Central Pennsylvania presents an ancient landscape, worn down by the grind of ice, water, wind, and time. The ridge lines of the Appalachian Mountain chain, once formidable, are now gentle folds rising over fertile valleys. Ice age glaciers shaped the land, smoothing out the mountains and depositing rich soil as the ice melted away. While the ice has done its work, this natural-color image, taken by the Landsat satellite, reveals another powerful natural force that has had a hand in sculpting the landscape: the Susquehanna River system. The river flows generally south from its headwaters in upstate New York to the Chesapeake Bay. In this image, the river cuts right through several ridge lines, apparently without regard to rock or gravity. Contrary to how things may appear at first glance, the mountains do shape the river’s course. In two places in this image, the river bends west along a ridge line until it finds a gap through which it cuts south. In every place where the river flows through the mountains, it is pouring through a gap that must have existed before. Located north of Harrisburg, Pennsylvania, which is just below the lower edge of the image, the region is called the Susquehanna Water Gap.
The image also illustrates more rapid changes. The trees along the ridge lines are gold, orange, and red, hinting that the coolness of autumn had settled over the region on October 21, 2001, when the image was taken. At lower elevations, the trees remained dark green. Forest once covered the entire landscape, but now, the fertile valleys are filled with squares of pink, tan, and green agricultural fields. Many crops had been harvested, leaving behind golden stubble or red-brown bare earth. Other signs of human habitation are visible in the image. Bright white roads line both the Susquehanna and Juniata Rivers. Small cities—Marysville near the bottom edge of the image and Duncannon near the confluence of the two rivers—are concentrated dots of white-gray. The vegetation along much of the Kittatinny Ridge is a patchwork of various shades of green, a good indication that the landscape has been developed.