Geological History of Jamestown, Rhode Island

Building the Northern Appalachian Mountains
and New England

Relief map of parts of present-day New England
To view the entire Appalachian chain from Maine to Georgia, click on the image above. (1.3 KB file)

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 focuses on important mountain-building and landscape altering milestones in the history of the Northern Appalachians and New England. It utilizes a set of illustrative cross sections to depict the cumulative effects of these milestone events. The example cross-section runs parallel to, but slightly north of the present southern coasts of Connecticut, Rhode Island and Massachusetts, and extends westward into New York state and eastward beyond Cape Cod. Each geological event mentioned on this page is described more fully on other pages on this site; and those pages, in turn, have links to external resources.

Each of these mountain building events (orogenies) included incorporation of new land masses seaward of the former coast line, distortion of existing land forms and metamorphosis of existing rocks. Erosion of the each newly-formed mountain chain resulted in deposition of massive amounts of sediment - both inland and along the coast.

Over the more recent past (tens of thousands of years) the region has been further shaped by two major glacial advance and retreat cycles.

To assist in visualizing the impacts of these cumulative events, a consistent color code has been used to indicate the ages of rocks depicted in each sketch. (See description near bottom of page.)

The Process Begins More than 1 Billion Years Ago with Creation of Grenville Mountains

Creation of the Appalachians began more than 1 billion years ago when an extensive chain of mountains  was formed as a result of a series of collisions between the largest land masses of the time. Approximately 85% of the world's current land mass had formed by this time in the Earth's history; however, the various pieces were arranged quite differently than they are now (see sketch below-left). 

A series of collisions known as the Grenville events occurred over a prolonged period - about 350 million years.  They resulted in formation of an early supercontinent called Rodinia with Laurentia occupying a central position.  The core of the present North American land mass was hit from all sides during formation of this "supercontinent"; and the grand scale of the Grenville collisions resulted in the creation of colossal mountain ranges on many continents.

The large, roughly triangular-shaped land mass (labeled "Laurentia") located in the center of the sketch of the components of Rodinia  is the continental core of present North America.  It includes much of the present United States, Canada and Greenland.  Many parts of the United States - most of the land west of the Rocky Mountains and east of the Appalachians - were not part of this early land mass (see sketch on right).    

"Laurentia" or "Proto-North America" (two names commonly used to refer to predecessors to our familiar continent) had been built up over the previous 1 1/2 billion years by agglomeration of many smaller land masses (see the purple, dark blue and dark teal-green areas of the map below-right.)  Between each of these early collisions, there were periods of crustal stretching, marine sedimentation, and erosional deposition.

Rodinia - the earliest supercontinent - about 1 billion years ago - click for larger version  

Proto-North America (at the center of the drawing of Rodinia on the left) was located at the equator and rotated almost 90 degrees from its current orientation.

Impacts with parts of Africa, South America and Europe, pushed a huge wedge of continental margin and ocean sediment onto Proto-North America.  This resulted in a wide chain of mountains from northern Mexico through Canada to Greenland (red band at right.)

  Laurentia or Proto-North America about a billion years ago.  It was formed by agglomeration of many smaller land masses.

During these collision, the "Northeast US" collided with Amazonia, which is part of present-day South America. The impact was against the western side of present-day South America - where Peru is currently located. The "Northeastern US" coastline was near the present Hudson Valley.

Circa 1.3 Billion Years Ago - Proto-North America is Surrounded by Sediment - Amazonia Approaches
Proto - North America (Coast Near Hudson Valley) Part of Present-day South America
Please visit our Prelude page for more information on what was happening at this time.
 
Circa 1.2 to 1.0 Billion Years Ago - The Grenville Orogenies - Focus on the Present "East Coast" of the United States - Part of Present-day South America Collides with Laurentia
Proto - North America Mountains Within Rodinia Supercontinent Part of Present-day South America
Please visit our Prelude page for more information on what was happening at this time.

