Cover Image Credits

Braintree Weymouth Tunnel leading north to the Deer Island Sewage Treatment Plant in Boston Harbor, Massachusetts (photo by Theresa Green, Wellesley '02). The tunnel measuring 13 feet (3.9 m) in diameter passes through dark gray, laminated deposits of the Cambridge Formation, which is regionally the most extensive sequence in the Boston Basin, but poorly exposed at the surface. Integrated structural, petrographic and geochemical data from this and other Boston-area tunnels reveal more than 5 km of argillaceous strata that include widespread volcanic components and rest on a significant unconformity. A ~551 Ma CA-TIMS date from an ash bed near the top of this section demonstrates that Cambridge deposition is significantly younger than previously reported. This revision necessitates re-consideration of tectono-stratigraphic linkages between southeastern New England and other northern Appalachian Avalonian terranes. See paper by Thompson and Crowley, p. 405–449.