The coastline, under the constant action of the waves, tides and currents, is undergoing changes from day to day. On calm days, when winds are slight, waves do little damage to the shoreline and may instead help to build up beaches and other depositional features. It is in storms that the ravages of the waves reach their greatest magnitude. The average pressure of Atlantic waves on adjacent coasts is about 600 lb. per square foot in the summer and treble that in winter. During storms, the pressure exerted is more than 6,000 lb. or 3 tons per square foot! Movements of such intensity will wear down not only the cliffs but also sea walls and buildings. Tides and currents, on contact with the shores, make very little direct attack on the coastline. Tides affect marine erosion mainly by extending a line of erosion into a zone of erosion. This zone corresponds to the area between the low water level and the high water level. Currents help to move eroded debris and deposit it as silt, sand and gravel along the coasts.