Geological History of Jamestown, Rhode Island

The Post-Glacial Period
Holocene Epoch

LOCAL GLACIERS MELT, LEAVING DEBRIS

As the world started to warm, the glaciers melted, leaving moraines of various types in the area. (Moraines are deposits of soil, rocks and gravel that were pushed or carried by the ice) Terminal moraines are large deposits marking where the leading edge of the glacier stopped moving forward; lateral moraines are deposits left along the edge of the moving glacier; and ground moraines mark places where the movement of the glacier halted for some time, allowing material to drop out of the ice.

Block Island was formed from debris left about 55,000 years ago. Other remnants of the same moraine extend eastward to Martha's Vineyard and Nantucket Island, and westward to southern Long Island and into New Jersey.

Long Island shares the glacial history of Rhode Island. It has, in a sense, two "backbones", which are the remnants of two almost-parallel terminal moraines. The southern one (the Ronkonkoma moraine) is older, and is part of the moraine that created Block Island. The more recent deposits are part of the "Charlestown moraine"- a series of ridges on the northern side of Long Island that extends to Fisher's Island in Connecticut, then to Westerly, Charlestown and Narragansett in Rhode Island.

Glacial till (soil composed of many sized particles) that deposited behind the latest moraine is responsible for the many ponds and swamps found around Charlestown and Point Judith in Rhode Island . Several additional lines of moraine deposits are found in Connecticut and western Rhode Island, marking places where glacial movement hesitated or temporarily retreated.

PLANTS AND ANIMALS RETURN FROM SOUTHERN REFUGES

Once global warming began, glacial retreat was rapid. Rhode Island was ice free about 14,000 years ago. All of the United States located south of Canada was free of permanent ice approximately 10,000 years ago, except for a few isolated glacial remnants at high altitudes.

Moving over the land, glaciers scoured away soil, leaving, in many areas, barren rock. As they melted, they left deposits of till in some areas. (Till is a mix of materials that had been carried by the glaciers - everything from clay to boulders.) In other areas, particle-laden rushing melt water left glacial outwash (fluvial) deposits. Some lower-lying areas are now covered with lacustrine deposits - very fine textured silt that fell to the bottom of glacier-created lakes.

Many areas would have remained barren for a long time except for an interesting phenomenum. Bare rock surfaces close to retreating glaciers heated up in the sunlight, causing thermal uplift of warmed air, which was replaced by dense cold air sweeping outward from the base of the glacier. The high thermal gradient between the sun-warmed and "ice cold" air resulted in very high winds that lifted small stones and fractured them at they hit against each other. This resulted, in many parts of New England, in an eolian (wind-blown) mantle of silt and sand-sized particles measuring ten to 40 inches thick.

Moss, grasses and other types of cold-hearty plants began to re-vegetate the area. Animals also moved north. The first arrivals were types that we now associate with more northern regions, such as caribou.

As growing conditions became amenable, deciduous trees and other now-common plants (and animals) moved northward from southern refuges. The process took several thousand years.

THERE HAS BEEN A GENERAL WARMING TREND FOR 18,000 YEARS

About 18,000 years ago, a prolonged warming cycle began. The rise in tempeature has not been continuous however. The Younger Dryas Cold Episode between 11,000 and 10,000 years ago caused new icecaps and forest land to change to tundra. Then it ended suddenly.

The period starting about 10,000 years ago is called the Holocene epoch. This is also known as the time of modern man.

Even in the Holocene, there have been sudden and significant temperature (and local climate shifts).

THE SEA RISES FROM FAR BELOW CURRENT LEVELS

Rhode Island became glacier-free 14 to 15,000 years ago.

The local glaciers melted many years before the huge masses of ice located further north. Consequently, when Rhode Island became ice-free a significant amount of the world’s water was still frozen.

Sea level was initially about 350 to 400 feet (100 to 150 meters) lower than it is today. It rose rapidly until about 8,000 years ago, and more slowly since.

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The future Narragansett Bay was tens of miles from the sea. The glacial cuts through the Narragansett Basin were inland valleys with streams and rivers.

Long Island and Block Island Sounds initially filled with fresh water. The lakes were separated from the ocean by moraines.

HUMANS ARRIVE, THE SEA CONTINUES TO RISE

The first humans are believed to have arrived in Rhode Island about 10,000 to 12,000 years ago. These early visitors and inhabitants undoubtedly spent most of their time near rivers and streams.

As more glacial ice melted in Canada, Europe and Asia, the sea rose; flowing into lower-lying areas.

Long Island and Block Island Sounds became salt water sounds when the sea breached the moraines that formed their southern boundaries.

As the sea continued its rise, it eventually entered the area we know as Narragansett Bay.

In the rising Atlantic Ocean, large portions of the terminal moraines became submerged, leaving strings of islands stretching from New York to southern Massachusetts.

As the water level in Narragansett Bay rose, the bay became wider and extended further inland. People and animals moved to higher elevations. The rising water submerged most traces of the first human inhabitants.

The oldest human artifacts found on Conanicut Island are about 5000 years old.

The next Geological History topic

Guide to Bedrock

Map of bedrock in the Narragansett Bay area

Go directly 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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