Small, ovoid hills are common throughout eastern Massachusetts, especially in the vicinity of Boston. Viewed from the air, one sees that the long axis of these hills are aligned in the same direction (NW to SE), indicating that they were all formed by the same, large-scale process. These hills are drumlins, which typically occur in groups known as "drumlin fields."
Drumlins are composed of glacial till and form on the underside of a moving glacier. Aligned in the direction of the ice flow, they usually have a blunt northern (or ice-ward) edge and are tapered toward the south (or lee side). While the exact mechanism of their formation is a subject of ongoing debate among geologists, all agree that the drumlin fields of New England, Canada and the Northern Midwest are remnants of the Wisconsin Glacier, which scoured the region from 60,000 to 12,000 years ago.
Those in the Boston Area include Bunker Hill, Breed's Hill, the Blue Hills (south of the city) and the numerous islands in Boston Harbor; the latter islands are the summits of drumlins that were enclosed by the rising sea as the climate warmed and the Glacier retreated northward.
