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THE WORLDS'S COASTS: ONLINE - Puerto Rico

More than 45 small islands and islets are scattered around Puerto Rico. Some like Isla Magueyes are less than 50 meters from the island of Puerto Rico others are 10 (Caja de Muertos) to 100 km (Mona) offshore. The largest five of these are discussed in this section. Vieques and Culebra are populated, the other three are under the supervision of the Department of Natural and Environmental Resources. Desecheo is closed to the public, but Mona and Isla Ratones are open to visitors.

Desecheo

Isla Desecheo is a small island (mean diameter 2 km) in Mona Passage between Puerto Rico and Hispaniola (17 km offshore). The Great Southern Puerto Rico Fault that passes south of Rincon also passes south of Desecheo. The island is a fault-controlled outcrop of the same volcanoclastic rocks (middle Eocene) as at Punta Jiguero. The shoreline is volcanoclastic and Pleistocene marine terrace rocks.






































111. The Desecheo shoreline has Pleistocene marine terraces

Mona

Mona and Monito are very different from the rest of Puerto Rico. They appear as great flat white slabs floating on the sea bounded by high cliffs rising vertically out of the water. Mona is seven by eleven kilometers. Monito , about 3 miles northwest of Mona, is comparatively tiny, only 40 acres in area.

At sea level the cliffs which may rise from depths of 30 m underwater are undercut by pounding waves. Huge boulders , former sections of the cliffs, lie in the surf where they have fallen. Deep cracks on the upper surface of the island near the cliffs mark the next rocks to cede to the relentless force of the sea.

Only on the more protected sides of these islands is the sea sufficiently shallow for the development of coral reefs. On Mona they protect a narrow coastal plain partially fringed with sandy beaches. Mona is too small to generate daytime showers or a nocturnal land breeze, so the weather is essentially that which would prevail over the sea at this location if it didn't exist, much drier than that of most of Puerto Rico.

Isla de Mona and Isla Monito lie on two upthrown structural blocks that are delineated by several near-vertical faults. The blocks are separated by a shallow graben (Kaye, 1959). The eastern boundary of the structural block containing Isla Monito is a north-trending fault that projects south across the graben and appears to be the same as the prominant fault in the north-central cliffs of Isla de Mona (Briggs and Seiders, 1972).

Mona is composed of stratified limestone and dolomite, reef rock, and boulder rubble- that range in age from lower or middle Mocene (about 18-25 million years old) to Holocene (Recent). The bulk of the island is composed of two generally flat-lying rock units, the Isla de Mona Dolomite and the Lirio Limestone.

Reef terraces of late Pleistocene age form the shoreline along 14 km of the south and west coasts of Isla de Mona. Beach sands and beach rock form shoreline from Playa Sardinera southeastward to Playa del Uvero, and from Punta los Ingleses northeastward to Playa de Pajaros, and slightly beyond. The beach deposits are composed of fine to medium-grained grayish-pink calcite and aragonite sand, chiefly cornminuted shell debris. The rest of the shoreline is limestone cliffs.





































112. At Sardinero limestone cliff shoreline is joined by a fringing reef
















113. Sardinero is at the end of one of the larger lowland areas that lies beneath limestone cliffs. The shoreline is beach and beachrock This extends to the southwest corner of Mona forming a sandy point.
















114. The coastal plain continues east and narrows to limestone cliffs at the shoreline. The shoreline is a beach.

















115. Limestone cliffs continue to the east except for low plains east and west of Pajaros where the shoreline is beach plain and Pleistocene marine terrace.















116. The coast to the lighthouse and around the north coast back to Sardinero is limestone cliff. Caves are at the contact between the grey Lirio Limestone, and the underlying buff-colored Isla de Mona Dolomite



















117. Sea cliffs form the north shore of Mona. The contact between the Lirio Limestone and the Isla de Mona dolomite and caves at that contact are high in the cliff. The cliffs continue underwater for more than 25 m and shallow sea caves have formed at water level.

Caja de Muertos

Caja de Muertos is volcaniclastic and limestone, with Pleistocene marine terraces. The shoreline is beach plain and rock, including reef terrace.






































118. The northeast coast is a sloping plain to a coastline of marine terrace and beach

















119. At the southwest a low plain lies between the main rock island and a small outcrop at the soutwest tip. Both sides of this are beach plain.

Vieques

Isla Vieques, five by twenty-nine kilometers, is 13 km off the east coast of Puerto Rico. The island has Tertiary limestones, Cretaceous volcanics and plutonic intrusives with alluvial plains, beaches and mangrove swamps. The coastline is rock, mangrove and beach.




































120. Green Beach is located on a sandy point. A large shoal submarine sand body extends northwestward from the point.

















121. The southwest corner has rocky points separating pocket beaches.























122. The Esperanza shoreline is beach in front of an alluvial plain, with a few rocky points.























123. East of Esperanza the shoreline is low lying plain with rocky points that has been shaped into beaches and mangrove lagoons. One of the larger of these is Ensenada Hondo.
















124. The low lying east end of Vieques is rock and mangrove shoreline with short beaches.
















125. The north coast is rocky shoreline except for beaches developed below the rocky land. Some of the beaches are partially protected by fringing reefs . West of Isabel Segundo, the pattern of rocky points separate pocket beaches on the narrow coastal plain.























126. West of Isabel Segundo, the coastline is a series of pocket beaches between rocky points.
photo USGS

















127. The western part of the north coast has a unique structure; a long jetty extends out from a small plain in front of the rock hills.

















128. The northwest lagoon on the island has a seaward mangrove and beach shoreline.

Culebra

The volcanic island of Culebra has shorelines that are rock, beach and fringing reefs.








































129. Dewey, the only town, is behind a small beach set between rocky shoreline. Immediately southwest of here is Isla Martin Peņa, one of three islands near Culebra.

















130. Dakity is a fringing reef shoreline in front of the island at the entrance to Ensenada Honda, a mangrove and rock lined lagoon.


















131. The north coast is rock coastline with a few beaches. Culebrita , northeast of Culebra is one of the nearby islets.

















132. Flamingo Beach is a striking fine carbonate sand beach near the west end of the north shore.



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