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104-520: Lying on the northwesternmost point of the islet of Old San Juan , Castillo San Felipe del Morro is named in honor of King Philip II of Spain . The fortification, also referred to as el Morro or 'the promontory ,' was designed to guard the entrance to the San Juan Bay , and defend the Spanish colonial port city of San Juan from seaborne enemies. In 1983, the citadel was declared a World Heritage Site by

208-455: A Mohs hardness of 2 to 4, dense limestone can have a crushing strength of up to 180 MPa . For comparison, concrete typically has a crushing strength of about 40 MPa. Although limestones show little variability in mineral composition, they show great diversity in texture. However, most limestone consists of sand-sized grains in a carbonate mud matrix. Because limestones are often of biological origin and are usually composed of sediment that

312-566: A World Heritage Site by the United Nations . In honor of the Quincentennial of the voyages of Columbus in 1992 the exterior esplanade was cleared of palm trees that had been planted by the U.S. Army in the Fort Brooke era, and restored to the open appearance this "field-of-fire" for El Morro's cannon would have had in colonial Spanish times. Parking lots and paved roads were also removed, and

416-457: A bloom of cyanobacteria or microalgae . However, stable isotope ratios in modern carbonate mud appear to be inconsistent with either of these mechanisms, and abrasion of carbonate grains in high-energy environments has been put forward as a third possibility. Formation of limestone has likely been dominated by biological processes throughout the Phanerozoic , the last 540 million years of

520-452: A hornwork , crossing the headland , to protect the landward side of the existing tower and water battery. Two half- bastions , one on the Atlantic side called "Tejeda", and another on the harbor side called "Austria", were connected by a curtain wall fronted by a moat , and spanned by a drawbridge in the center. The gate and drawbridge were protected by a ravelin , and just inside the gate

624-434: A carbonate rock outcrop can be estimated in the field by etching the surface with dilute hydrochloric acid. This etches away the calcite and aragonite, leaving behind any silica or dolomite grains. The latter can be identified by their rhombohedral shape. Crystals of calcite, quartz , dolomite or barite may line small cavities ( vugs ) in the rock. Vugs are a form of secondary porosity, formed in existing limestone by

728-612: A central quartz grain or carbonate mineral fragment. These likely form by direct precipitation of calcium carbonate onto the ooid. Pisoliths are similar to ooids, but they are larger than 2 mm in diameter and tend to be more irregular in shape. Limestone composed mostly of ooids is called an oolite or sometimes an oolitic limestone . Ooids form in high-energy environments, such as the Bahama platform, and oolites typically show crossbedding and other features associated with deposition in strong currents. Oncoliths resemble ooids but show

832-449: A change in environment that increases the solubility of calcite. Dense, massive limestone is sometimes described as "marble". For example, the famous Portoro "marble" of Italy is actually a dense black limestone. True marble is produced by recrystallization of limestone during regional metamorphism that accompanies the mountain building process ( orogeny ). It is distinguished from dense limestone by its coarse crystalline texture and

936-949: A composition reflecting the organisms that produced them and the environment in which they were produced. Low-magnesium calcite skeletal grains are typical of articulate brachiopods , planktonic (free-floating) foraminifera, and coccoliths . High-magnesium calcite skeletal grains are typical of benthic (bottom-dwelling) foraminifera, echinoderms , and coralline algae . Aragonite skeletal grains are typical of molluscs , calcareous green algae , stromatoporoids , corals , and tube worms . The skeletal grains also reflect specific geological periods and environments. For example, coral grains are more common in high-energy environments (characterized by strong currents and turbulence) while bryozoan grains are more common in low-energy environments (characterized by quiet water). Ooids (sometimes called ooliths) are sand-sized grains (less than 2mm in diameter) consisting of one or more layers of calcite or aragonite around

1040-412: A considerable fraction of the limestone bed. At depths greater than 1 km (0.62 miles), burial cementation completes the lithification process. Burial cementation does not produce stylolites. When overlying beds are eroded, bringing limestone closer to the surface, the final stage of diagenesis takes place. This produces secondary porosity as some of the cement is dissolved by rainwater infiltrating

1144-483: A drop of dilute hydrochloric acid is dropped on it. Dolomite is also soft but reacts only feebly with dilute hydrochloric acid, and it usually weathers to a characteristic dull yellow-brown color due to the presence of ferrous iron. This is released and oxidized as the dolomite weathers. Impurities (such as clay , sand, organic remains, iron oxide , and other materials) will cause limestones to exhibit different colors, especially with weathered surfaces. The makeup of

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1248-621: A few million years, as this is the most stable form of calcium carbonate. Ancient carbonate formations of the Precambrian and Paleozoic contain abundant dolomite, but limestone dominates the carbonate beds of the Mesozoic and Cenozoic . Modern dolomite is quite rare. There is evidence that, while the modern ocean favors precipitation of aragonite, the oceans of the Paleozoic and middle to late Cenozoic favored precipitation of calcite. This may indicate

