Irregular Coasts display a great variety of forms due to the manner of their origin, and several subdivisions are employed to express this diversity of origin. All the forms, however, have this in common, that they .are produced by the depression and submergence of land-surfaces, and it is the variety of the latter which causes the manifold differences of the irregular coasts. On such coasts the numerous bays penetrate far into the land, sometimes diminishing regularly in width, but frequently of varying width, now expanding into lake-like form, now contracting to a strait, often winding and even branching, and always ending in a land valley. Islands are numerous and are in line with the land between the bays. The land may be high or low, gently or steeply sloping, and the subaerial slope is continued beneath the water without change, until the flat sea-bottom is reached. The type of an irregular coast is given by its bays, which vary from short, funnel-shaped indentations, to long, narrow, winding, and branching channels.

The subdivisions of the class are made in accordance with these variations.