Drumlins are a hill-like structure formed by glaciers thousands of years ago; many can be found on the surrounding islands and on mainland Boston, including Bunker Hill. These “drowned drumlins” (any drumlins in water and intersecting a coastline) are rare: the only others in the world lie off the coast of Northern Ireland.

Tombolos are sandy spits of land connected to islands or drumlins, often covered up by high tide. Spectacle Island used to be two drumlins connected by a tombolo; the entire area where the tombolo once was has been filled in by the landfill and subsequent Big Dig dirt that covered the garbage.