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Boston lies near the geographic center of the Boston Basin, a roughly triangular area of sedimentary and volcanic rocks of late Precambrian and Cambrian age that is surrounded by older, contemporary, and younger granites and related rocks. The basin rocks are mostly argillites, sandstones, and conglomerates interlayered with rhyolitic and spilitic volcanics and volcaniclastic sediments. The rocks are deformed into large east-west folds and are much broken by faults of several orientations. Dikes and sills abound. Deep alteration of rock to a soft clayey aggregate is widespread. The irregular bedrock surface is the result of differential glacial erosion of rocks of varying hardness, and under most of the central Boston area, bedrock is buried by thick drift. The stratigraphy of the drift is complex and includes four stratigraphically distinct tills and outwash units, representing several advances and retreats of glacial ice. These are provisionally interpreted to consist of two glacial stades of the early Wisconsinan and two of the late Wisconsinan, including a read-vance that overrode much of the Boston area about 12,000 years B.P. The effects of the geology on local engineering practices are briefly described.

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