Recent geologic work in eastern Massachusetts and Connecticut has demonstrated that the rocks of the area are broken into a series of fault blocks. From east to west, the faults bounding these blocks are the North Boston Basin fault (in Massachusetts and possibly extending south into Rhode Island), the Bloody Bluff (Massachusetts) and the Honey Hill-Lake Char (Connecticut) fault system, and the Clinton-Newbury fault (Massachusetts). All of these faults, except possibly the North Boston Basin, trend southwest toward a common area in southern Massachusetts that has not yet been mapped in detail, so the relationship between them has not yet been established. Although early work in the area lumped various rock units across the faults, the modern work indicates that distinct groups of rocks occur within each fault block, and repetition of units across the major faults has not been demonstrated.
The ages of the rocks in the area are not yet established. Fossil-bearing rocks from Cambrian to Pennsylvanian in age occur in fault-bounded blocks; their relationship to the metamorphosed rocks cannot be determined.