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.
|
 |
.
|
 |
| 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
Go
directly to any Geological
History page:
Search
for a particular topic:
|
|
Jamestown RI - Rhode Island
Visitor Information Home Page
Direct
questions and comments on this site to Webmaster
|
