Boston Harbor contains one of the few drumlin coasts in the world.  The drumlins formed during the late Wisconsinan stage of glaciation and were drowned by rising sea level during the Holocene transgression approximately 5,000 years ago.  Modification of the drumlins accelerated as rising sea level encroached upon the island shorelines.  Storm waves attacked the base of the drumlins, causing mass wasting and slumping of sediment downward.  The mobile silt and clay were removed from the shoreline and deposited in low energy environments.  The fine gravel and sand were transported along shore forming beaches, spits, and beach ridges.  The former extent of the drumlin is defined by a boulder-lag pavement.

Present island morphology reflects the distribution of drumlins, the reworking of drumlin sediment by wave energy, basement controls, and ongoing sea level rise.  Individual islands may consist of as many as five drumlins that have been connected by spits.  The spits, which built across topographic highs from the eroded drumlin sediment, are responsible for marsh and pond development.  Active glacial bluffs along many of the islands are a product of narrow abutting beaches and rising sea level.  This condition allows storm waves to break against the base of the drumlin slope perpetuating the retreat of the glacial bluffs.  In a regime of accelerated sea level rise, the rate of bluff erosion will likely increase.  Although this process will increase the long-shore contribution of sediment and temporarily maintain the integrity of the spit systems, ultimately the longevity of individual drumlin islands is a function of future sea-level trends, island size, exposure to wave energy, and existence of protective engineering structures.