The gentle fold in the siltstone beds pictured above is not some kind of optical illusion created by the camera lens. It is real, and quite unusual. Folded rock is fairly rare in Ohio. Geologic folds, called anticlines or synclines, normally occur in high mountain ranges. They are the result of enormous stresses, deep burial and plastic deformation when continent-sized crustal plates collide against one another. The process is called plate tectonics and can take millions of years to create huge, tight folds in what were once horizontal beds. Nothing like that happened here. In stark contrast, stream anticlines are modern features formed in hardened rock. They are caused by erosion. The rapid removal of thousands of tons of rock and sediments above the streambed causes the thin siltstone and shale layers that underlie the stream to flex upward. There are eye-witness reports of steam anticlines in the process of formation, slowly migrating upstream at a walking pace, accompanied by loud roaring and popping sounds.
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