1352-455: A few thousand years. As rainwater mixes with groundwater, aragonite and high-magnesium calcite are converted to low-calcium calcite. Cementing of thick carbonate deposits by rainwater may commence even before the retreat of the sea, as rainwater can infiltrate over 100 km (60 miles) into sediments beneath the continental shelf. As carbonate sediments are increasingly deeply buried under younger sediments, chemical and mechanical compaction of

1456-624: A force of 7,000–13,000 men, invaded the island of Puerto Rico. Captain General Don Ramón de Castro and his forces repelled the attack. Abercromby and Harvey were defeated in the Battle of San Juan (1797) . El Morro's last active fight occurred during a naval bombardment by the United States Navy during the 1898 Spanish–American War , ending the age of naval warfare in the Caribbean, at least in

1560-428: A free trolley provided limited service to Old San Juan seven days a week but is not available any longer. People get around the city on foot or by car. There have been plans to turn the historic district into a pedestrian-only zone , and to either rebuild the trolley system or build a new light-rail train system in the recent years. Old San Juan is the setting for films, books, song lyrics and music videos, including

1664-490: A high percentage of the mineral dolomite , CaMg(CO 3 ) 2 . Magnesian limestone is an obsolete and poorly-defined term used variously for dolomite, for limestone containing significant dolomite ( dolomitic limestone ), or for any other limestone containing a significant percentage of magnesium . Most limestone was formed in shallow marine environments, such as continental shelves or platforms , though smaller amounts were formed in many other environments. Much dolomite

1768-502: A large U.S. Army post, called Fort Brooke . In the early 20th century, the U.S. military filled up the esplanade (the green space in front of "El Morro") with baseball diamonds, hospitals, officers' quarters, an officers' club and even a golf course. On March 21, 1915, Lt. Teófilo Marxuach was the officer of the day at the El Morro fortress. The Hamburg America Line cargo liner Odenwald tried to leave port without permission from

1872-435: A limestone sample except in thin section and are less common in ancient limestones, possibly because compaction of carbonate sediments disrupts them. Limeclasts are fragments of existing limestone or partially lithified carbonate sediments. Intraclasts are limeclasts that originate close to where they are deposited in limestone, while extraclasts come from outside the depositional area. Intraclasts include grapestone , which

1976-449: A local civic leader who had successfully prevented the demolition of the Capilla del Cristo in favor of a traffic redesign. He advised mayor Rincón de Gautier in having local zoning laws changed to favor remodeling and the incorporation of Spanish colonial motifs in any new construction. This helped preserve the city's architectural profile, and has been a key to San Juan's current status as

2080-466: A lower Mg/Ca ratio in the ocean water of those times. This magnesium depletion may be a consequence of more rapid sea floor spreading , which removes magnesium from ocean water. The modern ocean and the ocean of the Mesozoic have been described as "aragonite seas". Most limestone was formed in shallow marine environments, such as continental shelves or platforms . Such environments form only about 5% of

2184-525: A lower diversity of organisms and a greater fraction of silica and clay minerals characteristic of marls . The Green River Formation is an example of a prominent freshwater sedimentary formation containing numerous limestone beds. Freshwater limestone is typically micritic. Fossils of charophyte (stonewort), a form of freshwater green algae, are characteristic of these environments, where the charophytes produce and trap carbonates. Limestones may also form in evaporite depositional environments . Calcite

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2288-403: A massive concrete bunker to the top of El Morro to serve as a Harbor Defense Fire Control Station to direct a network of coastal artillery sites, and to keep watch for German submarines which were ravaging shipping in the Caribbean. A lighthouse, rebuilt by the U.S. Army in 1906–08 is the tallest point on El Morro, standing 180 feet (55 m) above sea level. Flagpoles on El Morro today customarily fly

2392-523: A mechanism for dolomitization, with one 2004 review paper describing it bluntly as "a myth". Ordinary seawater is capable of converting calcite to dolomite, if the seawater is regularly flushed through the rock, as by the ebb and flow of tides (tidal pumping). Once dolomitization begins, it proceeds rapidly, so that there is very little carbonate rock containing mixed calcite and dolomite. Carbonate rock tends to be either almost all calcite/aragonite or almost all dolomite. About 20% to 25% of sedimentary rock

2496-444: A neighborhood of Old San Juan and its reputation has recently transformed from a dangerous place into a cultural hub. It has appeared on films, literature, TV shows and music videos . With its abundance of shops, historic places, museums, open air cafés, restaurants, colorful homes, tree-shaded plazas, and its old beauty and architectural peculiarity, Old San Juan is a main spot for domestic and international tourism. The district

