Calas Coasts are typically displayed in the Balearic Islands and are marked by numerous short, semicircular, and rather shallow bays, separated by narrow peninsulas. On the coasts of the Red Sea the bays have a more or less rectangular outline not narrowing inland. Obviously, coasts of this class differ but little from the serrate regular coasts into which they grade; their mode of origin, however, renders it important to make the distinction. Calas coasts owe their irregularities not to wave erosion, but to the submergence of land valleys; those of the typical kind are due to the depression of mountain slopes, furrowed by numerous short ravines. The coasts of the Red Sea type arise on the depression of desert mountains, in which valleys are few and small.

The irregular coasts are thus in all cases due to the submergence of land, and their characteristic features are to be explained by the differences of the land-surfaces before submergence, which in turn are determined chiefly by the subaerial agents.

While the classification of coastal forms serves a useful purpose, it must not be supposed that it is always easy to refer a given coast to a definite type. In travelling along a coast, one type is frequently found to change and give way to another, as in eastern North America, for example. This entire coast has recently been depressed, and from the end of Florida to 450 N. lat. the subsidence is still in progress, but the effects of the submergence are very different in accordance with the former land-surfaces. The depressed margin of the coastal plain is a regular coast, with very few islands, while that of Maine, Nova Scotia, and New Brunswick is highly irregular and has numerous rocky islands, while the Hudson River, Delaware and Chesapeake Bays are drowned valleys. Between the two types the transition is gradual. Here one kind of coast succeeds another, but two or more may occur together; calas are frequent on rias coasts, and rias are found among fjords. In such cases the general character of the coast determines its reference.