Boston Harbor Islands National Recreation Area

Geology

Education Themes: Geology

Island Formation

  • Boston Harbor is part of the Boston Basin, a topographic lowland underlain by sedimentary layers deposited at the end of the Precambrian time.
  • Where bedrock is exposed (Calf Island, the Brewsters, and small islands near Hingham), it is a shaly to slaty formation called Cambridge Argillite which was deposited on the muddy floor of an ocean dating back some 570 million years.
  • In the past 100,000 years, two separate periods of Pleistocene glaciation formed the hills that cap most islands of Boston Harbor and created the local drainage system, consisting of the Charles, Mystic, and Neponset watersheds.
  • The cores of many harbor islands are drumlins. Drumlins are glacier-formed, asymmetrical, elongate masses of till formed into smooth-sloped hills on the Boston Basin lowlands. In profile, they look like upside-down teaspoons.
  • As the climate warmed and the glacier receded from the Boston area some 15,000 years ago, the melting of glacial ice raised the level of the ocean, eventually creating this section of the basin and isolating the islands.
  • Drumlins may occur as scattered single hills, or in so-called "swarms." The Boston Harbor Islands are a geological rarity, part of the only drumlin swarm in the United States that intersects a coastline. This "drowned" cluster of about 30 of more than 200 drumlins in the Boston Basin are not all elongate in shape, as most other drumlins are (molded in the direction of glacial flow). Geologists believe the islands illustrate two separate periods of glacial action. Many of the islands have more than one drumlin.

Shaping of the Islands

  • Natural coastal processes continue to reshape the island landforms, from sea level rise (as part of climate changes) to northeast storms.
  • Rates of erosion on the islands can be dramatic. In general, the highest rates of beach erosion occur along beaches facing north and east, which are the dominant directions for winds and seas in these storms. The shifting shores of Thompson Island illustrate this process of erosion and sedimentation.
  • Human use of the islands also effects erosion by removal of vegetative cover promoting erosion, or by structures built to prevent erosion.

Geology of Specific Islands

  • Every island within the park, except for those composed largely of bedrock, has beach areas lining portions of its shores.
  • The beaches generally most attractive to recreational users in the park are found on Spectacle (recently replenished), Long, Lovells, and Gallops islands and are primarily sandy and possess comparatively few biological resources.
  • Rocky beaches, however, such as at Peddocks, provide excellent habitat for invertebrates and the animals that feed on them.
  • Small barrier beaches have been identified on portions of Great Brewster, Gallops, Peddocks, Bumpkin, Long, Rainsford, and Thompson islands.
  • Two islands within the park, Lovells and Long, have dunes. Lovells has the more extensive dune system, whereas Long Island's dunes are in one discrete area on its southern shore.
  • At 188 acres, Peddocks is one of the largest, most diverse islands in the harbor. With the longest shoreline of any harbor island, Peddocks is composed of four headlands, connected by sand or gravel bars called tombolos.
  • The slate bedrock (underlying the entire harbor region) is visible as an outcrop on the bluff at Grape Island. Bedrock is also exposed on Calf Island, the Brewsters, and small islands near Hingham.

Soils

The soils of the Boston Harbor Islands have been classified into three major types: Hinckley-Merrimac- Urban Land; Canton-Charlton-Hollis; and Newport-Urban Land. The Hinckley-Merrimac-Urban Land soils (found on the southwestern end of Thompson) are very deep, nearly level to steep, excessively drained soils formed in sandy and loamy glacial outwash overlying stratified sand and gravel, and areas of urban land. These soils are usually found in major stream valleys and on coastal plains. The Canton-Charlton-Hollis soils (found on Outer and Middle Brewster, Calf, and George’s) are very deep and shallow, gently sloping to steep, somewhat excessively drained and well drained, loamy soils formed in glacial till and in ice-contact, stratified drift. These soils are generally found on uplands and low hills. The Newport-Urban Land soils (found on Long, Deer, Spectacle, Lovell’s, Gallop’s, Great Brewster, Grape, and Slate) are very deep, gently sloping to moderately steep, well-drained soils formed in friable, loamy glacial till overlying a firm substratum, and areas of urban land. These soils are generally found on steep hillsides in the Boston Basin.

