notorious for its polluted waters. Sources of pollution to Boston Harbor included the discharge of sewage treatment plant effluent, sanitary sewer overflow, stormwater runoff, and combined sewer overflows (CSOs). In 1994, there was discharge to Boston Harbor from two major sewage treatment plants as well as 81 CSOs within the Boston Harbor watershed (Leo, et al., 1995). Recent infrastructural improvements have greatly improved water quality within the harbor (see Boston Harbor Project section). For discussion purposes within this report, the Boston Harbor Islands national park area islands, former islands, and peninsulas are grouped into four geographic sub-areas: (1) the Inner Harbor group, (2) the Quincy Bay group, (3) Hingham Bay group, and (4) the Outer Harbor group (National Park Service, 1994). These are described briefly below.

Estuarine Intertidal Wetlands

Estuarine intertidal wetlands are the predominant wetland type in the shoreline habitats of the majority of the Inner Harbor, Quincy Bay and Hingham Harbor areas within the Boston Harbor Islands national park area. These include the shorelines of Bumpkin Island, Button Island, Gallops Island, Georges Island, Grape Island, Hangman Island, Long Island, Moon Island, Nut Island, Peddocks Island, Raccoon Island, Ragged Island, Rainsford Island, Sarah Island, Sheep Island, Slate Island, Spectacle Island, Thompson Island, and Worlds End, as well as the western shores of both Deer Island and Lovell Island (USF&WS 1995a; 1995b). Estuarine wetlands may also be found in low-lying areas adjacent to the shoreline which may be partially or fully enclosed by land, but have regular or sporadic access to tidally influenced harbor water. These habitats include low-lying regions on Thompson Island, Spectacle Island, Deer Island, Long Island, Moon Island, Peddocks Island, Sheep Island, Worlds End and Calf Island (USF&WS 1995a; 1995b). Salinity in estuarine intertidal wetlands is influenced by freshwater dilution caused by the river systems discharging into Boston Harbor. Salinity in these habitats may fluctuate significantly, being influenced not only by freshwater input but also by winds and tidal factors, and in some cases by evaporation in those estuarine wetlands which are periodically cut off from the tidal cycle. Classes of estuarine intertidal wetlands within the national park area include unconsolidated, emergent, aquatic bed and rocky shore estuarine intertidal wetlands (USF&WS 1995a; 1995b). A review of the general geomorphological, physical, and biological characteristics of the estuarine wetland resource habitat types found in the northeastern United States has recently been published by Roman et al. (2000). While most of the estuarine intertidal wetlands fringing the properties of the national park area are quite small in extent, the estuarine intertidal wetlands found at Worlds End comprise the northern edge of a larger system, the Weir River Area of Critical Environmental Concern (ACEC) as defined by the Massachusetts Department of Environmental Management. The Weir River ACEC, which contains both Straights Pond and the tidally influenced portion of the Weir River, is one of the most extensive salt marsh systems in the greater Boston metropolitan area. Designated in 1986, the Weir River ACEC is located adjacent to the boundaries of the national park area. It is regionally significant both for its size and its importance in providing a relatively undisturbed marshland wildlife habitat (Massachusetts Department of Environmental Management, 1986). The Weir River ACEC contains a significant shellfish resource, supports an active anadromous fish run, and provides extensive nursery and feeding habitat for a wide variety of finfish including alewives (herring), smelt, flounder, bluefish, and striped bass. It also provides important habitat for over 100 species of migratory and indigenous birds and serves as an important food source for migratory waterfowl (Massachusetts Department of Environmental Management, 1986).

Similarly, two additional ACECs and part of a third, are located in the general proximity of the national park area. The Weymouth Back River ACEC, designated in 1982, comprises a natural area of approximately 950 acres in the midst of an urban/suburban environment (Massachusetts Department of Environmental Management, 1982). Approximately 180 acres of the Weymouth Back River ACEC are tidal waters flushing into Hingham Bay and support productive clam flats as well as nursery and feeding areas for finfish ecologically important to Boston Harbor. The ACEC also includes extensive salt marsh and salt pond habitats, and the lower portion of Herring Brook, Hingham's Fresh River, and several unnamed tributary streams, which provide spawning sites for an annual anadromous fish (herring) runs (Massachusetts Department of Environmental Management, 1982).

