The sea-eroded terrain of Houao in Xiyin is rich in variety, with trenches, arches, and pillars that make it perfect for observing such features. The sea-eroded caves here were formed by the waves working at a cape and wearing away at cracks or soft junctions between layers of rock, finally digging out large holes. If the erosion continues long enough, the holes will be dug all the way through to form natural arches. If the tops of the arches collapse because of weak support, or if the middle of the cape is eroded away to leave the outer part separated from the land, that part becomes a sea stack.