Illinoian Till in Boston Harbor

Provided and copyright by: Rob Sheridan
Summary authors & editors: Rob Sheridan

The 34 Boston Harbor Islands scattered about the Boston Basin have a similar anatomy. All are overlain with more or less glacial till, generally depending on the height of their base and the degree of their exposure to the open Atlantic and its storms.

Perhaps 10 cycles of glaciation have occurred in what is now New England over the past million years. The most recent cover of glacial ice, the Laurentide Ice Sheet, receded about 15,000 years ago at the end of the Wisconsinan period, and is responsible for most visible glacial till in the Boston Basin. However, there's a lower till layer that was deposited about 150,000 years earlier, during the Illinoian period. In a few Boston Harbor Island locations, the erosion of island drumlins has exposed this lower till layer, as is the case above.

The lower (Illinoian) till is darker and more compacted than the upper (Wisonsinan) till. It also contains shell fragments of the marine hard-shell clam (Mercenaria mercenaria). This photo shows a section of the eroded Prince's Head, a drumlin of Peddocks Island. It provides a wide exposure of the lower layer of Illinoian till, gradually eroding into the sea. The heavier elements of the till are retained at the base of the dissolving cliff, while the sand and lighter particles have been washed out to sea, explaining the rocky beach at the foot of the cliff.