PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY
Sea stack and arch in limestone cliffs. The sea arch formed as wave action eroded part of the rock in the headland faster than the rest. Caves formed on each side of the headland, cutting deeper until they merged to form the arch. A sea stack is what is formed when an arch collapses. The limestone rock (white) of the cliffs is a sedimentary rock that formed in layers (strata, horizontal bands). These layers are clearly seen here. The limestone is mostly calcium minerals, formed from calcium in the skeletons of billions of microscopic plankton in an ancient sea. These settled on the ancient seabed and over millions of years this rock formed and was uplifted to form the cliffs seen here. Photographed in Etretat, Normandy, France.
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