The bedrock geology of the Maynard quadrangle of east-central Massachusetts was examined through field and petrographic studies and mapped at a scale of 1:24,000. The quadrangle spans much of the Nashoba terrane and a small area of the Avalon terrane. Two stratigraphic units were defined in the Nashoba terrane: the Cambrian to Ordovician Marlboro Formation and the Ordovician Nashoba Formation. In addition, four igneous units were defined in the Nashoba terrane: the Silurian to Ordovician phases of the Andover Granite, the Silurian to Devonian Assabet Quartz Diorite, the Silurian to Devonian White Pond Diorites (new name), and the Mississippian Indian Head Hill Igneous Complex. In the Avalon terrane, one stratigraphic unit was defined as the Proterozoic Z Westboro Formation Mylonites, and one igneous unit was defined as the Proterozoic Z to Devonian Sudbury Valley Igneous Complex.

Two major faults were identified: the intra-terrane Assabet River fault zone in the central part of the quadrangle, and the south-east Nashoba terrane bounding Bloody Bluff fault zone. Petrofabric studies on fault rocks in two areas indicated final motion in those areas: the sheared Marlboro Formation amphibolites indicated dextral transpressive NW over SE motion, and the Westboro Formation Mylonites indicated sinistral strike-slip motion.

Fracture characterization of entire quadrangle where attributes (orientation, trace length, spacing, and termination) of fractures and joints were used to identify dominant sets of fractures that affect the transmissivity and storage of groundwater. Orientations of SW – NE are dominant throughout the quadrangle and consistent with regional trend.