Waves are generally larger in the southwest of the British Isles than the southeast due to larger fetch, or the distance over which wind can blow over open water to generate waves. Waves break when the wavelength reaches the sea floor, causing the bottom of the wave to slow down while the top continues forward. Coastal erosion processes include abrasion by materials carried in waves, hydraulic action as waves enter cracks and cause small explosions, and corrosion as acids in sea water dissolve rocks. These processes form landforms like headlands and bays, cliffs and wave-cut platforms, and over more time, caves, arches, stacks, and stumps.