The Drumlin Islands of Boston Harbor, shared by the NASA Earth Observatory, offer a glimpse into the geological forces that have shaped the northeastern United States over the past 20,000 years. These islands are the remnants of the Wisconsin Glaciation, where a massive ice sheet more than a mile thick once covered the land. As the ice melted, it left behind distinct landforms known as drumlins — elongated hills formed from glacial debris. Today, these drumlins are submerged, creating a unique archipelago, one of only three partially submerged drumlin fields in the world. This distinctive feature is now preserved within the Boston Harbor Islands National and State Park, making it a vital historical and ecological resource.
Glacial Origins of Boston Harbor Islands
The formation of the Drumlin Islands traces back to the height of the Wisconsin Glaciation, which occurred around 20,000 years ago. During this period, a massive ice sheet, over a mile thick in some places, covered the region that is now Boston Harbor. As the ice slowly advanced, it scraped the landscape, depositing debris and sediments. When the glaciers began to retreat, these sediments formed the distinctive elongated hills known as drumlins.
Unlike other glacial features, drumlins have streamlined shapes, with one side steeper than the other. This unique formation is a testament to the dynamic nature of glaciers. Geoscientist Jasper Knight, from the University of Witwatersrand in South Africa, explains the complexities of glacier movements: “Previous ideas of slow, steady advance, or retreat, really don’t hold. Glaciers are dynamic.” The retreating ice sculpted the land, leaving behind these residual hills, which would later become the Drumlin Islands as rising sea levels flooded the region. These islands, now fully or partially submerged, offer a rare view of a geologically significant phenomenon, forming one of only three submerged drumlin fields in the world.
The Boston Harbor Islands: Ecological Treasure
Today, the Boston Harbor Islands National and State Park consists of 34 islands and peninsulas, many of which were formed from these glacial drumlins. The park is a haven for diverse ecosystems, including salt marshes, seagrass beds, and tidal pools. These unique environments support a wide range of species, both marine and terrestrial. Lobsters, crabs, clams, and various fish such as striped bass, bluefish, and flounder inhabit the waters surrounding the islands. On land, the islands feature grasslands, hardwood forests, and mudflats, home to many bird, reptile, amphibian, and mammal species.
The conservation efforts within the park are vital to preserving these ecosystems. It serves not only as a protected space for wildlife but also as an educational and recreational resource. Visitors can explore the islands, with some accessible by car, others by seasonal ferries or private boat. These ecosystems provide a natural sanctuary for a variety of species, and the park’s protection ensures their continued existence.
A Rich History Embedded in the Islands
The Boston Harbor Islands are not only geologically and ecologically important but also hold significant historical value. Many of the islands have been shaped by human activity throughout the centuries, making them a crucial part of the region’s history. Long Island, for example, was once connected to the mainland via a highway bridge. Today, the island contains abandoned government buildings, a chapel, a crematorium, and other infrastructure that reflect its past as a site for government use.
Peddocks Island, one of the largest in the park, has privately owned cottages and the remnants of a coastal fort. Georges Island is home to Fort Warren, a pentagon-shaped fort built in the 19th century. This fort, named after Revolutionary War hero Joseph Warren, was used during the Civil War as a prison for Confederate officers and officials. It serves as a testament to the region’s military history and remains a key site for visitors interested in the history of the U.S. military.
One of the most iconic structures on the islands is Boston Light on Little Brewster Island, the oldest continually operating lighthouse in the United States. Built in 1716, it has been guiding ships into the harbor for centuries. The lighthouse’s original use of tallow candles has long been replaced by modern electric bulbs, but its historical significance remains unchanged.