Rodinia is believed to have existed as a coherent mass for about 350 million years. The lands to the north of Laurentia (Antarctica, Australia, India and parts of southern Africa) began to rift away around 750 million years ago, beginning formation of the Panthassa (later Pacific) Ocean. About 650 to 575 million years ago, the lands to the south rifted as well, as the Iapetus Ocean (predecessor to the Atlantic) formed. (Scroll back up to see the rift zones shown in white in the depiction of Rodinia, about 1 billion years ago.)

About 575 Million Years Ago:
Rifting Creates the Iapetus Ocean

The Iapetus Ocean (a forerunner to the Atlantic Ocean) was born as Laurentia separated from components of present-day Africa and South America and from Baltica (Proto-Western Europe).

Circa 575 Million Years Ago - Rifting Separates Laurentia and Southern Rodinia - Iapetus Ocean Forms
Proto- North America (Coast Re-forms Near Hudson Valley) Part of Present-day South America
Please visit our Prelude page for more information on what was happening at this time.

For the next several hundred thousand years, Laurentia drifted further north until the current eastern seaboard area was close to the Equator.

Most of the North American continent that is now south of Canada was covered by shallow tropical seas. Layers of limestone formed in many parts of Laurentia, including atop the now-greatly-eroded Grenville mountains.

Adding the Land that Became New England was an Extended, Multi-step Process

New England was added between 465 to 400 million years ago, when two very long island arcs, the Taconic and Avalonian (which had each formed near the junction of South America and Africa and then drifted away) were drawn toward the southern (now-eastern) coast of Proto-North America by a subduction zone that formed near the coast. (A subduction zone is a zone of sinking ocean crust that forms along a plate boundary.) As each island chain collided with Laurentia, it added land and raised mountains from Canada to the Carolinas.  (See the graphics and summary descriptions below).

The two-island arc collision model just described is, of course, a simplification of complex events that occurred over hundreds of millions of years.  Michael Brown, of the Laboratory for Crustal Petrology, University of Maryland, reports that on the basis of geological, geo-chemical and paleomagnetic evidence, "it is clear that the tectonic evolution of the Northern Appalachians was (more) complex".

Brown cites local variations in characteristics of land in the Newfoundland through New Brunswick-New England segment of the Northern Appalachians as evidence that "a number of independent, generally short-lived arc systems were present in the Iapetus Ocean, outboard of the margins of Laurentia and Gondwana during the Ordivician and Silurian". (Between about 505 and 408 million years ago). These small arc systems became incorporated into Proto-North America, independently or as part of the large-scale Taconic and Avalonian collision events, leaving evidence of their existence in local geological discontinuities.

About 450 Million Years Ago:
Formation of The Taconic Mountain Chain

Circa 470 Million Years Ago - the Taconic Island arc approaches the eroded, limestone-covered future "East Coast" of the United States
Proto- North America (Coast Near Hudson Valley) Taconic Island Arc (Volcanic)
Please visit our Taconic and Acadian Orogenies page for more on what was happening at this time.

The Taconic Orogeny took place about 450 million years ago. These collisions between the island chain and Proto-North America added the western portions of New England as well as portions of the Maritime provinces in Canada and land in northern New Jersey, south-eastern Pennsylvania, Virginia and North Carolina.

  The Taconic Mountains were created as the islands rode up and onto the Laurentian land mass. In the process, Grenville Mountain remnants were pushed up, distorted and overlaid by Taconic arc materials. Thrusting and folding led to huge amounts of sediment that washed into a basin that formed behind the mountain chains resulting in the "Queenstown delta" near Albany, New York.. The Taconic orogeny ended about 445 million years ago.
 
Circa 445 Million Years Ago - the Taconic Orogeny is Ending - The Avalonian Island Arc Approaches
Proto- North America (Coast East of Hudson Valley) Avalonian Island Arc (Volcanic)
Please visit our Taconic and Acadian Orogenies page for more on what was happening at this time.

