Houao of Xiyin has many rock formations affected by waves, such as sea caves, sea arches, and sea stacks. This is an ideal place for observing such formations.
A sea cave takes shape when a sea cape gets eroded by ocean waves along its broken lines or softer sections. Day after day, as the waves hit the cape, a cave is formed. If the process continues, it becomes a sea arch. If the top of the arch topples due to lack of support, or that the middle section of a cape falls off, separating its front part from the coastal land, a sea stack stands out in the sea alone.