The Neponset River Estuary ACEC, designated in 1995, consists of 1,260 acres between Lower Mills Dam and the mouth of the Neponset River. The central resource features of the Neponset River Estuary ACEC are the Neponset River and portions of its tributaries, estuarine wetlands including salt marsh, floodplains, and fisheries and wildlife habitat (Massachusetts Department of Environmental Management, 1995). Open water, salt marsh and other estuarine wetland habitats comprise about 830 acres of this ACEC and support substantial soft-shell clam beds, valuable anadromous fishery habitat, spawning areas, and bird and wildlife habitat surrounded by an urbanized setting.

Most of the 2800-acre Rumney Marshes ACEC, designated in 1988, is located north of Boston Harbor principally within the Saugus River / Pines River estuary. However, the Belle Isle Marsh area of the Rumney Marshes ACEC empties into Winthrop Bay along the northern shore of Boston Harbor. The Belle Isle Marsh contains 275 acres of salt marsh, salt meadow, and tidal flats (Massachusetts Department of Environmental Management, 1988). It is publicly owned by the Metropolitan District Commission and the municipalities of Boston, Winthrop and Revere and provides both important ecological habitat and flood water storage (Massachusetts Department of Environmental Management, 1988). While the ACECs are located almost entirely outside of the boundary of the national park area they are biologically connected and the habitats they provide are of great significance to the overall ecological health of the entire Boston Harbor area.

Geohydrology / Groundwater Resources

The Boston Harbor islands are composed primarily of glacial materials that directly overlie the bedrock surface. The surficial materials that overlie the weathered bedrock surface were deposited by continental ice sheets that covered New England twice during the Pleistocene Epoch, and by near-shore processes in the Holocene Epoch. The thickness of these materials, where present, ranges from less than 1 foot to about 300 feet thick (Masterson et al., 1996). Many of the islands of the Boston Harbor Islands national park area consist of glacially deposited drumlins composed of a thick, dense, homogeneous till core, which is overlain by a thin layer of stratified beach deposits (Masterson et al., 1996). The till is characterized as an unsorted matrix of sand, silt, and clay intermixed with variable amounts of stones and large boulders. The stratified deposits primarily consist of sorted and layered sand and gravel that accumulated and formed the beaches and tombolos of the islands (Masterson et al., 1996). In 1996, the US Geological Survey conducted an investigation of the geohydrology and of the potential for water supply development for six of the Boston Harbor islands including Bumpkin, Gallops, Georges, Grape, Lovell and Peddocks islands (Masterson et al., 1996). The primary purpose of this study was to investigate the possibility of developing permanent, small-capacity water supplies capable of supporting recreational activities such as hiking, camping, and swimming.

Masterson et al. (1996) found that the hydrology of each of the six islands studied was characterized by hydraulically independent freshwater-flow systems that consisted of about 10 to 30 feet of weathered till containing a dense substratum near the land surface that may perch water. The freshwater flow systems of the six islands is underlain by either bed rock or saltwater and they are also laterally separated from one another by the saline waters of Boston Harbor (Masterson et al., 1996). Thus, the sole source of freshwater to each of the islands studied is the infiltration of precipitation into the soils of the island. The amount of precipitation reaching the saturated zone (recharge) would be expected to vary spatially and temporally in response to climatic (e.g. precipitation patterns, etc.) and biologic (e.g. evapotranspiration) factors, as well as to the local topography. The recharge rates would also be affected by local differences in the infiltration capacity and other hydraulic properties in the unsaturated zone. Using estimates of recharge rates from previous investigations in similar hydrogeologic settings, Masterson et al. (1996) were able to model approximate groundwater movements for the six islands.

Masterson et al. (1996) determined that groundwater flow generally is radial from the center of the island towards the coast. Topographically high areas, such as the upper slopes and crests, are typically the recharge areas. Groundwater discharge would typically occur at the lower slope areas of the drumlins, the topographically low areas such as coastal marshes or ponds, or directly along the coast (Masterson et al., 1996).

The position of the water table relative to land surface is an extremely important consideration in selecting sites for water-supply development, especially given the difficulties associated with drilling in till material. However, Masterson et al. (1996) found that the depth to water table was impossible to estimate for each of the Boston Harbor islands due to the lack of available hydrologic data. Therefore, they developed a conceptual 2-dimensional cross-sectional groundwater flow model which indicated that nearly all of the saturated portion of the drumlinisland flow system occurred in the compact drumlin till, the beach-type deposits, and the underlying weathered-bedrock zone (Masterson et al., 1996).

