Seashores were first created when the continents and islands of the Earth were formed. Since then many changes have occurred. Some happened during prehistoric times, and others are still taking place.

Movement of the Earth's Crust

The underlying structure of the shoreline depends upon the shape of the land where it meets the ocean and the type of rock of which it is a part. Earthquakes and volcanoes during prehistoric times may have helped form many shorelines. An earthquake, for example, caused part of the California shoreline to sink. The sunken area became what is now San Francisco Bay and a new shoreline was created. In regions like northern California, where earthquakes and volcanoes still occur, the shoreline may undergo many more changes in the future.

Glaciers

During prehistoric times, glaciers (giant, slow-moving rivers of ice) may also have altered...