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Molly Shabica
August, 2004
A Beginner’s Geologic Exploration
of Martha’s Vineyard
Martha’s Vineyard, a small island off the coast of eastern Massachusetts,
became a growing vacation spot in the early 1900’s. Although connected in
formation with Staten Island, Long Island, Nantucket, and Cape Cod, Martha’s
Vineyard has unique features which add to her distinctive characteristics.
Martha’s Vineyard Island is considered new land, only 12,000 – 15,000 years
old. It was inhabited in the late Pleistocene era 10,000 years ago by hunters
and gatherers. More recently, at the turn of the 17th century, an
Algonquian tribe was the main occupant and guardian of the island. Europeans
arrived thereafter.
Martha’s Vineyard is a
continental island meaning it was created either by a rise in sea level
(burying most of the land except for the coastal highlands underwater), or by
the sea breaking through a peninsula and cutting the land off from the
mainland.
The
origins of the actual land can be traced from the last ice age and glacial
movement 10,000 years ago. Ice sculpts the land and removes material from one
place while depositing it elsewhere. These deposited materials that were
pushed or carried by the ice are known as Moraines. Lateral Moraines are
deposits left by the edges of moving glaciers. Ground Moraines are deposits
that dropped out of the ice as the movement of the glacier stopped. Finally,
Terminal Moraines are large deposits of this soil, rock, and gravel that mark
where the leading edge of the glacier stopped moving forward. These deposits
can also be known as till.
The Holocene Epoch (less than
10,000 years ago) is considered the post-glacial period. The world started to
warm. As temperatures increased the glaciers started to move and melt. An
ice sheet completely covered North America as far as Missouri. As glaciers
move back and forth they picked up the continental land and left sand, gravel,
other ground up rocks, and boulders in its path. The melting glaciers
released this land as they moved. Staten Island, Long Island, Nantucket, Cape
Cod, and Martha’s Vineyard are terminal moraines that mark the farthest point
of expansion of glaciers to the southeast. As the glaciers melted, sea level
rose and parts of the continental shelf became separated as seen in the figure
below.
http://www.geowords.com/h_/histbookpdf/b16.pdf
Martha’s
Vineyard Island soil is sandy. The sedimentary rock that forms the island is
logically dated to the Holocene and Pleistocene Epochs around the time of the
last glacier movement 10,000 years ago. In the figure below one can see Cape
Cod, Nantucket, and Martha’s Vineyard are sediments from the Pleistocene and
the Holocene Epochs. On the mainland of Massachusetts there is a wide range
of other sedimentary, igneous, and metamorphic rock. It makes sense that the
islands formed by the Terminal Moraines are the youngest in geology.
http://geology.about.com/library/bl/maps/blmassachusettsmap.htm
Although the above
figure states that Martha’s Vineyard Island is sedimentary rock from the
Pleistocene and Holocene Epochs, there is a unique piece of the island which
exhibits a much earlier formation. Gay Head or “Aquinnah” is the westernmost
end of the island. It is attached to the rest of the island by a barrier bar
and is famous for the Gay Head cliffs.
www.nps.gov/nero/nnl/gayheadcliffs.htm
The one
mile of exposed cliffs is a pre-glacial sedimentary formation that rises 150
feet above sea level. The cliffs show a cross section of strata from the
Cretaceous through the Pleistocene Ages. This documentation of the geologic
phases on the continental shelf dating from 100 million years ago is one of
the Island’s biggest tourist attractions. The cliffs in the past have been
sources for fossils as well as clay used for pottery. Recently there are
restrictions placed on access to the cliffs due to erosion. The natural
erosion from the ocean and weather takes about two feet off of the cliffs each
year. One can easily compare the black and white photograph taken in the late
1800’s to the more recent photographs taken in 1996 to illustrate this point.
www.nps.gov/nero/nnl/gayheadcliffs.htm
www.saralovering.com/gallery/000118.php
Martha’s Vineyard shares a history with Cape Cop, Nantucket, Long Island, and
Staten Island. Being a part of the Terminal Moraine formed around 12,000
years ago at the end of the last ice age; Martha’s Vineyard is considered to
be new land. Yet differing from the other areas it contains cliffs exhibiting
sediment formation from 100 million years ago. Terminal Moraines and the Gay
Head Cliffs in addition to other geologic occurrences demonstrate the ability
of our changing Earth to record history and keep track of events that happened
long ago.
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