2600-552: A plan to strengthen San Juan's defenses, which was approved by Charles III of Spain . San Juan became a Defense of the First Order, and one of the most powerful plazas in the Americas by 1790. El Morro's improvements included 3 cisterns under the main plaza containing 216,000 gallons of water collected from times of rain. The walls were strengthened to be 18–40 feet thick. These walls consisted of limestone and sandstone blocks forming

2704-448: A plausible source of mud. Another possibility is direct precipitation from the water. A phenomenon known as whitings occurs in shallow waters, in which white streaks containing dispersed micrite appear on the surface of the water. It is uncertain whether this is freshly precipitated aragonite or simply material stirred up from the bottom, but there is some evidence that whitings are caused by biological precipitation of aragonite as part of

2808-587: A project proposal suggested that the old Carmelite Convent in San Juan be demolished to erect a new hotel, the Institute had the building declared a historic building, and asked that it be converted to a hotel in a renewed facility. As in other Operation Bootstrap projects, the Woolworth family was invited in 1957 to reconstruct the former convent (which had more recently been a low-rent apartment building and city garage) into

2912-449: A radial rather than layered internal structure, indicating that they were formed by algae in a normal marine environment. Peloids are structureless grains of microcrystalline carbonate likely produced by a variety of processes. Many are thought to be fecal pellets produced by marine organisms. Others may be produced by endolithic (boring) algae or other microorganisms or through breakdown of mollusc shells. They are difficult to see in

3016-403: A tourist destination. Luis Muñoz Marín , as a senator and later as governor of Puerto Rico, supported and implemented what became known as Operation Bootstrap , which included the development of the tourism industry. In 1948 the government invited one of the organizers of Colonial Williamsburg to lead its tourism development efforts. These efforts were further supported by the 1955 creation of

3120-521: Is Castillo San Felipe del Morro, a fort built by the Spaniards when settled on the Island. Besides the magnificent fort, tourists are drawn to La Perla , a colorful neighborhood outside of the historic city wall. Old San Juan is one of the two barrios , in addition to Santurce , that made up the municipality of San Juan after 1864 and prior to 1951, in which the former independent municipality of Río Piedras

3224-577: Is a historic district located at the "northwest triangle" of the islet of San Juan in San Juan . Its area roughly correlates to the Ballajá , Catedral , Marina, Mercado , San Cristóbal , and San Francisco sub-barrios (sub-districts) of barrio San Juan Antiguo in the municipality of San Juan, Puerto Rico . Old San Juan is the oldest settlement within Puerto Rico and the historic colonial district of

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3328-560: Is also favored on the seaward margin of shelves and platforms, where there is upwelling deep ocean water rich in nutrients that increase organic productivity. Reefs are common here, but when lacking, ooid shoals are found instead. Finer sediments are deposited close to shore. The lack of deep sea limestones is due in part to rapid subduction of oceanic crust, but is more a result of dissolution of calcium carbonate at depth. The solubility of calcium carbonate increases with pressure and even more with higher concentrations of carbon dioxide, which

3432-482: Is an uncommon mineral in limestone, and siderite or other carbonate minerals are rare. However, the calcite in limestone often contains a few percent of magnesium . Calcite in limestone is divided into low-magnesium and high-magnesium calcite, with the dividing line placed at a composition of 4% magnesium. High-magnesium calcite retains the calcite mineral structure, which is distinct from dolomite. Aragonite does not usually contain significant magnesium. Most limestone

3536-402: Is carbonate rock, and most of this is limestone. Limestone is found in sedimentary sequences as old as 2.7 billion years. However, the compositions of carbonate rocks show an uneven distribution in time in the geologic record. About 95% of modern carbonates are composed of high-magnesium calcite and aragonite. The aragonite needles in carbonate mud are converted to low-magnesium calcite within

3640-506: Is characterized by numerous public plazas, which are filled with local shops that sell traditional craftwork, sculptures and typical musical instruments. Moreover, Old San Juan is known for its historical churches, such as San José Church and the Cathedral of San Juan Bautista , which contains the tomb of the Spanish explorer and first European to discover modern day Florida , Juan Ponce de León . Tourists' main spot to visit at Old San Juan

3744-474: Is clusters of peloids cemented together by organic material or mineral cement. Extraclasts are uncommon, are usually accompanied by other clastic sediments, and indicate deposition in a tectonically active area or as part of a turbidity current . The grains of most limestones are embedded in a matrix of carbonate mud. This is typically the largest fraction of an ancient carbonate rock. Mud consisting of individual crystals less than 5 μm (0.20 mils) in length

