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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• Geology of the "Fourth River"

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 Allegheny River

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 Mon's West Virginia origins

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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• Pittsburgh's origins at the Forks of the Ohio