Chepiwanoxet is underlaid by sedimentary rocks of the Coal Age (Pennsylvanian period, about 300 million years old). This rock formation is long, extending from Hanover, Massachusetts, south to the mouth of Narragansett Bay, and wide from Providence to Fall River. It underlies the entire Narragansett Bay, including the Providence River, Greenwich Bay, Fall River, the Sakonnet River, plus both East and West passages out to the ocean. Chepiwanoxet and East Greenwich are just inside the western edge of the Rhode Island formation.