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  • 1403. Great Falls Overlook

    Look out over the Great Falls. You are looking at a formation half a billion years in the making.
    The rocks you stand on were formed in the muddy currents at the bottom of the ocean over 500 million years ago. And about 300 million years ago, the North American and African continental plates collided, creating heat, pressure and an upward movement that formed the Appalachian Mountains. This mountain building caused the soft mud sandstone to harden, and created the metamorphic and sedimentary rocks you see today.
    The Potomac River formed about 2 million years ago. During the onset of the Ice Age, the sea level dropped, so the Potomac's path to the sea grew longer. The drop in elevation also made the current more powerful and caused the river to carve its channel faster and deeper. It was easy for the Potomac to cut through the softer rock around the coastal plain, but harder to cut through the bedrock in the Piedmont area. As the ice receeded and sea levels rose, and over the last 20,000 years or so, the Potomac has slowly eroded the bedrock, causing the falls to move upstream from an earlier point about 10 miles east, where the Chain Bridge stands.
    The river's power can be seen not only in the waterfall, but also in some features around you. In the rock outcroppings here, you can see circular potholes formed by swirling vortices of sand and silt that the water ground into cracks and depressions, these creating these holes. Remember, at certain times in geologic history, the Potomac floweed over these rocks, and there is major flooding every 12-15 years.
    These major floods can carry whole trees down the river and deposit them on the rocks. Furthermore, these floods affect plant life. The forest growing around you is a unique ecosystem called a bedrock terrace forest. The trees and plants here must adapt not only to floods, but also to droughts and shallow or no soil. These difficult living conditions make this and other islands in the Potomac a significant national conservation area, as many of the plants and animals cannot be found anywhere else.
    The Potomac, Great Falls, and Mather Gorge offer a rich and dynamic enviornment. The beautiful result of this ever changing landscape attracts million of visitors every year.
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