About 425 Million Years Ago:
Creation of More Mountains: Collision with Avalonia

The Acadian Orogeny started about 430 to 425 million years ago when the eastern end of the Avalonian arc (together with Baltica, or Proto-Western Europe) impacted against the northeast coast of Proto-North America, in what is now Canada. Ongoing collisions continued for about 50 million years as more of the island chain impacted against Laurentia. The impact zone progressed from east to west.  Parts of the Avalonian island chain came ashore as far south as the Carolinas.

As the Avalonian chain approached Laurentia, the basaltic bed of the Iapetus Ocean was pulled under Laurentia (subducted) and melted. Volcanoes formed in and near the narrowing ocean.

The Avalonian Island Arc Pushes Ocean Crust Toward Shore - Circa 410 Million Years Ago
Proto- North America (Coast East of Hudson Valley) Avalonian Island Arc (Volcanic)
Please visit our Taconic and Acadian Orogenies page for more on what was happening at this time.
 
  As Avalonian terrane rode up and onto the coast, it pushed a wide swath of seabed material onto the land, compressing, bending, lifting and pushing existing rock formations. The new mountains were initially very high.
A large amount of sand and gravel that had been pushed ashore washed westward from New England and southward from Canada into a basin inland of the mountains, where thick layers of sedimentary rock built up in the Catskill Delta region of New York and Pennsylvania. Additional sediment was generated by erosion of the new mountains, and it added to the build up of the land in this region, which is often referred to as the Appalachian Plateau..
 

Most of Maine, New Hampshire and Vermont plus major parts of Massachusetts and Connecticut were created by the large volumes of volcanic material and Iapetus Ocean sediments that were pushed onto the margin of Laurentia. This Iapetus Ocean Terrane (land created from ocean bottom materials) is colored light grey in this sketch.

Avalon Terrane (the original volcanic arc material) is shown in dark grey.

All of Rhode Island is underlain by Avalon Terrane.

While the Acadian orogeny (sometimes also called the Appalachian or Avalonian orogeny) is given credit for creating the Northern Appalachian mountains - the story of these mountains is far from complete.

About 350 Million Years Ago:
Collision with Gondwana Creates More Mountains

More than 425 million years ago, subduction zones developed in the oceans on all sides of Laurentia and around Gondwana (the large continent that contained most of modern Africa and South America). As ocean crust was consumed, the giant land masses were drawn closer together. By about 390 million years ago the gap between the zones had been consumed. Proto-North America was more buoyant. Gondwanda was pulled toward the subduction zone (and, inevitably, toward Laurentia).

Circa 375 Million Years Ago - With Complex Mountains Lining the Coast, Collision Looms with Gondwana
Proto- North America (Coast Near Present Location) Gondwana (Africa plus S. America)
Please visit our Alleghenian Orogeny page for more information on what was happening at this time.

Once collision began, about 350 million years ago, the thick continental crust of Gonwanda rode up and onto the edge of Proto-North America. In New England, Avalonian Terrane was thrust under Iapetus Terrane and the remnants of older mountains were pushed, crushed and buried.

Circa 350 Million Years Ago - CRUNCH TIME! - Collision of North America With Africa (Gondwana)
Proto- North America Gondwana (Africa, S. America and more)
Please visit our Alleghenian Orogeny page for more information on what was happening at this time.
 
Depiction of Pangea, courtesy of Ron Blakey, Northern Arizona University.    Along the impact zone between Proto-North America and Gondwana, a vast mountain range arose extending from eastern Mexico (which collided with northwestern South America) along the east coast of North America into Canada. Near-simultaneous collisions occurring between parts of Europe and  Northeastern Canada and Greenland, extended the mountain range into Northern Europe.

Collision energy deformed and metamorphosed rocks that had already been altered by earlier mountain building episodes. Effects penetrated far beyond the Acadian and Taconic mountain ranges, to most of the area underlain by the remnants of the ancient Grenvilles.

