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NPS Geodiversity Atlas—Boston Harbor Islands National Recreation Area, Massachusetts

Geodiversity refers to the full variety of natural geologic (rocks, minerals, sediments, fossils, landforms, and physical processes) and soil resources and processes that occur in the park. A product of the Geologic Resources Inventory, the NPS Geodiversity Atlas delivers information in support of education, Geoconservation, and integrated management of living (biotic) and non-living (abiotic) components of the ecosystem.

boston harbor islands gri report with island image
In-depth geologic information is contained in the baseline inventory products of the Geologic Resources Inventory, see table below.

Geologic Features and Processes

Boston Harbor Islands National Recreation Area was authorized as a unit of the National Park System on 12 November 1996, incorporating the Boston Harbor Islands State Park and additional lands. The Boston Harbor Islands Partnership manages 34 islands and peninsulas (former islands) in Boston Harbor across Norfolk, Plymouth, and Suffolk counties in Massachusetts, east of Boston. The islands and peninsulas are owned or administered by eight federal, state, municipal, and non-profit agencies. The National Park Service facilitates resource management and visitor programs across the national recreation area.

Boston Harbor Islands are within an ancient geologic structure—the Boston Basin. This topographically low area in eastern Massachusetts began as an extension basin within a landmass that tectonic forces pushed and pulled across ancient seas before it was accreted onto the eastern margin of North America during the Paleozoic Era, more than 252 million years ago. The fault-bounded basin had collected sediments and volcanics more than 500 million years ago that would prove less resistant to erosion than the igneous and metamorphic rocks surrounding it. When glaciers scoured the landscape during the ice ages of the Pleistocene Epoch (2.6 million to 11,700 years ago), they carved the lowland of Boston Harbor, but also left vast amounts of sediment behind to form the “whaleback” mounds called drumlins. Some areas of bedrock persisted, particularly those underlain by intrusions of resistant igneous rocks. When global sea level rose after glacial retreat, the drumlin field was eventually drowned, and coastal processes of wave erosion, longshore transport, and barrier migration began to shape the coastal features that persist at Boston Harbor Islands today.

Boston Harbor Islands are the only drumlin field along a coast in the United States (National Park Service 2002; FitzGerald et al. 2011).

Bluff Retreat
The unconsolidated glacial deposits that make up the drumlins are erodible. Since their shaping by ice age glaciers, drumlins have been exposed after the glaciers receded and submerged by rising sea level. As a result, the drumlins at Boston Harbor Islands have been eroding via a process known as “bluff retreat.” Today, many drumlins have multiple exposures forming eroding scarps.

Glacial Landforms

Glacial features such as drumlins define the landscape of Boston Harbor Islands National Recreation Area. Massive ice sheets repeatedly advanced from the Arctic during Pleistocene glaciations (ice ages) ending approximately 10,000–12,000 years ago. Those glaciers scoured and reshaped the landscape of the northeastern United States, including Boston Harbor Islands National Recreation Area.

Within the recreation area, the diversity of deposits and features associated with glacial ice includes:

  • Till
  • Moraines
  • Drumlins
  • Kettles
  • Grooves
  • Striations
  • Glacial Erratics

Glaciofluvial deposits and features (deposited by rivers flowing beneath or out of glaciers) include:

  • Kames
  • Eskers
  • Braided Streams
  • Deltas
  • Lake Deposits
  • Outwash Fans

Additionally, glaciolacustrine deposits (deposited in lakes near glaciers) formed broad plains where the underlying sediments are fine-grained and rhythmically deposited, and commonly contain erratics dropped from icebergs.

Also see the park Geologic Resources Inventory Report for details on glacial features at Boston Harbor Islands and the Series: Glacier Landforms for definitions and examples from around the NPS.

Beaches and Coastal Landforms

The diversity of coastal features at Boston Harbor Islands, includes:

  • Glacial-till Bluffs
  • Sand Beaches
  • Gravel Barrier Beaches
  • Retrograding Barrier Beaches Anchored to Drumlins
  • Spits
  • Tombolos
  • Gravel Ridges
  • Salients (Cuspate Forelands)
  • Welded Bars
  • Salt Marshes
  • Retreat Platforms
  • Raised Sea-level Terraces
  • Intertidal Substrates and Assemblages

Because the islands’ shorelines are dynamic, many of the coastal features are temporary and can be transitional, changing from one type to another, or physically connected.

Also see the park Geologic Resources Inventory Report for details on coastal features at Boston Harbor Islands.

Abandoned Mineral Lands

For hundreds of years, humans have utilized Boston Harbor Islands by quarrying slate, removing of sediment for ballast, and a variety of military, settlement, and quarantine purposes. Artificial fill occurs as coastal engineering structures and in places like Spectacle Island. That island was reclaimed for park use through capping of a municipal landfill then topping with clean fill from the harbor tunnel associated with Boston’s “Big Dig,” and finally covered with topsoil and
planted.

NPS AML sites can be important cultural resources and habitat, but many pose risks to park visitors and wildlife, and degrade water quality, park landscapes, and physical and biological resources. Be safe near AML sites—Stay Out and Stay Alive!

Geology Field Notes

Students and teachers of college-level (or AP) introductory geology or earth science teaching courses will find that each park's Geologic Resource Inventory report includes the Geologic History, Geologic Setting, and Geologic Features & Processes for the park which provides a useful summary of their overall geologic story. See Maps and Reports, below.

Regional Geology

Boston Harbor Islands National Recreation Area is a part of the New England Physiographic Province and shares its geologic history and some characteristic geologic formations with a region that extends well beyond park boundaries.

Maps and Reports

The Geologic Resources Inventory produces digital geologic maps and reports for more than 270 natural resource parks. The products listed below are currently available for this park, check back often for updates as many maps, reports, and posters are still in progress.
  • Scoping summaries are records of scoping meetings where NPS staff and local geologists determined the park’s geologic mapping plan and what content should be included in the report.
  • Digital geologic maps include files for viewing in GIS software, a guide to using the data, and a document with ancillary map information. Newer products also include data viewable in Google Earth and online map services.
  • Reports use the maps to discuss the park’s setting and significance, notable geologic features and processes, geologic resource management issues, and geologic history.
  • Posters are a static view of the GIS data in PDF format. Newer posters include aerial imagery or shaded relief and other park information. They are also included with the reports.
  • Projects list basic information about the program and all products available for a park.

Source: Data Store Saved Search 2728. To search for additional information, visit the Data Store.

A NPS Soil Resources Inventory project has been completed for Boston Harbor Islands National Recreation Area and can be found on the NPS Data Store.

Source: Data Store Saved Search 2736. To search for additional information, visit the Data Store.

visitors and lighthouse on rocky bluff
View of interbedded deposits of Cambridge Argillite (light gray layers) and intrusive diabase (brown layers) bedrock at the Boston Light, Little Brewster Island. Visitors for scale. (NPS photo by Trista L. Thornberry-Ehrlich)

Related Links

Related Articles

Boston Harbor Islands National Recreation Area

National Park Service Geodiversity Atlas

The servicewide Geodiversity Atlas provides information on geoheritage and geodiversity resources and values within the National Park System. This information supports science-based geoconservation and interpretation in the NPS, as well as STEM education in schools, museums, and field camps. The NPS Geologic Resources Division and many parks work with National and International geoconservation communities to ensure that NPS abiotic resources are managed using the highest standards and best practices available.

For more information on the NPS Geodiversity Atlas, contact us.

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Boston Harbor Islands National Recreation Area

Last updated: August 16, 2021