The sea-coast is not merely a line, but a zone of varying breadth, sloping toward the sea, and with a subaerial and a submarine portion. The submarine portion of the coast frequently continues the slope of the subaerial portion, with an interruption formed by the actual beach, upon which the surf breaks. 'On flat, gently sloping coasts, the beach is generally broad, especially if the range of the tides is great, and a beach wall is present (see p. 247), frequently with a belt of sand-dunes behind it, while on steep, rocky coasts the beach is narrow and may occasionally be absent altogether.

The coasts of the different parts of the world display a great variety of form and structure, but they may all be included in a small number of classes. An obvious primary division is into (1) regular, (2) irregular, and (3) lobate coast-lines.