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.