3848-416: Is commonly white to gray in color. Limestone that is unusually rich in organic matter can be almost black in color, while traces of iron or manganese can give limestone an off-white to yellow to red color. The density of limestone depends on its porosity, which varies from 0.1% for the densest limestone to 40% for chalk. The density correspondingly ranges from 1.5 to 2.7 g/cm . Although relatively soft, with

3952-420: Is controlled largely by the amount of dissolved carbon dioxide ( CO 2 ) in the water. This is summarized in the reaction: Increases in temperature or decreases in pressure tend to reduce the amount of dissolved CO 2 and precipitate CaCO 3 . Reduction in salinity also reduces the solubility of CaCO 3 , by several orders of magnitude for fresh water versus seawater. Near-surface water of

4056-545: Is converted to low-magnesium calcite. Diagenesis is the likely origin of pisoliths , concentrically layered particles ranging from 1 to 10 mm (0.039 to 0.394 inches) in diameter found in some limestones. Pisoliths superficially resemble ooids but have no nucleus of foreign matter, fit together tightly, and show other signs that they formed after the original deposition of the sediments. Silicification occurs early in diagenesis, at low pH and temperature, and contributes to fossil preservation. Silicification takes place through

4160-503: Is deposited close to where it formed, classification of limestone is usually based on its grain type and mud content. Most grains in limestone are skeletal fragments of marine organisms such as coral or foraminifera . These organisms secrete structures made of aragonite or calcite, and leave these structures behind when they die. Other carbonate grains composing limestones are ooids , peloids , and limeclasts ( intraclasts and extraclasts  [ ca ] ). Skeletal grains have

4264-460: Is described as coquinite . Chalk is a soft, earthy, fine-textured limestone composed of the tests of planktonic microorganisms such as foraminifera, while marl is an earthy mixture of carbonates and silicate sediments. Limestone forms when calcite or aragonite precipitate out of water containing dissolved calcium, which can take place through both biological and nonbiological processes. The solubility of calcium carbonate ( CaCO 3 )

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4368-617: Is described as micrite . In fresh carbonate mud, micrite is mostly small aragonite needles, which may precipitate directly from seawater, be secreted by algae, or be produced by abrasion of carbonate grains in a high-energy environment. This is converted to calcite within a few million years of deposition. Further recrystallization of micrite produces microspar , with grains from 5 to 15 μm (0.20 to 0.59 mils) in diameter. Limestone often contains larger crystals of calcite, ranging in size from 0.02 to 0.1 mm (0.79 to 3.94 mils), that are described as sparry calcite or sparite . Sparite

4472-462: Is distinguished from micrite by a grain size of over 20 μm (0.79 mils) and because sparite stands out under a hand lens or in thin section as white or transparent crystals. Sparite is distinguished from carbonate grains by its lack of internal structure and its characteristic crystal shapes. Geologists are careful to distinguish between sparite deposited as cement and sparite formed by recrystallization of micrite or carbonate grains. Sparite cement

4576-560: Is one of the first minerals to precipitate in marine evaporites. Most limestone is formed by the activities of living organisms near reefs, but the organisms responsible for reef formation have changed over geologic time. For example, stromatolites are mound-shaped structures in ancient limestones, interpreted as colonies of cyanobacteria that accumulated carbonate sediments, but stromatolites are rare in younger limestones. Organisms precipitate limestone both directly as part of their skeletons, and indirectly by removing carbon dioxide from

4680-473: Is otherwise chemically fairly pure, with clastic sediments (mainly fine-grained quartz and clay minerals ) making up less than 5% to 10% of the composition. Organic matter typically makes up around 0.2% of a limestone and rarely exceeds 1%. Limestone often contains variable amounts of silica in the form of chert or siliceous skeletal fragments (such as sponge spicules, diatoms , or radiolarians ). Fossils are also common in limestone. Limestone

4784-472: Is produced by decaying organic matter settling into the deep ocean that is not removed by photosynthesis in the dark depths. As a result, there is a fairly sharp transition from water saturated with calcium carbonate to water unsaturated with calcium carbonate, the lysocline , which occurs at the calcite compensation depth of 4,000 to 7,000 m (13,000 to 23,000 feet). Below this depth, foraminifera tests and other skeletal particles rapidly dissolve, and

4888-514: Is secondary dolomite, formed by chemical alteration of limestone. Limestone is exposed over large regions of the Earth's surface, and because limestone is slightly soluble in rainwater, these exposures often are eroded to become karst landscapes. Most cave systems are found in limestone bedrock. Limestone has numerous uses: as a chemical feedstock for the production of lime used for cement (an essential component of concrete ), as aggregate for

4992-558: The Battle of San Juan (1595) . In 1598, the English attacked again, led by George Clifford , 3rd Earl of Cumberland. Clifford succeeded because he attacked San Juan over land instead of trying to enter through the San Juan Bay. However, an epidemic of dysentery forced him to flee the island after the Battle of San Juan (1598) . In 1625, the Dutch , led by Boudewijn Hendricksz , also attacked

