Roxbury Conglomerate

Conglomerate, sandstone, siltstone, argillite, and melaphyre. Consists of Brookline, Dorchester, and Squantum Members. Roxbury Conglomerate forms base of Boston Bay Group. Divided into Brookline, Dorchester, and Squantum Members. Conglomerate in Brookline Member contains clasts of Dedham Granite, quartzite (possibly from Westboro Formation), and volcanic rock from underlying Mattapan Volcanic Complex. Dorchester Member consists of interbedded argillite and sandstone and forms an intermediate unit between Brookline Member and overlying Cambridge Argillite. Uppermost Squantum Member is a distinctive diamictite which appears to pinch out in northern part of basin. Brighton Melaphyre lies within Brookline Member and consists of mafic volcanic rocks (quartz keratophyre, keratophyre, and spilite). Roxbury clearly lies nonconformably on Dedham Granite near Hull, MA; can be traced continuously over Mattapan Volcanic Complex. Age is Proterozoic Z and possibly Early Cambrian (Goldsmith, 1991).
State Massachusetts
Name Roxbury Conglomerate
Geologic age Proterozoic Z to earliest Paleozoic
Lithologic constituents
Sedimentary > Clastic > Conglomerate
Sedimentary > Clastic > Sandstone
Sedimentary > Clastic > Siltstone
Igneous > Volcanic > Mafic-volcanicmelaphyre
Metamorphic > Metasedimentary > Metaclastic > Argillite
Comments Part of Milford-Dedham Zone (Tertiary and Older Rocks). Secondary unit description per MA023.
NGMDB product
Counties Middlesex - Norfolk - Plymouth - Suffolk