The Ridge-and-Valley Appalachians in Pennsylvania

iss074e0315618 (Feb. 13, 2026) --- The Ridge-and-Valley Appalachians in Pennsylvania were formed from folded sedimentary rock and erosion patterns roughly 250 to 480 million years ago during the late Paleozoic Era. The International Space Station was orbiting 260 miles above the Keystone State when this photograph was taken. Credit: NASA/Chris Williams