5096-719: The Chagres River , Panama City , Havana , and St. Augustine . San Juan construction began in March 1589 with skilled artisans, 12 stonecutters , 18 masons, 2 smiths, a cooper , metal founder, and an overseer assigned to the task, with the help of 150 slaves. Captain General Diego Menéndez de Valdés, who was the governor of Puerto Rico, took over construction after Tejeda and Antonelli left to start construction of Santo Domingo's fortifications. Capt. Pedro de Salazar took over construction in 1591. The new fortifications consisted of

5200-596: The Institute of Puerto Rican Culture ( Instituto de Cultura Puertorriqueña ) head by Alegría. He sought legal and administrative changes that would allow for major remodeling efforts to be successful. At the time, most real estate in Old San Juan had devalued under appraised values because the city was perceived as unsafe (particularly because of building disrepair and social ills such as prostitution ) and not profitable for business (because of rent control statutes, as well as

5304-569: The UNESCO World Heritage Site list since 1983. Historically the mixed-use commercial and residential real estate in the main street s of Calle Cristo and Calle Fortaleza from Calle Tanca to the Governor’s Mansion is the most valuable in the area and it has kept its value and increased steadily through several years despite the past economic turmoils. Old San Juan is located on a small and narrow island which lies along

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5408-574: The United Nations in conjunction with the San Juan National Historic Site . Over two million visitors a year explore the castillo , making it one of Puerto Rico's leading tourist attractions. Facing the structure, on the opposite side of the bay, a smaller fortification known as El Cañuelo complemented the castillo's defense of the entrance to the bay. The construction of the citadel and its surrounding walls began in 1539 on orders of King Charles V of Spain . The original fortress

5512-752: The United States flag , the Puerto Rican flag and the Cross of Burgundy flag, also known in Spanish as las Aspas de Borgoña, a standard which was widely used by Spanish armies around the world from 1506 to 1785. In 1961, the United States Army officially retired from El Morro. The fort became a part of the National Park Service to be preserved as museums. In 1983, the Castillo and the city walls were declared

5616-695: The Zona Antigua Histórica Casco de San Juan ( English : Historic Old Town Area of San Juan). In 1972, the same area was recognized by the National Register of Historic Places as the San Juan Historic Zone ( Spanish : Zona Histórica de San Juan ). In 2012, the National Register increased the boundaries and renamed the district to Old San Juan Historic District ( Spanish : Distrito Histórico del Viejo San Juan ). Old San Juan

5720-546: The 16th and 17th centuries—when Puerto Rico was a Spanish possession. Near Fort San Felipe del Morro, is the Casa Blanca , a palace built on land which belonged to the family of Ponce de León . In 1508, Juan Ponce de León founded the original settlement, Caparra (named after the Province of Cáceres , Spain , birthplace of the then-governor of Spain's Caribbean territories, Nicolás de Ovando ). The ruins of Caparra are known as

5824-622: The Customs Collector. Lt. Marxuach gave the order to open fire on her, which forced her to stop. The shots ordered by Lt. Marxuach are widely regarded to be the first shots fired by the United States in World War I, although the first actual wartime shot fired by the US came on the day war was declared, during the scuttling of SMS Cormoran off Guam . During World War II the United States Army added

5928-496: The Earth's history. Limestone may have been deposited by microorganisms in the Precambrian , prior to 540 million years ago, but inorganic processes were probably more important and likely took place in an ocean more highly oversaturated in calcium carbonate than the modern ocean. Diagenesis is the process in which sediments are compacted and turned into solid rock . During diagenesis of carbonate sediments, significant chemical and textural changes take place. For example, aragonite

6032-487: The El Morro lighthouse repaired and restored to its original appearance. The 1963 film Thunder Island featured an extensive foot chase and shootout scene filmed at El Morro. El Morro was also used as a film set in the 1996 motion picture Amistad . Steven Spielberg used it to represent a fort in Sierra Leone where African slaves were auctioned in 1839. African slave labor was used in addition to local labor to help build

6136-468: The Pueblo Viejo sector of Guaynabo , behind the almost land-locked harbor just to the west of the present San Juan metropolitan area. In 1509, the settlement was abandoned and moved to a site which was called at the time "Puerto Rico" (meaning "rich port" or "good port"), a name that evoked that of a similar geographical harbor in the island of Gran Canaria , Canary Islands . In 1521, the name "San Juan"

6240-649: The Spanish government of the island, El Morro, also known as Castillo de San Felipe, survived several attacks from foreign powers on various occasions. In 1593 Portuguese soldiers, sent from Lisbon by order of Phillip II, composed the first garrison of the San Felipe del Morro fortress in Puerto Rico. Some brought their wives, while others married Puerto Rican women, and today there are many Puerto Rican families with Portuguese last names. In 1595, Englishman Sir Francis Drake unsuccessfully attacked San Juan with his fleet in

