Making mountains
Formation
of the region's mountains began 570 million years ago. Seas covered
the land that is now Western Pennsylvania and deposited layers of limestone
thousands of feet thick. Over the next 400 million years, tectonic
plates – that is, huge plates of the earth's crust –
moved, pushing together the masses of land that became known as the
continents. The continents collided three times, each time with enough
force to literally "wrinkle" the land, just as an automobile's
hood might be wrinkled in a fender-bender. Those "wrinkles"
of land became mountains! Pennsylvania's Allegheny Mountains--part of
the Appalachian chain--were formed in the final collision.
300 million
years ago, the Carboniferous Period was a time of lush vegetation in
the area now known as Pennsylvania. As these early plants died, they
become submerged in swamps where lack of oxygen kept them from decaying.
The result was peat. Water levels fluctuated covering the peat with
sand and slit under pressure. After a long time under pressure the peat
turned into a low grade coal called lignite. The longer it stayed under
pressure the more carbon and less moisture the coal contained and the cleaner it would burn--from lignite to bituminous to anthracite.
Bituminous is found throughout Pennsylvania, but anthracite only in
the eastern part of the state. Many other gem stones and precious metals
formed over time, although none would figure as prominently as coal
in Western Pennsylvania's industrial heritage.
In the past
century, human-engineered highways cutting through the mountains, expose
the layers of rock that are clues to these ancient geologic events.
The next time you drive through the mountains, look for these clues
that tell scientists about how the landscape was formed so many years
ago. Note that the layers of rock are not always horizontal. Sometimes,
when the earth shifted with enough force, the land literally flipped,
so that some layers of rock are horizontal, some diagonal, and some
even are vertical! You may even see some seams of coal that have managed
to miss being mined. Geologists can "read" these layers of
rock to discover the story of the land over millions of years.
Also note,
as you take this drive in the mountains, that you are, in fact, driving
in the mountains. A couple of centuries ago, such a feat would have
been unimaginable! Another impossible-to-imagine task would be crossing
the region's many rivers in a matter of mere seconds – thanks to
the plentiful bridges that take us from one shore to another.
Rivers run through it
Watching
our rivers flow, and seeing the gentle ripples and occasional crest
on a windy day, one can overlook the power of the currents beneath the
surface. Water is very efficient. Runoff from rain will find the fastest
way to flow downhill, creating streams that cut a path through rocks
and soil (and wash away the loosest ones) to join other streams--tributaries--that
feed the river to carve out a riverbed. (The Grand Canyon in Arizona
is a massive example. The Grand Canyon of Pennsylvania is the older
Appalachian version.) This erosion contoured Western Pennsylvania's valleys, a process that you can see
at work when burbling rivulets turn to raging torrents after a heavy
rain. This weathering has worked a long time to give the Appalachian
Mountains their characteristic rounded shapes in contrast to the craggy,
younger Rocky Mountains.
Downtown
Pittsburgh earned its nickname, the "Golden Triangle," from
the distinct triangular shape defined where the Allegheny River and
the Monongahela River meet to form the Ohio River. Here in modern-day
Pittsburgh, some people speak of a fourth river that runs underground,
and feeds the fountain at Point State Park. This actually is more of
an urban legend than a geological fact. This mysterious "fourth
river" really refers to and aquifer,
a layer of water absorbed by the layers of sand and gravel beneath the
riverbed and that is the water pumped up to supply Point State Park's
fountain.
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All three
of the rivers take their names from their old Native American names.
"Allegheny," for example, means "fair waters." This
water's source is located in north central
Pennsylvania, near the small town of Coudersport. From there, the Allegheny
River flows north, briefly passing through New York state, and then
south again into the Kinzua Dam, built in 1960 by the Army Corps of
Engineers to combat devastating flood waters. From the Kinzua Dam, the
river winds its way to the Pittsburgh area, where it meets the Monongahela
River and forms the Ohio. It's steep banks have limited large industrial
development, though lumber has been floated in great rafts downriver
to mills in Pittsburgh. In the early days of petroleum, oil also made
its way downriver in special boats called guiphers, because steamboats
could only navigate 60 miles upriver from Pittsburgh.
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The Monongahela's
Native American name, meaning "falling banks," comes from
its muddy appearance, which is easily visible when its waters meet the
waters of Allegheny at the Point. The Mon – as it is affectionately
called by Pittsburghers – begins in Fairmont, West Virginia, and
even though the water is flowing north, the river's path is downhill.
Like the Ohio River in Pittsburgh, the Mon is formed by two rivers joining
at a fork. The site where the Tygart and West Fork Rivers meet is undeveloped,
and may resemble what Pittsburgh's Golden Triangle looked like a long
time ago. The Mon's broad floodplains were particularly suitable for
factories, which late in the 1800s would so monopolize its banks that
it would be called the "Steel Valley."
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The meaning
of the word "Ohio" again can be traced to the Native Americans.
It translates to "beautiful river," "frothy waters,"
or "something big" – and by looking at the river one
can see that it is all three of those things! In the 1700's, French
armies in the area thought of the Allegheny and the Ohio as one, and
they referred to it as La Belle Riviere – or, "the beautiful
river." Even a young General
George Washington, who visited the area, believed this and referred
to La Belle Riviere in his journal. From its starting point in Pittsburgh,
the Ohio River winds 981 miles through the country's Midwest, to Cairo,
Illinois, where it empties into the Mississippi River. Pittsburgh's
direct connection to the Mississippi River and eventually New Orleans
and the Gulf of Mexico.
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Making
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