South of MayagŁez , the shelf is relatively broad and reefs and shoals with depths of less than 10 meters extend 20 kilometers offshore. The same pattern of ridge with rocky shoreline and valley with sand beach is present, but there are also areas of mangrove coastline and fringing reef coast because of the protection offered by the shoal continental shelf area.
The southwest coast is very irregular, with projecting brush- covered points of
between shallow coves and bays. There are fringing reefs along part of this coast. Except for the eastern and western ends of the south coast, and the Guanica area, the land is generally low near the shore. The shoreline development is closely related to whether the adjacent land area of the coast is Cretaceous-Tertiary limestones, igneous rock, or low Tertiary sediment fans and alluvial plains. The limestone outcrops generally form a
with small, very local sand or gravel beaches. In many places, this type of shoreline has been altered by the growth of mangrove and the subsequent development of a
The alluvial plains are either beach areas or unconsolidated cliffs that are retreating under the attack of strong wave action.
The southeast coast of Puerto Rico is mainly rocky coastline with some broad beach under the attack of strong wave action areas developed in front of alluvial valleys and a few areas of fringing reef coastline. This pattern extends along most of the east coast where irregular projecting rocky bluffs separate numerous small shallow coves and bays. In these areas, as on the west coast, the hills are within a mile of the coast. Shoal water extends offshore as part of the platform connecting Puerto Rico and the Virgin Islands.
There is fairly widespread mangrove coast along the northern half of the east coast. The north coast from the east end of the island to San Juan is low and sandy , except for occasional eolianite bluffs . The low land extends three to seven kilometers inland. The coast is indented by many coves that are protected by reefs and rocky islets lying only one to two kilometers offshore. Most of the coast is sandy beach. The large mangrove areas that lie east of San Juan do not form the coast, but lie several hundred meters behind the sand beaches and dune system.
From San Juan to Arecibo, the coast lies in front of a low-lying coastal plain . Beach sand, cemented dunes, beach rock, and mangrove swamps dominate this region. There are sandy beaches and dunes with occasionally rocky eolianite coast and beach rock coast from San Juan to Vega Baja. The mangroves lie close to shore on the coastal plain but do not form the coastline along any part of the northern coast. From Vega Baja to Arecibo, the coast is dominantly eolianite with numerous small lunate bays bordered by beaches. There is some movement of the beach sand from one bay to another behind the eolianite ridges.
The north coast from Arecibo to Aguadilla is a series of rocky cliffs with sand beaches and dunes between them. The prominent features are the high hills in the interior and high cliffs along the coast. Where low coastal plains with beach are present, they are less than a kilometer in width.
The north shelf is only two to four kilometers wide and there are few offshore reefs beyond a half kilometer from shore. In most places the open ocean waves break directly against the shore. It is a high energy coast with a rugged shoreline and active beach systems.