Waves are formed by wind blowing over the surface of the sea. The height and energy of waves depends on wind strength, duration, and fetch. There are two main types of waves - constructive waves which build land, and destructive waves which erode land. Coastal geology also influences landscapes. Discordant coasts have bands of rock at 90 degrees to the sea, creating headlands and bays as softer rocks erode faster. Concordant coasts have parallel rock bands, where erosion of harder rocks leads to fast erosion of softer rocks behind.