The Narragansett Basin contains a calculated thickness of 12,000 feet of clastic sedim entary rocks including conglomerates, sandstones, and shales. Sedimentary structures indicate deposition in an alluvial environment for many of the sediments. The lowermost formation in the western and northern portion of the basin is the Pondville, characterized by polymict conglomerate overlying older igneous and metamorphic rocks with sharp unconformity. Basal Pennsylvanian rocks along the eastern margin of the basin are highly altered arkoses that locally grade imperceptibly into underlying weathered granite. Overlying the basal rocks is the Rhode Island Formation, a thick and extensive unit characterized by sandstone with numerous beds of conglomerate and siltstone. Granitic detritus, prominent in many of the sandstones and conglomerates, was probably derived from western highlands. A quartzite source region lay to the east and contributed detritus to rocks of the southeastern part of the basin. The Wamsutta Formation interfingers with the Rhode Island Formation in the northwestern part of the basin. It contains reddish sedimentary rocks, some of which are poorly sorted, contain considerable felsite debris, and may have been emplaced by subaerial mudflows. Occurrence of extrusive and intrusive igneous rocks within the Wamsutta provides further evidence that volcanic activity attended deposition of the sedimentary rocks. The uppermost formation in the basin is the Dighton Conglomerate. The lower part of the basin sequence has been florally dated as Pennsylvanian. Radiometric age calculations raise the possibility that deposition may have continued into the Permian.

Composition of conglomerates within the Narragansett Basin contrasts sharply with that of vein quartz-pebble conglomerates deposited along the western front of the central Appalachians from Ordovician through Pennsylvanian time. The differences are attributed to a combination of different sources and different depositional histories.