6344-470: The Spanish on 18 November 1898. Upon the advice of Gonzalo Fernández de Oviedo y Valdés , a battery was constructed on the rocky promontory called "the Morro", when the location of La Fortaleza was deemed unsuitable. This battery consisted of a tower with 4 embrasures , and a Water Battery at the foot of the slope for 3 guns. By 1555, Morro had 8 bronze cannons, as a defense against French privateers . During

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6448-430: The accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life. About 20% to 25% of sedimentary rock is carbonate rock, and most of this is limestone. The remaining carbonate rock is mostly dolomite , a closely related rock, which contains

6552-434: The base of roads, as white pigment or filler in products such as toothpaste or paint, as a soil conditioner , and as a popular decorative addition to rock gardens . Limestone formations contain about 30% of the world's petroleum reservoirs . Limestone is composed mostly of the minerals calcite and aragonite , which are different crystal forms of calcium carbonate ( CaCO 3 ). Dolomite , CaMg(CO 3 ) 2 ,

6656-657: The beds. This may include the formation of vugs , which are crystal-lined cavities within the limestone. Diagenesis may include conversion of limestone to dolomite by magnesium-rich fluids. There is considerable evidence of replacement of limestone by dolomite, including sharp replacement boundaries that cut across bedding. The process of dolomitization remains an area of active research, but possible mechanisms include exposure to concentrated brines in hot environments ( evaporative reflux ) or exposure to diluted seawater in delta or estuary environments ( Dorag dolomitization ). However, Dorag dolomitization has fallen into disfavor as

6760-546: The castillo. El Morro was a defensive military fortification and a major component of San Juan's harbor defense system. Puerto Rico as such was considered by the Spanish crown as the "Key to the Antilles"; no enemy ship could navigate its waters without fear of capture. An annual artisans festival is held at the fort every July. The fort is home to a resident colony of around 200 stray cats that are believed to have been descended from cats introduced either during colonial times or in

6864-481: The city expansion across the Puerta de Tierra which included the plan for demolishing the city walls along the eastern side. On May 28, 1897, the wall demolition was officially started after a proclamation was issued by Queen Maria Christina . By the year 1899, the population of Puerta de Tierra had risen to 5,453; while the area comprising the old walled city had a civilian population of roughly 18,103 inhabitants. During

6968-480: The city of San Juan. This historic district is a Puerto Rico Registered Historic Zone and a National Historic Landmark District , Old San Juan Historic District , and is also listed on the United States National Register of Historic Places . Several historical buildings and structures, particularly La Fortaleza , the city walls , and El Morro and San Cristóbal castles, have been inscribed in

7072-403: The city walls occupying the east side of Old San Juan Island, was almost uninhabited. In 1838 the so-called area of Puerta de Tierra (or "Land Gate") had a population of 168 residents, mainly of African descent. According to a census made in 1846, the population had risen to 223 inhabitants living in 58 houses. On March 3, 1865, the municipal government of San Juan approved a resolution promoting

7176-670: The classical sense. During the Spanish–American War, the castle was attacked at least three times by American naval forces, the largest being the Bombardment of San Juan on May 12, 1898. The war ended with the signing of the Treaty of Paris , in which Spain ceded ownership of the islands of Puerto Rico , Cuba , Guam, and the Philippines to the United States. El Morro and many other Spanish government buildings in Old San Juan became part of

7280-462: The depositional fabric of carbonate rocks. Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles, based on criteria such as whether the grains were originally in mutual contact, and therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk scheme, Dunham deals with the original porosity of

7384-469: The deposits are highly porous, so that they have a spongelike texture, they are typically described as tufa . Secondary calcite deposited by supersaturated meteoric waters ( groundwater ) in caves is also sometimes described as travertine. This produces speleothems , such as stalagmites and stalactites . Coquina is a poorly consolidated limestone composed of abraded pieces of coral , shells , or other fossil debris. When better consolidated, it

7488-406: The earth's oceans are oversaturated with CaCO 3 by a factor of more than six. The failure of CaCO 3 to rapidly precipitate out of these waters is likely due to interference by dissolved magnesium ions with nucleation of calcite crystals, the necessary first step in precipitation. Precipitation of aragonite may be suppressed by the presence of naturally occurring organic phosphates in

7592-520: The entrance to the harbor, rise the battlements of Fort San Felipe del Morro , and its lighthouse . The San Antonio Channel ( Caño de San Antonio ) lies to the southeast, where the island of Old San Juan connects to the mainland through Santurce , by three bridges: Dos Hermanos ( Ashford Avenue ), G. Esteves ( Ponce de León Avenue ) and San Antonio ( Fernández Juncos Avenue ). The city is characterized by its narrow, blue cobblestone streets, and flat-roofed brick and stone buildings, which date back to

