From Castle Island, you have one of the best mainland views of Boston Harbor’s drowned drumlin field; a rare geological landscape where glacially sculpted hills were later partially submerged by rising seas.
Across the harbor, you can clearly see several drumlin islands, including Thompson Island, Spectacle Island, and Long Island. Their smooth, rounded, elongated shapes are classic examples of drumlins formed beneath the massive ice sheet that covered New England during the last Pleistocene glaciation.
Geological Story
About 20,000 years ago, the entire Boston area was buried beneath the Laurentide Ice Sheet, a massive glacier that flowed across New England from the north and northwest. As this ice moved, it reshaped the landscape beneath it, molding loose sediment (called glacial till) into smooth, streamlined hills known as drumlins.
Drumlins form beneath actively moving ice, where pressure and shear forces deform and redistribute sediment. The glacier erodes and compresses the up‑ice (stoss) side, creating a steep, blunt slope, while dragging and depositing sediment along the down‑ice (lee) side, forming a long, gentle taper. This process produces the classic teardrop shape seen in drumlins across Boston Harbor.
When the Ice Age ended, the melting Laurentide Ice Sheet released enormous amounts of water into the world’s oceans. Sea levels rose more than 300 feet globally, flooding the low valleys between drumlins. Only the highest drumlin crests remained above water, becoming the modern Boston Harbor Islands. This created one of the best examples of a drowned drumlin field in the world, where the original glacial landscape is still visible; just partially submerged.
From Castle Island, you can clearly see this story in the landscape:
- Steep northern ends of islands like Spectacle and Thompson show where the glacier pushed against them.
- Longer southern slopes point in the direction the ice flowed.
- The water between the islands marks the former lowlands that were flooded as sea level rose.
This vantage point allows you to reconstruct the Ice Age landscape simply by observing the shapes and orientations of the islands.
References
Information above was referenced from the following sources:
Activities
From your position at Castle Island, look across the harbor and make the following observations. Note that the ilsands will be in the order, from left to right, of Harbor (in the distance East), Spectacle (SouthEast) and Thompson (South):
- From your viewing point, choose one island and describe a feature of its shape that is visible from this angle (for example: a break in slope, a ridge, a low point between hills, or a change in the outline) and explain how you think it was formed
- Looking at the island’s profile, identify where you see the steepest slope and where you see the gentlest slope. Describe what you observe
- Using what you can observe from Castle Island, describe one piece of visual evidence that suggests the island was shaped by flowing ice (for example: a streamlined outline, a tapered end, or a smoothed crest)
- Observe the shoreline of one island. Describe what you see at the waterline (rock, beach, vegetation, riprap, etc.) and explain how this visible shoreline supports the idea that sea level rose and flooded lower areas between drumlins
- Compare two islands from this viewpoint. Describe one difference in their visible shapes that helps you understand how drumlins can vary even when formed by the same glacier.