For the purposes of their investigation, Masterson et al. (1996) assumed that drawing water from a standard "pitcher" pump would be most efficient at a depth to water of less than 20 feet below the land surface. Thus, their model indicated that a freshwater water table at this depth would be located within 240 feet of the shoreline, which generally coincides with the high hydraulic conductivity zones of beach-type and weathered till deposits on the fringes of each drumlin island (Masterson et al., 1996). However, Masterson et al. (1996) did not model the possible effects of ground-water withdrawal on the position and movement of this interface or whether groundwater withdrawals from these areas would be affected by saltwater intrusion.

Source National Park Service Water, Resources Division


Leo, W.S., A.C. Rex, S.R. Carroll, and M.S. Connor. 1995. The State of Boston Harbor 1994: connecting the Harbor to its Watersheds. ENQUAD Report No. 1995-12. Massachusetts Water Resources Authority, Boston , MA.

Massachusetts Department of Environmental Management. 1982. Designation of the Weymouth Back River as an Area of Critical Environmental Concern. Commonwealth of Massachusetts , Secretary of Environmental Affairs, Boston , MA .

Massachusetts Department of Environmental Management. 1986. Designation of the Portions of the Towns of Cohasset, Hingham , and Hull as the Weir River Area of Critical Environmental Concern. Commonwealth of Massachusetts , Secretary of Environmental Affairs, Boston , MA

Massachusetts Department of Environmental Management. 1988. Designation of the Rumney Marshes as an Area of Critical Environmental Concern. Commonwealth of Massachusetts , Secretary of Environmental Affairs, Boston , MA .

Massachusetts Department of Environmental Management. 1995. Designation of the Neponset River Estuary as an Area of Critical Environmental Concern: Commonwealth of Massachusetts , Secretary of Environmental Affairs, Boston , MA.

Masterson, J. P., B. D. Stone, and R. R. Rendigs. Geohydrology and Potential Water-Supply Development on Bumkin, Gallops, Georges, Grape, Lovell, and Peddocks Islands , eastern Massachusetts . U.S. Geological Survey Open-File Report 96-117. Massachusetts - Rhode Island District , U.S. Geological Survey, Marlborough , MA .

National Park Service. 2000. Boston Harbor Islands National Park Area Draft General Management Plan and Draft Environmental Impact Statement. Prepared for the Boston Harbor Islands Partnership by the Boston Support Office, Northeast Region, National Park Service, Boston , MA .

U.S. Fish and Wildlife Service. 1995a. National Wetlands Inventory Map for East Half of Boston South Quadrangle (1:58000 scale). National Wetlands Inventory , U.S. Fish and Wildlife Service, Washington , DC .

U. S. Fish and Wildlife Service. 1995b. National Wetlands Inventory Map for West Half of Hull, Massachusetts Quadrangle (1:58000 scale). National Wetlands Inventory , U.S. Fish and Wildlife Service, Washington , DC.

park maps subheading

The General park map handed out at the visitor center is available on the park's map webpage.

For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.

photo album subheading

For information on other photo collections featuring National Park geology, please see the Image Sources page.

books, videos, cds subheading

Currently, we do not have a listing for a park-specific geoscience book. The park's geology may be described in regional or state geology texts.

Please visit the Geology Books and Media webpage for additional sources such as text books, theme books, CD ROMs, and technical reports.

Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
ISBN 0-393-92407-6
9" x 10.75", paperback, 550 pages, full color throughout

The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!

Ordering from your National Park Cooperative Associations' bookstores helps to support programs in the parks. Please visit the bookstore locator for park books and much more.

geologic research subheading


For information about permits that are required for conducting geologic research activities in National Parks, see the Permits Information page.

The NPS maintains a searchable data base of research needs that have been identified by parks.

A bibliography of geologic references is being prepared for each park through the Geologic Resources Evaluation Program (GRE). Please see the GRE website for more information and contacts.

selected links subheading

NPS Geology and Soils Partners

NRCS logoAssociation of American State Geologists
NRCS logoGeological Society of America
NRCS logoNatural Resource Conservation Service - Soils
USGS logo U.S. Geological Survey

teacher feature subheading

Currently, we do not have a listing for any park-specific geology education programs or activities.

General information about the park's education and intrepretive programs is available on the park's education webpage.

For resources and information on teaching geology using National Park examples, see the Students & Teachers pages.
updated on 01/04/2005  I   http://www.nature.nps.gov/geology/parks/boha/index.cfm   I  Email: Webmaster
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