Perturbed by the wind, particles of water roll on the surface and a wave-form oscillation of water is propagated. The waves continue as long as the wind does not weaken and no obstacle impedes them. When the swell reaches the coast, it is slowed by the rise in level of the seafloor and the waves change shape. The crests get closer together and the wavelength diminishes, although the period (the interval of time between two crests) remains the same. The waves get taller and the movement of the water particles becomes elliptical. When they can no longer move in this way, the waves break. Energy is dispersed as the particles are ejected forward in a rush of foam.