Radioactivity patterns in southeastern Connecticut, adjacent Rhode Island, and New York closely reflect bedrock units. This relationship is not greatly affected by surficial deposits of Pleistocene age because most deposits are locally derived and bedrock outcrops are relatively abundant. Most bedrock units are medium- to high-grade metasedimentary, metavolcanic, and metaplutonic rocks that range in age from possible Precambrian to middle Paleozoic. The rest are postmetamorphic granite and pegmatite of late Paleozoic age. The area is on the west flank of an anticlinorium of granitic gneiss and subordinate metasedimentary and metavolcanic rocks at a place where the anticlinorium changes trend from north to east. The Honey Hill and Lake Char faults mark a zone of low-angle faulting between the granite-gneiss terrane and stratified rocks of the Merrimack synclinorium to the north and west. The east-trending Honey Hill fault marks the southern end of the Merrimack synclinorium.
Four fields denoting different levels and patterns of radioactivity coincide with major geologic provinces. The granite-gneiss terrane, which contains abundant potassic, ledge-forming granitic rocks (the Sterling Plutonic Group), coincides with a field of relatively high radioactivity, in which subordinate lows reflect outcrop areas of quartzitic rocks of the Plainfield Formation. This terrane is flanked to the west and north by layered metavolcanic rocks of mafic to intermediate composition belonging mostly to the Quinebaug Formation. These rocks coincide with a field of relatively low to moderate levels of radioactivity. The lowest level within this field is produced by the Preston Gabbro, which is at the right-angle bend in the structure.
A sequence of metasedimentary schist and gneiss of the Tatnic Hill Formation, Hebron Formation, Scotland Schist, and the sill-like Canterbury Gneiss overlies the Quinebaug Formation. These rocks produce a field of moderate levels of radioactivity but have nodes of high radioactivity where pegmatite is abundant. A field of moderate intensity south of the east-trending anticlinorium of granitic gneiss coincides with layered and massive granodioritic and quartz-dioritic gneiss and subordinate amphibolite equivalent stratigraphically to the layered metavolcanic rocks north of the anticlinorium. The late metamorphic to postmetamorphic, low-angle Honey Hill and Lake Char faults, which follow the bend in the structure at the base of and within the metavolcanic rocks on the north and west sides of the anticlinorium, respectively, can be recognized only by the radiometric pattern that reflects the tectonic pinch-out of the Quinebaug Formation westward along the Honey Hill fault.
A pluton of Narragansett Pier Granite of late Paleozoic age, which has associated dikes of Westerly Granite, is reflected in a sub field of high radioactivity in southern Rhode Island. Material derived from these rocks is apparently responsible for a relatively high level of radioactivity in the Charlestown moraine immediately south of the out crop area.