Coastal erosion. This image of a coastal promontory off the coast of Kythira, Greece, has been manipulated to show how the arch keystone might have appeared before its collapse. C025/8986 shows the same feature as it appears in the present day. A sea arch is a natural opening eroded out of a cliff face by marine processes and may take many hundreds of years to form. Arches are therefore ephemeral geological landforms. They develop where waves attack a plane of weakness which cross-cuts a promontory. Caves produced on either side of a promontory might be joined over time to become a tunnel and, finally, an arch. Ultimately the keystone itself will erode leaving only a seastack. The architecture of an arch and the speed of its erosion is a reflection of its lithology and structure.