Wave pounding is an erosion process which involves the force of the wave hitting rock and wearing it down, this particularly occurs in soft rock such as sandstone. Hydraulic action may also work with this to get between cracks and release trapped air, causing enlargement of cracks. Another process is solution which is where acidic sea water mixes with rock sediment causing a chemical reaction, forcing rock to break apart. Attrition can also have an effect as rocks are carried by powerful waves (especially in winter and high tides), causing sediment to break away from weakened cliffs. These erosion processes can help to form various land-forms such as Caves, arches, stacks and stumps. It can also create headlands and bay's where the coastline is discordant.