MICHAEL MARTEN / SCIENCE PHOTO LIBRARY MICHAEL MARTEN / SCIENCE PHOTO LIBRARY
Sea arch silhouetted against sunlight reflecting from the surface of the sea. This is Durdle Door, near Lulworth Cove in Dorset, UK. A sea arch forms when wave action erodes part of a land promontory quicker than the rest. Initially, caves form on each side of the promontory, cutting deeper until they merge to form an arch. The arch will continue to be eroded by waves, eventually collapsing. In this case the arch has remained while surrounding soft chalk rocks have been eroded away. The name Durdle Door, and the structure itself, is thought to be at least 1000 years old.
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