Ash-flow tuff in the Stony Brook Reservation contains clasts as much as 450 m long. The pyroclastic matrix of this breccia and the large size of its clasts are characteristic of “megabreccias” found in terrains of ash-flow volcanism and caldera collapse. The breccia of the Stony Brook Reservation is intruded by shallow-level (Westwood?) granite, suggesting that both caldera collapse and magmatic resurgence were important processes in the history of this part of the late Precambrian plutonic-volcanic terrain in southeastern New England.

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