Chepiwanoxet Island (Rhode Island)

Geology

Geologically, Chepiwanoxet and its island are located in the East Greenwich 7.5 quadrangle in Rhode Island. Bedrock and surficial geologic maps for East Greenwich were published by the US Geologic Service and Rhode Island State Geologist.

Chepiwanoxet (locally called "Chepi") is underlain by sedimentary rocks of the Coal Age (Pennsylvanian Period, about 300 million years old). This rock formation is 60 miles long, extending from Hanover, Massachusetts, south to the mouth of Narragansett Bay, and 18 miles 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 then ground 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 Block Island and Charlestown (hills along the southern part of the state). [1976, Dr. Alonzo Quinn: Rhode Island Geology For The Non-Geologist] .

The new Bedrock Geologic Map of Rhode Island [Hermes, 1994] gives more details of what underlays 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.

The Island possibly was created by a brief pause in the last glacial retreat. Since the land appears to be largely sand, having few native boulders and rocks, it is also likely to have been an outwash deposit, perhaps from melt water runoff from the high hills (the Love Lane and Cowesett Hills area) down the gully, now a marsh swamp.

Early bay maps (e.g. Charles Blaskowitz, 1777) clearly show Chepiwanoxet as an island. It became a peninsula when the Gallaudet seaplane factory was constructed in around 1915. For trucks to reach the factory, fill was dumped into the marsh, making a causeway access which still remains.

The Soil Survey of Rhode Island [Rector, 1981] lists the Chepiwanoxet Island and nearby shores as Hinckley gravelly sandy loam — a very porous, well drained soil of glacial origin which is of poor topsoil quality. Only in the marsh does the soil type change to a bit of Walpole sandy loam associated with severe wetness.

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