The main cause of coastal erosion is the kinetic energy of wind. The main medium of transmission of wind energy is through wind generated water waves. Occasionally waves are also generated by other forms of energies such as earthquakes (sometimes at sea beds). The waves in turn cause erosion upon breaking on the shoreline or by way of wave induced currents. In coasts where substantial tidal variation occurs (as in Mumbai) there may be marginal erosion. While erosion of loose materials like sand is visible clearly, it is not so with hard materials like rock. It may be interesting to note, that where the water is still and calm, as in a lagoon, the coastline is be stable. Also to be noted, is the phenomenon of beach formation due to littoral drift (as in Chennai Marina), stopped by the harbour structures jutting into the sea.