The new Bedrock Geologic Map of Rhode Island gives more details of what underlies the unconsolidated surface material. Chepiwanoxet sits atop the western edge of the Rhode Island Formation of the stratified Narragansett Bay Group of the Pennsylvanian period (between 323 and 290 million years ago). The Narragansett Bay Group rocks are part of the Esmond-Dedham subterrane of the Southeastern New England Avalon zone. Uphill from the Post Road (US Route 1) is the boundary between the Rhode Island Formation and the older Scituate Igneous Suite (granite and diorite/gabbro rock) of the Devonian period (408 to 362 million years ago), which extends westward to the Connecticut border.