Compression and folding produced a valley and ridge province far inland (most pronounced in Pennsylvania and other states located south of New York.)
 
In the Narragansett Bay area and part of Massachusetts, folded rocks formed a large depression (or basin) during the Alleghenian Orogeny. Sediments accumulated in the Narragansett Basin forming shale, which makes up the upper layers of bedrock beneath much of the area.

Continuing collisions led to creation of the "supercontinent" Pangaea (meaning "all lands" in Greek) about 306 million years ago. (See color-coded sketch at right.) The present east coast of North America was far from the sea.

Components of Pangea - color coded to show present continental identities


About 220 Million Years Ago
:
Rifting Creates the Atlantic Ocean

By the time North America began to pull away from Africa, about 220 million years ago, weathering had greatly reduced the size of the huge mountains that had been created in the Alleghenian Orogeny. Land that had been compressed and metamorphosed by the energy of collision was now stretched, cracked and rearranged as the land masses separated.

Circa 220 Million Years Ago - Breakup of Pangaea - Atlantic Ocean Forms (then continues to widen)

North American Coast Re-forming Offshore of Present Location Gondwana (Africa, S. America and
 other lands)
Please visit our Atlantic Formation page for more on what was happening at this time.

The (Relatively) Recent Past:
Glaciations and Rising Seas Reshape the Coast

The most recent changes to the Appalachian mountains, and all of New England, have been driven by erosion and glaciations, rather than orogeny.

From about 1.8 million to about 11,000 years ago, a period known as the Pleistocene, much of the northern portion of the world was alternately covered with glacial ice and uncovered again during interglacial periods.  The last two glaciations reached the New England coasts about 55,000 and 18,000 years ago. Everything was covered, with slowly moving ice; even the tops of tall mountains. The map at right depicts the maximum reach of the last great glaciation, about 18,000 years ago.

As the ice sheets advanced they scoured rocks, uncovered Grenville formations and carved out Narragansett Bay. Plants and animals were driven far south. As the ice retreated it left debris - sometimes isolated rocks and sometimes huge accumulations of rock, sand and silt. Many well known geographical landmarks like Long Island, Long Island Sound, Block Island, Nantucket and Cape Cod were created by glacial action.

Glacial advance - about 18,000 years ago
 
The Northern Appalachians Today - After 200 Million Additional Years of Erosion, Deposition and Glaciation
The Present New England Coast Continental Shelf and
Expanding Atlantic Ocean
Please visit our Holocene and Reference pages for more information on what is going on NOW.
 

As temperatures have risen since the last ice age, so have ocean levels - by as much as 400 feet - moving the shoreline inland many miles from the edge of the continental shelf. As the glacial ice began to melt, Long Island and Block Island Sounds filled with fresh water. As the ocean level rose, fed by melted ice and snow, salt water eventually flowed over portions of the moraines that had contained these large lakes.  As the sea continued to rise, salt water eventually entered and filled Narragansett Bay as well.

The only significant mountain building event occurring in the Northern Appalachians the continuing rise of the ancient Grenvilles in the Adirondack region.  Vertical uplift has been going on for 10 to 20 million years.

For most of the region, erosion is the dominant transforming force.

 
Color Code for the event sketches above:
  • Ocean crust is shown in red
  • Pre-Grenville crust is mustard yellow
  • Crust created during the Grenville orogenies (and predecessor sediment deposits) is purplish-brown
  • Sediment laid down after the Grenville orogonies, but prior to the Taconic Orogeny, is pale yellow-green
  • Taconic arc materials are mustard brown
  • Sediment laid down between the Taconic and Acadian Orogenies is light yellow - green
  • Avalonian arc materials are olive green
  • African crust is light dusty rose

We are indebted to Roy W. Schlische, Department of Geological Sciences, Rutgers University, for posting a set of cross-section sketches for New Jersey (parts of which have been incorporated into this set) and to Andrew Birrell for making a set of relief maps available on the internet, from which the map at the top of the page has been excerpted.

 

Go 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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