Geological History of Jamestown, Rhode Island

Guide to Bedrock

Overview of New England Geology and Topography

The following chart, showing the New England area, is reproduced from the USGS site Tapestry of Time and Terrain. The USGS claims this map of the U.S. is the most detailed and accurate portrait of the land surface and the ages of its underlying rock formations yet displayed in the same image.

The USGS map resembles traditional 3-D perspective drawings of landscapes with the addition of a fourth dimension, geologic time, which is shown in color. In mutually enhancing the landscape and its underlying temporal structure, this digital tapestry outlines the geologic story of continental collision and break-up, mountain-building, river erosion and deposition, ice-cap glaciation, volcanism, and other events and processes that have shaped the mainland 48 states over the last 2.6 billion years.

 

An Overview of Bedrock in the Narragansett Bay Area

Rhode Island is now largely covered by sand and gravel. The bedrock shown in the map below is rarely seen except in road cuts and, as in the Jamestown area, along the periphery of the island where relatively recent glacial gouging cut through the younger deposits, exposing underlying formations.

Rhode Island Bedrock Identification Key

Color Description Geologic Age
Light Green Narragansett Pier Plutonic (Granites) Permian
(> 245 million yrs)
Shades of Yellow Narragansett Bay Stratified Group Pennyslvanian (290 million yrs)
Shades of Dark Green Conanicut
Group
Cambrian
(>439 million yrs)
Shades of Rose Granites, Gneiss Protozoic
(> 570 million yrs)
Shades of Purple Newport
formations
Protozoic
(> 570 million yrs)

A more detailed key is available

Click on image for larger version

Click here for the original (959 kb) image,
showing all of Rhode Island, prepared by
the Office of the Rhode Island State Geologist

Underlying all of Rhode Island is Avalon Terrane, but that does not mean that the bedrock is uniform throughout the area. The original volcanic arc was several thousand miles long and consisted of many individual islands. Within Rhode Island there are two subterranes, separated by a arcing shear zone located close to the western boundary of the state.

The shear zone extends into Connecticut near the center of the western state boundary, passes to the east of Hope Valley and terminates near Charlestown in the southern part of the state. Most of the state is to the east of this shear zone, in the Esmond-Dedham subterrane. The subterrane to the west of the shear zone is the Hope Valley subterrane.

The oldest bedrock in the Jamestown/ Newport area is shown in the lower center of the map - colored in shades of purple and rose. These rocks are located in the Fort Wetherill area of Jamestown and the southwestern portion of Newport. They date to the Late Proterozoic period or earlier (more than 570 million years ago) - when Jamestown was part of the original volcanic arc in the southern hemisphere, off of Africa.

The large area of pinkish-rose coloration (west of Narragansett Bay and inland from it) represents granites, gneiss and granadiorite of similar Late Proterozoic age.

West and north of the Fort Wetherill area on Conanicut Island - everything south of Great Creek (Marsh Meadows) - is Cambro-Ordovian era bedrock, which is colored in shades of dark green. These rocks were formed during the period when Avalonia collided with Laurentia approximately 425 million years ago.

The large area in shades of yellow are rocks formed after Avalonia was united with Laurentia and include the period when the area was united with Northern Africa - when Pangaea existed. These rocks were created from sediment laid down in the Pennsylvanian period, roughly 290 to 325 million years ago in a large depression created by the forces of continental impact.

The light green strip near Narragansett Pier represents Permian period granites formed as magma made its way to the surface near the end of the Alleghenian Orogeny that occured in conjunction with formation of the Pangaea supercontinent. They were formed between 245 and 290 million years ago.

The small amount of light blue in the upper left corner is a portion of a relatively large area (about ten miles by fifteen miles) of Triassic and Jurassic bedrock located west of West Warwick. These rocks, primaily vein quartz, are 146 to 245 million years old.

The USGS is currently developing The North America Tapestry of Time and Terrain, which will include Canada and Mexico. Check on the status at the USGS website.

Learn more:

Or, go back to any Geological History page:

Introduction and Summary: 565 Million Years of Jamestown's Geological History
Prelude: The Earth's first 4 billion years - forming Proto North America, Rodinia, Gondwana
Avalonia: Rhode Island was once part of a micro-continent called Avalonia
Acadian Orogeny: Avalonia collides with the mainland of Proto North America (Laurentia)
Alleghenian Orogeny: North America collides with Africa, forming Pangaea
The Atlantic Forms: Pangaea breaks up, the Atlantic forms, the Appalachians erode
Glaciation: Glaciers form and rework the land
The Holocene Epoch: Post-glacial Rhode Island - rising seas - the time of modern man
Building the Northern Appalachians: Significant event summaries with links to more information
Guide to Bedrock in and around Jamestown and Narragansett Bay
Additional Information and References

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