This term is derived from northwestern Spain, where the Ria de Vigo, de la Corufia, del Ferrol, and several others, form long, fjord-like bays, though branching little, which extend far into the land. Coasts of this type have frequently been regarded as fjord coasts, but there are essential differences; the bays are shorter, more funnel-shaped, broadening and deepening seaward, and are not nearly so deep as fjords. The excavation was not glacial, and hence the rias are not confined to high latitudes, but occur abundantly in the temperate and tropical regions, as in Brittany, Cornwall, Ireland, the east shore of the Adriatic, Brazil, southern China, and eastern Australia.

Rias coasts are most frequent at the margin of low mountains and lands of moderate height, but they may occur along areas of any elevation. Nor are they associated with any special type of geological structure; in Spain, Brittany, Cornwall, and other regions the' rock is a granite without recognizable structure. In other instances, structure has exerted an evident control, as in the southwest of Ireland, where the bays follow the strike of the rocks and thus occupy longitudinal valleys, which are cut out along the soft sandstones, while the intervening ridges are made up of more resistant limestones. In still other cases the rias are found in valleys of folding, where the coast-line intersects the line of strike. An unusual, case is the Bay of San Francisco which is T-shaped and fills a longitudinal strike-valley parallel to the coast, with a transverse connection with the Pacific by way of the Golden Gate.

The mode of origin of rias coasts is by the depression and partial submergence of short slopes cut by deep valleys of sub-aerial origin, whether excavated by rivers or formed by tectonic processes. The rivers were short and carried no great load of sediment, hence the bays were not filled up with silt and mud during the slow submergence. In some examples, as in those of Brittany, the valleys are continued for some distance across the sea-floor, a circumstance which in itself is an evidence of depression. The characteristic difference between fjord and rias coasts is that the former are due to glaciation and the latter are not.