The above soils are all excessively well drained to well drained soils (i.e., exclusively upland soils). There is also a range of wetland and intermediate soils on the islands as well as beach sands and mudflat muds.

Prime and Unique Farmlands
The U.S. Department of Agriculture defines prime farmland as the land that is best suited for food, feed, forage, fiber, and oilseed crops; unique farmland produces specialty crops such as fruit, vegetables, and nuts. Five soils classified as prime or unique farmland types occur within Boston Harbor Islands: Canton fine sandy loam, Merrimac fine sandy loam, Newport silt loam, Pittstown silt loam, and Sudbury fine sandy loam. There are no historically farmed areas still in active agricultural use.

Of the islands within the park, Thompson has the greatest percentage and variety of prime agricultural soils. About three-quarters of the island is composed of a patchwork of all five prime agricultural soil types. About two-thirds of Long Island and about half of Grape Island are covered with both Newport silt loam and Pittstown silt loam. Small portions of Gallop’s, Lovell’s, and Great Brewster are covered with Newport silt loam. As of September 1989 (Natural Resources Conservation Service’s soil survey date) about one third of Deer Island was composed of Pittstown silt loam, and about one-third of Spectacle Island was composed of Newport silt loam. The construction of the sewage treatment plant on Deer Island and the deposit of fill on Spectacle Island have since dramatically altered the soil content on those islands. Groundwater All islands with soil provide a receptacle for some groundwater. Because there is a shallow, relatively lowvolume water table in the highly permeable soils on all of the drumlins, the groundwater is vulnerable to contamination from failed septic systems, chemical spills, leaching dumpsites, fuel spills, and saltwater intrusion.

Geologic Formations

Boston Harbor is part of the Boston Basin, a topographic lowland underlain by sedimentary layers deposited at the end of the Precambrian time. Where bedrock is exposed (Calf Island, the Brewsters, and small islands near Hingham), it is a shaly to slaty formation called Cambridge Argillite which was deposited on the muddy floor of an ocean dating back some 570 million years.


In the past 100,000 years, two separate periods of Pleistocene glaciation formed the hills that cap most islands of Boston Harbor and created the local drainage system, consisting of the Charles, Mystic, and Neponset watersheds. The cores of many harbor islands are drumlins-glacier-formed, asymmetrical, elongate masses of till formed into smooth-sloped hills on the Boston Basin lowlands. In profile, they look like upside-down teaspoons. As the climate warmed and the glacier receded from the Boston area some 15,000 years ago, the melting of glacial ice raised the level of the ocean, eventually creating this section of the basin and isolating the islands.

Drumlins may occur as scattered single hills, or in so-called "swarms." The Boston Harbor Islands are a geological rarity, part of the only drumlin swarm in the United States that intersects a coastline. This "drowned" cluster of about 30 of more than 200 drumlins in the Boston Basin are not all elongate in shape, as most other drumlins are (molded in the direction of glacial flow). Geologists believe the islands illustrate two separate periods of glacial action. Many of the islands have more than one drumlin.

Natural coastal processes continue to reshape the island landforms, from sea level rise (as part of climate changes) to northeast storms. Rates of erosion on the islands can be dramatic. In general, the highest rates of beach erosion occur along beaches facing north and east, which are the dominant directions for winds and seas in these storms. The shifting shores of Thompson Island illustrate this process of erosion and sedimentation. Human use of the islands also effects erosion by removal of vegetative cover promoting erosion, or by structures built to prevent erosion.

Every island within the park, except for those composed largely of bedrock, has beach areas lining portions of its shores. The beaches generally most attractive to recreational users in the park are found on Spectacle (recently replenished), Long, Lovells, and Gallops islands and are primarily sandy and possess comparatively few biological resources. Rocky beaches, however, such as at Peddocks, provide excellent habitat for invertebrates and the animals that feed on them. Small barrier beaches have been identified on portions of Great Brewster, Gallops, Peddocks, Bumpkin, Long, Rainsford, and Thompson islands. Two islands within the park, Lovells and Long, have dunes. Lovells has the more extensive dune system, whereas Long Island's dunes are in one discrete area on its southern shore.