Moon Island Rills
December 08, 2008
Photographer: Rob Sheridan
Summary Author: Rob Sheridan
In the photo above, rills and gullies can be seen along the south face of Moon Island in Boston Harbor, Massachusetts. Moon Island, and most of the other 33 Boston Harbor Islands, is formed of mixed glacial till on a bedrock base. The till was deposited during one or both of the last two glacial periods here, about 150,000 and 15,000 years ago. You can see the thin veneer of soil covering the glacial till, the rills and gullies eroding the till, and how the erosion process sorts the till by size. Fine particles and clays are washed out to sea, contributing to the silt bed of Quincy Bay, Hingham Harbor, and Boston Harbor. Sands and gravels tend to wash alongshore, contributing to area beaches. Larger boulders and heavy gravel tend to stay at the foot of the eroding drumlin, forming the rocky base seen in the photograph.
Eventually, Moon Island may be eroded down to its Boston Slate and Roxbury Conglomerate base, perhaps disappearing below low tide level, as a few other "sinking" Harbor Islands have done, most famously Nix's Mate, which has eroded to non-existence in the 200 years since Boston Harbor has been an active international port. Photo taken on March 17, 1996.