When Water
Moves...
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When water moves it really is quite a
powerful force on this planet. A vast quantity of
water on Earth
is continually moving between the ocean, the atmosphere
and the
land. Water is stored in large quantities in the
ocean, on the
land in lakes and ponds to which it arrives after
traveling over the
land in rivers and streams, and in the form of ice in
large polar ice
caps. Additionally, throughout history large
quantities of water
have been trapped on the land in the form of great sheets
of ice during
many "Ice Ages" in times past. During these times,
the most
recent of which began about two million years ago, large
continental
glaciers slowly ground over the land, shaping the
landscape and
depositing large quantities of material on the land and on
the ocean
floor. Over the past two million years or so, the
ice sheets have
advanced and retreated over twenty times. The most
recent period
of glaciation ended about twelve thousand years ago.
Water has the ability to affect the landscape in either
its liquid or
its frozen form. Liquid water moving over the land
causes rocks
to be weathered,
or broken down into smaller pieces, transported over
the land in moving water through the process of erosion,
and deposited
in other locations, forming new rocks, in the
processes of
deposition
and lithification.
Wave action may also break down and transport the rocks
upon which they
crash. Wave erosion and deposition are responsible
for forming
many coastal features and depositing shoreline structures,
including
beaches.
The slow and steady force exerted on Earth by Ice moving
slowly across
its surface is responsible for much of the landscape that
you see
around you. During the most recent period of
glaciation during
the last Ice Age, large glaciers, as much as two miles
thick, covered
much of the northern hemisphere, in this area, the ice
extended as far
south as New York. The glacier that covered Maine
and New England
was nearly two miles thick in areas, and it moved
ponderously under its
own weight. Though glaciers such as this don't move
very fast,
they have a lasting effect upon the land. The
glaciers moving
across Maine erased most geological features that were
formed earlier,
leaving behind a new landscape loaded with clues about its
icy
origin. Long scratches, called Glacial
Striations, can be seen in many large areas of
rock. These
scratches were caused when the glacier slowly ground its
way over this
rock, leaving behind scratches caused by rock embedded in
the
ice. The direction of these scratches shows the
direction that
the glacier was moving. Maine's mountains tend to
have a profile
where the sides are gently sloping to the northeast, and
sharply jagged
on the southwest. This is caused by the slow
movement of the
glacier in this direction. The sides of the mountain
facing the
glacier were slowly worn away until they were the gentle
slopes we see
today. The side of the mountain facing the back of
the glacier is
often jagged, and many cliffs are present. These are
caused by
the glacial action known as "plucking",
where the retreating ice rips large chunks out of the
stone as it
passes. Torrents of water flowing beneath the
glaciers often
deposited sediments in long ridges called eskers.
Many
of Maine's earliest roads were built on the natural ridges
formed
by the eskers. As the glacier reached the sea, and
began to melt,
large chunks of boulders would be deposited in random
fashion.
These boulders had been carried by the ice sheet for many
miles, until
finally being deposited as the ice melted. These
large, seemingly
out of place, boulders are called "glacial
eratics".
Perhaps you've seen "Bubble Rock" in Acadia National
Park? Here,
a huge boulder the size of a house is perched right at the
edge of a
cliff several hundred feet high. Though the boulder
seems ready
to tumble down the slope at any moment, it's rested in
exactly the same
position since the melting glacier deposited it there,
about twelve
thousand years ago. The slow grinding movement of
the ice sheet
caused rock to be ground into smaller pieces. This glacial
till was
deposited in Maine and became some of our rich farmlands,
so important
to the economy of Maine today. Other glacial till
formed
economically important gravel pits, many still being used
today to
expand the network of roads in our state. The
melting ice sheet smoothed out our mountains and
hills, and gouged out our
coastline,
deposited
large amounts of till on the floor of the Gulf of Maine,
helping to
make it the incredibly productive fishing area it is
today.
The island of Long Island,
in
New York, was another result of the melting Ice
Sheet. As the
glacier melted, long ridges of material were deposited
near its
edge. These structures are called "End
Moraines",
and Long Island is a prime example. Rocky debris was collected
in the
glacier's steady southward march. As the ice melted,
this debris
was deposited at the glacier's edge. Some of this
material became
the underwater banks, such as Georges' Bank, which
separates the Gulf
of Maine from the open Atlantic Ocean, and helps to make
the Gulf of
Maine one of the world's most productive fishing
areas. Still
more of this rocky debris would one day become known as
"Long Island".
Look at Long Island on a map, you can see in it's outline,
the ancient
footprint
of the glacier that once would have covered this area in
over a mile of
ice.