Geological History of Jamestown, Rhode Island

Glaciation

Many of the most notable features of the Narragansett Bay region were formed during a period of repeated glacial formation and withdrawal known as the Pleistocene Epoch.

Massive Ice Flows Sculpted the Northern Hemisphere, Carving Out Narragansett Bay

Duting the last 80,000 years or so, glacial buildup repeatedly covered Canada, much of the northern United States, Europe and Asia with ice. This period is known as the Wisconsin Glaciation because of the massive impact on that state. There were two significant glaciation peaks within the Wisconsin - the first occured between about 75,000 and 55,000 years ago. The second advance reached its maximum about 18,000 years ago. In both cases, New England glaciers pushed southward onto the continental shelf, beyond the southern coasts of Connecticut, Rhode Island and Massachusetts. With so much water locked in ice sheets, ocean levels fell as much as 450 feet (about 150 meters). .

While, overall, the second major Wisconsin Glaciation is considered the more intense, local glaciers stopped their advance slightly short of the points reached during the first glaciation peak.

Glaciation began in higher altitude regions of the Canadian Rocky Mountains, in Greenland and central Canada. Ice then spread in all directions, following the contours of the land (and altering them over time).

Advancing from the Rocky Mountains and north-central Canada, during the last great glaciation event, the ice met in the center of the continent, creating a sheet that stretched from the Pacific to the Atlantic ocean. At the peak, glacial ice extended to a latitude of about 40 degrees North (see map at right).

European ice sheets initially formed in the Alps, in Scandinavia and the northern British Isles. In Asia, they began in northern Siberia. They ultimately reached about 45 degrees North latitude. The European and Asian ice sheets never merged.

As the ice thickened to a mile or more deep, it flowed under its own weight. The slowly moving mass crushed layers of rocks and ground the surface of the land; leaving, in some areas, scratches in the bedrock, called striations. Sand, gravel and boulders were pressed into the ice, moving with it. These pieces of rock and earth were sometimes carried for long distances - as much as several hundred miles. Large rocks that were carried great distances and then left when the ice retreated are known as erratics, as their composition and appearance is often obviously different from that of their surroundings. (Dr. Matt McConeghy, Johnson & Wales University, has prepared a page entitled Glaciers in Rhode Island presenting information on glacial effects that can be observed in the state.)

On the East Coast, glaciers advanced to just south of New York City, beyond the present coasts of Connecticut, Rhode Island and Massachusetts, and, inland, to the west, across New Jersey and eastern Pennsylvania. At the peak of glaciation the sea level was as much as four hundred feet lower than today and the shoreline was twenty-five to seventy-five miles from the current coast. Rivers flowing to this ancient shoreline cut canyons into now sea-covered land and deposited sediment into the deep ocean.

When glaciers reached the Narragansett Bay area, they encountered a fresh water lake contained within a geologically old sedimentary basin. As the glaciers flowed into and through the basin, they carved channels through the younger sediments and exposed much older bedrock.

Among the north-to-south cuts shaped by the ice are those that later became the West Passage that separates Conanicut Island from the western mainland and the East Passage that now separates Conanicut Island from Aquidneck Island (Newport).


Temperatures on Earth - The Historical Record

This chart, showing over 60 major temperature fluctuations during the Pleistocene, is based on Dr. R. Timothy Patterson's course on Climate Change from a Geological Perspective. Not all of the low temperature periods resulted in glaciation - however, several of the most recent cycles did.

Note the rapid rise in temperature since the last glacial period. The last 10,000 years are known as the Holocene Epoch, the time during which humans developed complex languages and lifestyles.

The intensity of the cycles, as measured by the difference between cyclic high and low temperatures, has grown greater over the past 600,000 years. The length of cycles has increased as well.


Proceed to the final Geological History narrative

The Holocene Epoch

Glaciers melt, leaving debris. Plants and animals return,
humans arrive, the sea rises

Or, 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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