Introduction to Physical Geology
The Shorelines
The rhythmic rise and fall of sea level along a coastline is called the tide. The tide is a result of the gravitational attraction exerted upon the earth by the moon, and to a lesser extent by the sun. Tides occur about fifty minutes later each successive day for about twenty-nine days, which completes one cycle. At the times of the new and full moons, the earth, sun, and moon are aligned, causing the greatest difference in tidal elevations, called spring tides. Neap tides, producing the least extreme tidal differences, occur midway between the spring tides.
A tidal current is the horizontal flow of water that accompanies the tides and flows in one of two opposite directions. Tidal currents preceding high tide are called flood currents; tidal currents preceding low tide are called ebb currents. The zone of coastline affected by the tidal currents is called the tidal flat. Tidal action often generates cross-bedded marine sedimentary deposits called tidal deltas.