7696-470: The executive mansion for the Governor of Puerto Rico . Many of the historic fortifications are part of San Juan National Historic Site . Old San Juan's land area roughly correlates to the following subbarrios of barrio San Juan Antiguo: Ballajá, Catedral, Marina, Mercado, San Cristóbal, and San Francisco. Old San Juan is the location for one of two transportation hubs serving Metropolitan San Juan. At one time,

7800-508: The exterior and interior, with rubble sandwiched in between. The Santa Bárbara battery became the main battery with 37 cannon, supported by casemate guns at a lower level, and backed by a great wall with casemates at a higher level. The top of the Castillo de San Felipe del Morro Lighthouse was destroyed during the 1898 bombardment of the city by the United States, and the American flag replaced

7904-399: The first refers to the grains and the second to the cement. For example, a limestone consisting mainly of ooids, with a crystalline matrix, would be termed an oosparite. It is helpful to have a petrographic microscope when using the Folk scheme, because it is easier to determine the components present in each sample. Robert J. Dunham published his system for limestone in 1962. It focuses on

8008-502: The following: Limestone Limestone ( calcium carbonate CaCO 3 ) is a type of carbonate sedimentary rock which is the main source of the material lime . It is composed mostly of the minerals calcite and aragonite , which are different crystal forms of CaCO 3 . Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as

8112-572: The formation of distinctive minerals from the silica and clay present in the original limestone. Two major classification schemes, the Folk and Dunham, are used for identifying the types of carbonate rocks collectively known as limestone. Robert L. Folk developed a classification system that places primary emphasis on the detailed composition of grains and interstitial material in carbonate rocks . Based on composition, there are three main components: allochems (grains), matrix (mostly micrite), and cement (sparite). The Folk system uses two-part names;

8216-429: The geologic record are called bioherms . Many are rich in fossils, but most lack any connected organic framework like that seen in modern reefs. The fossil remains are present as separate fragments embedded in ample mud matrix. Much of the sedimentation shows indications of occurring in the intertidal or supratidal zones, suggesting sediments rapidly fill available accommodation space in the shelf or platform. Deposition

8320-519: The island emulating George Clifford's overland invasion. To the amazement of the citizens, the invaders were able to pass in front of the castle's defenders and into the harbor, out of reach of the city's cannons. El Morro managed to resist the siege and eventually made the Dutch retire, although the attackers were able to sack and burn the city before leaving the Battle of San Juan (1625) . In 1797, British General Ralph Abercromby and Admiral Henry Harvey, with

8424-402: The late 1940s, disrepair in the old city was evident. The local authorities were considering development proposals for renovating the old city and incorporating modern architecture on new constructions. Anthropologist Ricardo Alegría vehemently advised against the idea of demolishing old colonial buildings in favor of contemporary building designs. He followed the example suggested by his father,

8528-467: The luxury Hotel El Convento . The Bank provided low interest loans to remodelers, and the government gave triple tax exemptions to commercial activities in the old city (they were exempted for a limited time, not only from property taxes and municipal taxes, but from income tax from activities generated in their properties as well). At least one retailer from the shopping district in Charlotte Amalie

8632-629: The mid-20th century as part of rat eradication programs. Following concerns over an increase in their population, in 2023 the National Park Service announced it would contract an animal welfare organization to remove the cats and threatened to hire a pest control agency if it were to fail. However, the plan was suspended following a lawsuit by animal welfare groups in 2024. Some other forts in San Juan: Old San Juan, Puerto Rico Old San Juan ( Spanish : Viejo San Juan )

8736-424: The mid-eighteenth century. La Fortaleza was built between 1533 and 1540, which was followed by the construction of a battery at "the Morro." Plans for the castle portion of San Felipe del Morro were made in 1584. Extensions to the Morro, plus construction of El Cañuelo , and El Boquerón , were made between 1599 and 1609. Circumvallation of the city commenced in 1630 and was concluded by 1641. San Cristobal fort

8840-459: The north coast, about 35 miles (56 km) from the east end of Puerto Rico, and is connected to the mainland of Puerto Rico by three bridges. It is bounded by the Atlantic Ocean to the north and to the south by San Juan Bay ( Bahia de San Juan )—which lies between the city and the mainland. On a promontory ( morro ) about 100 feet (30 m) high, at the west end of the island and commanding

8944-410: The ocean basins, but limestone is rarely preserved in continental slope and deep sea environments. The best environments for deposition are warm waters, which have both a high organic productivity and increased saturation of calcium carbonate due to lower concentrations of dissolved carbon dioxide. Modern limestone deposits are almost always in areas with very little silica-rich sedimentation, reflected in

