Mountainous interior of Puerto Rico

Mountainous interior of Puerto Rico

Topography is the set of forms that make up the surface of a region. The topography of the surface of Puerto Rico and its islands is the result of geological processes that took place millions of years ago. Climatic factors, such as rain and wind, have also played a part as they erode the soil and shape the surface. Changes in the morphology of the land may also be brought about by the actions of human beings.

The topography of the largest island is characterized by 40% of its surface being mountainous. Another 35 percent is hills and 25 percent plains. As to altitude, 55 percent of the area is under 150 m (492 feet); 21 percent is between 150 and 300 m (492 and 984 feet); and the remainder, 24 percent, is over 300 m (984 feet).

The mountainous interior of the island is made up of the Cordillera Central, the Sierra de Cayey, the Sierra de Luquillo, the Sierra de Guardarraya, the Cuchilla de Panduras, and the Sierra Bermeja. Cordillera Central is the name given to the mountains that cross the island from west to east, from Mayagüez to Aibonito. The highest point in the Cordillera Central is La Punta, a mountain that is 1,338 m (4,389 feet) high, located between Adjuntas and Villalba. To the northeast of the Cordillera, is the Sierra de Luquillo, which runs from Gurabo to Fajardo. Its highest peak is El Yunque, at 1,080 m (3,542 feet). The Sierra de Cayey is located to the southeast between Cayey and Humacao, and its highest point is La Santa, a mountain of 903 m (2,962 feet). Also to the southeast are Sierra Guardarraya and Cuchilla de Panduras located between Yabucoa and Maunabo. Sierra Bermeja, to the south of the Lajas Valley, has its highest point on Cerro Mariquita, a mountain of 301 m (968 feet).

In Puerto Rico it is common to find very steep mountain slopes of 45 degrees or more. Especially in the south of the largest island, the slopes are very pronounced, and the rivers are short and deep. Usually, the slopes are covered with a clay crust shaped by rain, wind, and changes in humidity and temperature. Some of the crust is earthy, red, rich in iron, which has developed over sedimentary rock or lava. This makes it susceptible to heavy rain and prone to rock falls and landslides.

In the mountains, there are some flat areas, the product of weathering, erosion and the movement of loose matter. These surfaces are stepped and are found at 750-800 m (2,460-2,624 feet), 470 m (1,524 feet), 330 m (1,082 feet) and 160 m (525 feet). They were formed as ancient coastal plains but were raised by upward and downward movements of the island that took place millions of years ago. These areas remain flat or gently undulating, and sometimes have deep river canyons cutting through them. Examples of these are San Cristóbal, cut by the Río Usabón in Barranquitas; Guajataca between Isabela and Quebradillas; and Tanamá in Arecibo, whose course may have been partially subterranean, as it still retains some caves and rocky arches. In the mountains, there are river valleys such as those in Cidra, Cayey and Caguas. These are low areas where eroded matter brought down by rain and wind has been deposited over the years.

In the southwest of the main island, between the Cordillera Central and the Sierra Bermeja, is the Lajas Valley, which at its center is only 13 m (43 feet) above sea level. This valley extends from Guánica to Cabo Rojo, which was, at some time in the past, a sound. The sea separated the Sierra Bermeja from the rest of the island, or linked them, depending on fluctuations of the sea level that took place during the Pleistocene, 1.8 million years ago.

To the north of the Cordillera Central as far as the northern coast, runs the Aguada-Loíza karst region. This is composed of plains, hills, and low mountains made of different kinds of carbonate rocks -some of which are limestone, calcareous shale, lutites, chalk and calcareous limestone. On its southern fringe, the karst region is found at altitudes higher than 400 m (1,320 feet) and up to 530 m (1,738 feet) at its highest. Then it sinks below the cover of loose or poorly consolidated sediment in the coastal plain, which was formed simultaneously by the action of the rivers and the sea. In some places in this zone, such as to the east of Arecibo, along the lower reaches of the Río de La Plata, as well as near the San Juan metropolitan area, its rocky interior is made up of a kind of buried or fossil karst.

These zones were covered with sediment brought down by rain and wind. Deep caves, ancient drainages related to lower sea levels than is typical today, have been discovered by geological researchers through drilling.

From the northern coastal plain rise residual hills, such as the mogotes, some forming chains, others standing alone, round or long or even irregular in shape, such as those in Loíza, Carolina, Guaynabo, Bayamón, Toa Baja, Vega Baja, Vega Alta and other parts of the northern area. As the region evolved, the ancient systems of drainage became disorganized and flowed underground. An example of this is the Cueva de Los Choros, in the Bosque Río Abajo in Arecibo, which belongs to the system of springs that constitute the underground supply of the Río Grande de Arecibo. The ancient river valleys become less well organized as they run north until in some places they are unrecognizable; but in the south, they can still be reconstructed with considerable precision.

In the limestone outcroppings in the mountains of the Cordillera Central, there are separate karst massifs, and the region is known as the isolated karst region of the Cordillera Central. In them, there are some noteworthy cave systems, such as the one in Sumidero Ward in the municipality of Aguas Buenas and the one in the Cerro de Las Cuevas between the municipalities of Juana Díaz and Villalba. To the south, in the coastal plains where there are karst hills, is the Juana Díaz-Cabo Rojo karst region, where the caves of Mapancha and El Convento are found in the municipalities of Peñuelas and Guayanilla. To the east, west, and southeast, there are also plains covered with alluvial and marine sediments, which have developed all along the lower reaches of the principal rivers.

Author: Dr. Manuel José Acevedo González
Published: September 05, 2014.

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