Chepiwanoxet Point - Geology

Geology

Chepiwanoxet (locally called "Chepi") is underlaid by sedimentary rocks of the Coal Age (Pennsylvanian period, about 300 million years old). This rock formation is 60 miles (97 km) long, extending from Hanover, Massachusetts, south to the mouth of Narragansett Bay, and 18 miles (29 km) 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.

The real beginning of Chepiwanoxet Island came as a result of the last great ice sheets, which bulldozed rock and stone from Canada to Rhode Island. Along the way, each glacier gouged up billions of cubic meters of underlying bedrock, and then grounded local and foreign rocks together, creating much of the glacial deposits existing today. When the Earth's temperature rose about 10,000 to 12,000 years ago, the glaciers retreated, when they finally began melting faster than they were moving forward. For extended periods, the ice advance equaled the rate of melt, and piles of sediment were deposited in terminal or end moraines, such as on Block Island and in Charlestown (hills in the southern part of the state).

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 to 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.