Boston Harbor Islands Geology: A Drowned Field of Drumlins

Geodiversity

Geodiversity is the full variety of a park's natural geologic resources — rocks, minerals, sediments, fossils, landforms and physical processes — and its soils. The Park Service's Geodiversity Atlas, a product of the Geologic Resources Inventory, supports education, geoconservation and management of a park's living and non-living resources together.

The park

Boston Harbor Islands National Recreation Area was authorized on 12 November 1996, taking in the Boston Harbor Islands State Park and other lands. The Boston Harbor Islands Partnership manages 34 islands and peninsulas (former islands) in Boston Harbor, across Norfolk, Plymouth and Suffolk counties, owned or run by eight federal, state, municipal and nonprofit agencies; the Park Service coordinates resource management and visitor programs.

How the islands formed

• An ancient basin: the islands lie in the Boston Basin, a low area of eastern Massachusetts that began as a basin pulled open within a landmass carried across ancient seas, before it was joined to North America's eastern margin in the Paleozoic, more than 252 million years ago. More than 500 million years ago, the fault-bounded basin collected sediments and volcanic rocks that erode more easily than the igneous and metamorphic rocks around it.
• Ice: during the Pleistocene ice ages (2.6 million to 11,700 years ago), glaciers carved the lowland of the harbor but also left vast amounts of sediment behind in whaleback-shaped mounds called drumlins. Some bedrock survived, especially where resistant igneous rock had intruded.
• Sea: as sea level rose after the ice retreated, the drumlin field drowned, and waves, currents moving sand along the shore, and migrating barriers began shaping today's coast.

The Boston Harbor Islands are the only drumlin field on a coast in the United States.

Eroding bluffs

The loose glacial deposits that make up the drumlins erode easily. Exposed once the glaciers melted, then flooded by the rising sea, they have been wearing back through bluff retreat, and many now show several eroding scarps.

Glacial landforms

Ice sheets advanced from the Arctic again and again during the Pleistocene, the last time ending about 10,000–12,000 years ago, and reshaped the Northeast. The park's glacial features include:

• from the ice itself: till, moraines, drumlins, kettles, grooves, striations and glacial erratics;
• from meltwater rivers: kames, eskers, braided streams, deltas, lake deposits and outwash fans;
• from glacial lakes: broad plains of fine, rhythmically layered sediment, often holding boulders dropped from icebergs.

A shifting shoreline

The islands' coastal features include glacial-till bluffs, sand beaches, gravel barrier beaches, retreating barrier beaches anchored to drumlins, spits, tombolos, gravel ridges, salients (cuspate forelands), welded bars, salt marshes, retreat platforms, raised sea-level terraces, and intertidal habitats. Because the shorelines keep moving, many of these features are temporary — changing from one type into another, or joining up.

Layered Cambridge Argillite (light gray) and intruded diabase (brown) at Boston Light, Little Brewster Island, with visitors for scale. NPS photo by Trista L. Thornberry-Ehrlich.

People and the land

For hundreds of years people have used the islands — quarrying slate, taking sediment for ballast, and for military, settlement and quarantine purposes. There is artificial fill in coastal engineering and on Spectacle Island, reclaimed for the park by capping a municipal landfill, topping it with clean fill from the harbor tunnel of Boston's "Big Dig," then covering it with topsoil and planting it. Abandoned mineral lands can be valuable cultural sites and habitat, but many are dangerous — stay out and stay alive.

Region

The park belongs to the New England Physiographic Province, sharing its geologic history with a much wider region.

The Geologic Resources Inventory

The Park Service's Geologic Resources Inventory produces digital geologic maps and reports for more than 270 natural resource parks:

• scoping summaries — records of the meetings where Park Service staff and local geologists planned the mapping and the report;
• digital geologic maps, for GIS software, with a user guide and ancillary information, and on newer products Google Earth and online map services;
• reports that use the maps to discuss a park's setting, significant features, management issues and geologic history;
• posters, a static view of the map data.

The park's Geologic Resources Inventory report details its glacial and coastal features, and a Soil Resources Inventory has been completed for it. See also the park's pages on geologic formations, coasts, wetlands and soils.

Sources

Based on "NPS Geodiversity Atlas—Boston Harbor Islands National Recreation Area, Massachusetts," National Park Service; a work of the United States government in the public domain. A thumbnail of the inventory report's cover is not reproduced.