Stratified rocks of northeastern Massachusetts that are known or considered to be of pre-Silurian age are (1) a sequence of nonfossiliferous marine metasedimentary and metavolcanic rocks at least 2,800 m thick, including the Westboro Formation (formerly known as the Westboro Quartzite) and the Middlesex Fells Volcanic Complex (new name), that probably are of Precambrian age; (2) a sequence of nonfossiliferous eugeosynclinal rocks having an aggregate thickness of about 18,800 m, which includes the Greenleaf Mountain Formation (new name), Burlington Formation (new name), Marlboro Formation, Shawsheen Gneiss (new name), Fish Brook Gneiss, Nashoba Formation, and Tadmuck Brook Schist (new name), all of undetermined age; and (3) some scattered remnants of unmetamorphosed, partly fossiliferous marine sedimentary rocks of Cambrian age.
The Westboro Formation, Middlesex Fells Volcanic Complex, and some older unnamed gneiss and quartzite strata were domed and locally subjected to minor folding during emplacement of the Dedham Granodiorite, which intrudes them but does not intrude any of the stratigraphically higher metasedimentary rocks. These intrusive and structural features, together with local omissions of the upper part of the above-named formations, suggest a significant unconformity. These stratified rocks are tentatively correlated with the Blackstone Series of Rhode Island.
The eugeosynclinal rocks constitute a generally homoclinal sequence, except where parts of the section are locally cut out or are repeated by faults or are repeated by minor folds. This sequence consists of at least 80 percent metamorphosed volcanic wackes; units characterized by strata of metamorphosed pelitic and carbonate sediments are interspersed through it.
The stratified rocks of Cambrian age form pendants in plutonic rocks and possibly also outliers. The Weymouth Formation of Early Cambrian age and the Hoppin Slate that possibly is correlative with it consist mainly of marine shale and limy shale interspersed with minor thin limestone strata. The Braintree Argillite, conformably overlying the Weymouth Formation, consists of marine shale, silty shale, and siltstone. The remnants of Cambrian rocks do not have any contacts with other stratified rocks.