Last updated: August 16, 2021
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.
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).
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 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:
- Glacial Erratics
Glaciofluvial deposits and features (deposited by rivers flowing beneath or out of glaciers) include:
- Braided Streams
- 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
- 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
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.
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.
- 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.
- Boston Harbor Islands—Geologic Formations
- Boston Harbor Islands—Coasts/Shorelines
- Boston Harbor Islands—Wetlands and Marshes
- Boston Harbor Islands—Soils
- Boston Harbor Islands—Park Home
- NPS—Fossils and Paleontology
- NPS—Glaciers and Glacial Landforms
- NPS—Beach and Coastal Landforms
- NPS—Plate Tectonics
- NPS—Geologic Time
- NPS—Explore Regional Geology
Related ArticlesBoston Harbor Islands National Recreation Area
National Park Service Geodiversity AtlasThe 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.