A geologic fold occurs when a flat surface is bent or contorted. For a rock or sediment to show its folds, it must have a set of parallel surfaces or different layers inside it. A very homogeneous rock mass with no internal variation would not show that any force had been applied to it to make it fold. Folds form in sediments and rocks generally from near the surface to as much as 45 km (kilometers) deep in the Earth, where elevated temperatures and the high rock overburden and fluid pressures cause the rocks to be ductile and change shape as they are subjected to tectonic or other forces from glaciers and surface processes. This allows their development in sediments and sedimentary rocks, in a ...

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