Bluff Evolution and Geomorphology of the Boston Harbor Drumlins
Emily A. Himmelstoss
Department of Earth Sciences
Department of Geology
Delivered at 2003 Boston Harbor Islands Science Symposium.
A series of drumlins deposited during the last glaciation form the characteristic shape of the Boston Harbor Islands. Many are partially submerged within the harbor and have actively retreating bluffs that have been impacted by wave attack. Comparisons between bluffs reveal variability in their height and length, as well as in the dominant mechanism causing their retreat. The bluffs erode by means of two distinct processes: (1) planar slopes undercut by waves, which result in episodic slumping events and (2) rilled or gullied slopes caused predominantly by rain splash. A model of drumlin bluff evolution is based upon features such as bluff height, slope morphology, and the orientation of the bluff with respect to the long axis of the drumlin and the topographic crest. The four stages of evolution were depicted for bluffs that formed both perpendicular and parallel to the long axis of the drumlin, as in Boston Harbor. A four-stage model of drumlin bluff evolution consists of:
(1) initial formation of bluff with retreat dominated by wave notching and slumping processes;
(2) rill and gully development as bluff heights exceed 10 meters and slumped sediment at bluff base inhibits wave attack;
(3) return of wave notching and slumping as bluff heights decrease, and
(4) final remnants of drumlin eroded by wave action.
This model captures the important physical processes of drumlin evolution in Boston Harbor, and may apply to other eroding coastal drumlin deposits.