Geological History of Jamestown, Rhode Island

Narraganset Bay offers many reminders that it was carved by glacial ice

The story of the land that is now Jamestown Rhode Island

Long ago - about 565 million years ago, and far away - near the South Pole - approximately where the coastline of Antarctica is located today - the basement rocks beneath Jamestown (and all of Rhode Island) were formed as part of a volcanic arc called Avalonia.

The world was very different then, in many ways. Land masses were arranged quite differently than they are now. (Over time continents move, at a very slow rate, roughly the rate at which finger nails grow; but over millions of years, huge rearrangements occur; and these will be traced in this narrative.) 

The Avalonian arc formed offshore of present day Morocco (northwest Africa) and Venezuela (northern South America) when Africa and South America were joined in a larger continent which we call Gondwana.

To illustrate how much the world has chanced since then, at that time, what is now the Sahara Desert, was located at the south pole!
Close up of Avalonia and Gondwana

Rhode Island's nearest neighbor (across-the-sea, as opposed to other parts of the Avalonian volcanic arc) was located to the south, closer to Gondwana, in a triangular bit of ocean. It was an island - now known to us as the state of Florida.

To the west, along the arc, were parts of what is now the eastern United States southward to the Carolinas. To the east were parts of present-day Massachusetts, Maine, Nova Scotia, New Brunswick, Newfoundland, Morocco, southern Ireland, southern England, Wales, Portugal, western Spain, northwest France, Belgium, Holland and northern Germany.

There was no life on the land. Living things were found only in the sea, and they were just beginning to evolve from single-cell organisms into some of the earliest soft-bodied, multi-cellular organisms.

To provide some sense of what came before Avalonia formed, we have included a Prelude that provides information on the first 4 billion (4,000 million) years of continental formation and evolution..

The main narrative traces the journey of Jamestown's land across oceans and through continental collisions to the relatively recent past, when Rhode Island and surrounding areas were sculpted by glacial action and Narragansett Bay was created by the rising sea as the glacial ice melted.

As you will see, quite literally, Jamestown's land has been at the leading edge of much of the world's history.

565 Million Years of RI Geological History, and More
A Very Brief Introduction and Summary
All of Rhode Island was part of a volcanic arc or micro-continent called Avalonia, which formed south of the equator more than 565 million years ago, off of the coasts of both Africa and South America, when they were joined as part of a larger continent. At that time, the Earth was about 4 billion years old and living things were just starting to evolve from single-cell organisms into some of the earliest soft-bodied, multi-cellular organisms.

Avalonia drifted northward, away from Africa, over the next 100 million years.

The Avalonian arc collided with Proto North America about 400 million years ago. The impact contributed most of the land that has become the New England states and created most of the northern Appalachians, as we know them.

The Appalachians are actually a complex mix of mountains formed by a series of continental collisions, beginning with the (now heavily eroded and often buried) Grenville mountains (formed more than 1 billion years ago), the Taconic mountains (formed by another island chain collision about 50 million years prior to the collision with Avalonia) and newer mountains formed about 350 million years ago when North America and North Africa collided, raising portions to great heights. This collision was one of several that formed a supercontinent, Pangaea about 306 million years ago.

For more than 100 million years most of the world's land was united. Jamestown was located thousands of miles from the ocean, deep within that huge supercontinent. This was the time when dinosaurs appeared. The layers of shale and coal that are common in the Narraganset Bay area were formed during this period.

About 200 million years ago, Pangaea broke up. The Atlantic Ocean formed and slowly widened.

As the Appalachians eroded, land on both sides of the mountain chain was covered with sediment. To the east and south, the layers of sediment formed the coastal plain and the (now underwater) continental shelf.

During the past 75,000 years, Jamestown was visited by glaciers at least twice. They gouged out channels in the Narragansett Basin and left deposits of rocks and soil throughout the area.

The last glaciers left the area about 12,000 years ago.

At that time, sea level was hundreds of feet lower than today. As additional glaciers melted around the world, the sea rose.

Vegetation and animals that had been driven south, returned to the barren-post glacial area.

Humans arrived. They were driven from coastal areas and river valleys along now-submerged former coastlines, as the rising sea covered the continental shelf and eventually filled Narragansett Bay.

You can start with an introductory "Prelude"

Prelude: The Earth's First 4 Billion Years
Formation of Proto North America

Formation and growth of early continents.
Assembly of Proto North America and the supercontinent Rodinia.
Breakup of Rodinia, formation of Gondwana.

Or click on this link for a page that begins with formation of the land that has become Jamestown Rhode Island


More than 550 million years ago, Rhode Island was part of a micro-continent named Avalonia, which formed off of Africa and South America, far south of the equator.
Avalonia drifted north and west, toward Proto North America.

Or, go directly to any of our Geological History pages:

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

Special Overview Page!!

Building the Northern Appalachian Mountains and New England

The Appalachians are a complex mix of mountains formed by a series of continental collisions that took place over a period of more than 1 billion years.. This page illustrates what was happening at a number of important event milestones.

A set of illustrative cross sections portray the cumulative effects of multiple events along a line from southern New York to Cape Cod and beyond.  Links on this page will take you to other pages containing more information on each event.

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Updated 2016.  Direct questions and comments on this site to Webmaster