Coastal erosion. This image shows a present day seastack off the coast of Kythira, Greece. C025/9001 is a simulation of the same feature at an earlier stage showing how the connecting arch (keystone) prior to collapse, might have looked a few centuries previously. A sea arch is a natural opening eroded out of a cliff face by marine processes, 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.