The Bedrock Geology of Boston Harbor and Outer Harbor Islands
Joseph P. Kopera
Delivered at the 2011 Boston Harbor Islands Science Symposium
The Boston Basin, a geologic feature, underlies Boston Harbor and its surrounding area. Over 150 years of research by geologists has led to a modern understanding of the structure and origins of this feature. The sedimentary and volcanic rocks which comprise the Boston Basin were deposited nearly 600 million years ago in the southern mid-latitudes off the coast of the continent Gondwana, as part of a larger group of rocks now called the Avalon terrane. Almost 250-300 million years later, Avalon made its way north to collide with the continent Laurentia (now North America) during the Acadian and Alleghenian orogenies. These events culminated in the formation of the supercontinent Pangea, deforming the Boston Basin into large open folds and lacing it with faults. At several points during the Basin's journey, the sedimentary rocks were intruded by dikes and sills of mafic igneous rock. As Pangea later split apart the Boston Basin was further cut by faults and intruded again, but stayed attached to North America to form the bedrock under Boston today. The sedimentary rock in Boston Harbor area is dominated by the erosionally soft Cambridge argillite, which forms the depression of the Harbor itself, while the more erosionally resistant mafic igneous rocks form the outer Harbor islands. Detailed mapping of the bedrock geology of the Harbor Islands, compilation of years of subsurface geotechnical data, and borings under the harbor have allowed the Mass. Geologic Survey to construct the most detailed map to date of the bedrock geology of the Harbor, gaining new insights into the structure of the Boston Basin.