Many Pennsylvanians call the Allegheny Mountain Range home.
As a part of 6 News’ Road Trippin' series, we're diving into some deep geological history that surrounds us every day.
Helen Delano, Senior Geologic Scientist with the Pennsylvania Department of Conservation and Natural Resources, explains how the Allegheny Mountains were born.
“The Appalachian, and specifically the Allegheny Mountains, over millennia. The rocks began to be deposited about 550 million years ago and continued until around 250 million years.”
So, for 300 million years we had kind of alternating oceans, and low land, and inland seas, and rivers and swamps building up a huge stack of sediments that got buried and became sedimentary rocks.”
“And then, about 300 million years ago, a mountain building time, we call it the Alleghenian orogeny began, and what was going on globally is that Africa was closing up an ocean and running into the North American continental plate. All of this is being driven by a set of processes we call plate tectonics, which operates all around the globe.”
“So, all of eastern North America, and parts of Africa and Europe were experiencing the same general kinds of activity.”
"The mountains we see now would have been under thousands of feet of sediment that is now eroded and out in the ocean. They were formed when they were much deeper in the earth, and conditions were hotter. As you go down in the earth, and get closer to the center, it gets hotter and there’s more pressure. There would have been mega earthquakes going on at the time these changes were happening, and those rocks were getting folded. But they were being folded thousands of feet down in the earth, and what we’re seeing now has been exposed because the overlying rocks have been eroded.”
Part of the Appalachian Mountains are made of sedimentary rocks, and the high parts are held up by the most resistant of those rocks, which are sandstones. They don’t dissolve very easily, and they don’t break down into physical components easily. Under those, there is a lot of shale and it’s protected by this sort of cap rock of the sandstones that are on top of it. There’s both shale and siltstone, mudstone and limestone that form the valleys and the side-slopes in Pennsylvania.”
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