Geology of Rhode Island

Rhode Island is divided into two distinct physiographic regions. The Eastern Highlands occupy the northwestern interior of the state, near the state line with Connecticut while the majority of the state's land area is grouped with the larger Atlantic Coastal Plain that extends along the eastern seaboard of the US.

Rhode Island is the only state in New England formed entirely atop basement rock from the microcontinent Avalonia. The bedrock of Rhode Island first took shape with the emergence of a volcanic arc near a subduction on the margin of the supercontinent Gondwana, with the earliest rocks likely formed in the late Neoproterozoic.

During the early Cambrian period, as multi-cellular life rapidly increased on Earth, Avalonian basement rocks moved northward from Gondwana following the breakup of the supercontinent Pannotia. Rhode Island became landlocked during the formation of Pangaea and experienced substantial folding during the Alleghanian orogeny leading to the creation of the Narragansett Basin. This shallow water depositional environment filled with eroded sediments and organic material. Accumulation of sediments and organic matter accelerated during the Carboniferous period, ultimately forming small coal deposits throughout the basin.

During the breakup of Pangaea Rhode Island was positioned alongside a marine environment once again as extensive erosion of the Appalachian Mountains deposited sediments out on the continental shelf, beginning the creation of the Atlantic coastal plain.^[1]

Rhode Island was glaciated in the past 2.5 million years of the Quaternary, in the Pleistocene period. The region was glaciated twice between 75,000 and 55,000 years ago. Glaciers encountered a freshwater lake in the Narragansett Bay area. The ice carved through younger sediments exposing more ancient rock. A large terminal moraine was formed in the ocean at the edge of the ice sheet during the first pulse of glaciation, creating Block Island along with Long Island, Martha's Vineyard and Nantucket. The last Wisconsin glaciation began to retreat 18,000 before the present, leaving behind a sediment strewn landscape. Much of the surficial geology of the towns of Westerly, Charlestown and Narragansett resulted from the extensive Charlestown moraine, stretching in a line from Long Island to Fishers Island to the southern coast of Rhode Island.

The region became entirely ice free by 14,000 before the present, shortly before the initial migration of humans to North America. Rhode Island has become substantially influenced by human activity since the formation of the Colony of Rhode Island and Providence Plantations in 1636.^[2]

Rhode Island's bedrock is divided into six large categories. The oldest rocks are metamorphic and date to the Precambrian or early Paleozoic. Metamorphic rocks in the southwest of the state have been correlated to Cambrian and Ordovician rocks in Connecticut, while the quartzite, marble and greenschist of the Blackstone Group are common in northern Rhode Island. Other metamorphic rocks are exposed in Newport and Tiverton.

Much of the state's land area, including most of the central, western and interior areas of Rhode Island are made up of intrusive plutonic rocks from the Paleozoic. This most extensive grouping of bedrock is made up of gneiss, granite and quartz diorite. Notable examples of plutonic rock groups include Hope Valley alaskite gneiss, Metacom granite gneiss, Esmond Granite, Bulgarmarsh Granite, Ponaganset Gneiss and Scituate Granite Gneiss.

Some locations in northeastern Rhode Island and in the East Greenwich Group are granite and volcanic igneous rocks. Sedimentary rocks are common in the Narragansett Basin in North Scituate and Woonsocket, where rocks date to the Pennsylvanian and are clastic sedimentary rocks, with different levels of metamorphism.^[3]

In the southern part of the state, the Westerly Granite and Narragansett Pier Granite belong to a series of granitic rocks from the Pennsylvanian and later. The youngest bed rock is found beneath Block Island where clay and sand are exposed from the Cretaceous.^[4]

Rhode Island's surficial geology includes sediments, soil and groundwater near the surface and lying atop the bedrock lithology. As a result of Pleistocene glaciation, much of the state is covered in glacial till including virtually all of the towns of Tiverton, Middletown, Newport, Portsmouth, Bristol, Jamestown, Burrilville and Foster. Glacial outwash dominates in Providence, Pawtucket, Cranston, Woonsocket, Pawtucket, Warwick and sections of the coast and interior.

The soils of Rhode Island are a part of the state's surficial geology, but are often studied separately because of their role in development planning, construction and agriculture. Approximately 65% of the soils developed from glacial till. Most soils are loamy sand and sandy loam, with medium or coarse textured grains of acid crystalline rock. A narrow strip along the western shore of Narragansett Bay originated from Carboniferous rocks including slate and shale. The soils on the western shore are dark colored, silt loam. Twenty percent of soil in Rhode Island resulted from glacial outwash and tend to be thick, well sorted layers of sediment with stratified layers of sand and gravel. The remaining soil in the state are 10% fine, wind-blown loess sediments that range from 6 inches to 4 feet in thickness, with an average thickness of 30 inches. Loess soils hold large quantities of water and are deemed high quality agricultural soils. An additional 5% of soils entirely organic, forming in freshwater wetlands, with thicknesses between one foot and 20 feet. Only 1% of soils are found in saltwater marshes, with 16 inches of organic material layered over sand.^[5]