9048-426: The reaction: Fossils are often preserved in exquisite detail as chert. Cementing takes place rapidly in carbonate sediments, typically within less than a million years of deposition. Some cementing occurs while the sediments are still under water, forming hardgrounds . Cementing accelerates after the retreat of the sea from the depositional environment, as rainwater infiltrates the sediment beds, often within just

9152-482: The relative purity of most limestones. Reef organisms are destroyed by muddy, brackish river water, and carbonate grains are ground down by much harder silicate grains. Unlike clastic sedimentary rock, limestone is produced almost entirely from sediments originating at or near the place of deposition. Limestone formations tend to show abrupt changes in thickness. Large moundlike features in a limestone formation are interpreted as ancient reefs , which when they appear in

9256-604: The reluctance of commercial banks to fund remodeling). Under combined efforts by the institute and the Government Development Bank for Puerto Rico, "model" remodelings were made to show potential property owners how their renewed properties could look. Strict remodeling codes were implemented to prevent new constructions from affecting the common colonial Spanish architectural themes of the old city. As with other Operation Bootstrap projects, mainland corporate investors were lured in with tax breaks and other incentives. When

9360-460: The rock. The Dunham scheme is more useful for hand samples because it is based on texture, not the grains in the sample. A revised classification was proposed by Wright (1992). It adds some diagenetic patterns to the classification scheme. Travertine is a term applied to calcium carbonate deposits formed in freshwater environments, particularly waterfalls , cascades and hot springs . Such deposits are typically massive, dense, and banded. When

9464-479: The sediments increases. Chemical compaction takes place by pressure solution of the sediments. This process dissolves minerals from points of contact between grains and redeposits it in pore space, reducing the porosity of the limestone from an initial high value of 40% to 80% to less than 10%. Pressure solution produces distinctive stylolites , irregular surfaces within the limestone at which silica-rich sediments accumulate. These may reflect dissolution and loss of

9568-662: The sediments of the ocean floor abruptly transition from carbonate ooze rich in foraminifera and coccolith remains ( Globigerina ooze) to silicic mud lacking carbonates. In rare cases, turbidites or other silica-rich sediments bury and preserve benthic (deep ocean) carbonate deposits. Ancient benthic limestones are microcrystalline and are identified by their tectonic setting. Fossils typically are foraminifera and coccoliths. No pre-Jurassic benthic limestones are known, probably because carbonate-shelled plankton had not yet evolved. Limestones also form in freshwater environments. These limestones are not unlike marine limestone, but have

9672-533: The water by photosynthesis and thereby decreasing the solubility of calcium carbonate. Limestone shows the same range of sedimentary structures found in other sedimentary rocks. However, finer structures, such as lamination , are often destroyed by the burrowing activities of organisms ( bioturbation ). Fine lamination is characteristic of limestone formed in playa lakes , which lack the burrowing organisms. Limestones also show distinctive features such as geopetal structures , which form when curved shells settle to

9776-553: The water. Although ooids likely form through purely inorganic processes, the bulk of CaCO 3 precipitation in the oceans is the result of biological activity. Much of this takes place on carbonate platforms . The origin of carbonate mud, and the processes by which it is converted to micrite, continue to be a subject of research. Modern carbonate mud is composed mostly of aragonite needles around 5 μm (0.20 mils) in length. Needles of this shape and composition are produced by calcareous algae such as Penicillus , making this

9880-414: Was a guardhouse. A pair of batteries behind the hornwork overlooked the sea and harbor. In 1634, construction of the city walls surrounding San Juan began. By 1650, the town was enclosed on the east, south and west, while natural battlements protected the city along the Atlantic. In 1765, Alejandro O'Reilly , Inspector General of Cuba, and Colonel Tomás O'Daly, San Juan Chief of Engineers, agreed on

9984-481: Was added, and the newer settlement was given its formal name of "San Juan Bautista de Puerto Rico", following the usual custom of christening the town with both its formal name and the name which Christopher Columbus had originally given the islands, honoring John the Baptist . Constructed in 1521, Casa Blanca served as the first fortification of the settlement and residence of Juan Ponce de León descendants, until

10088-479: Was annexed. The oldest parts of the district of Old San Juan remain partly enclosed by massive walls. Several defensive structures and notable forts, such as the Fort San Felipe del Morro , Castle of San Cristóbal , and el Palacio de Santa Catalina (also known as La Fortaleza ), acted as the primary defense of the settlement which has been subjected to numerous attacks. La Fortaleza continues to serve also as

10192-406: Was built under the direction of conquistador Diego Ramos de Orozco and its main purpose was to defend the port of San Juan by controlling the entry to its harbor. In order to have a viable defense while the rest of the fort was being completed, a small proto-fortress was erected during the first year of construction. It is estimated that this section comprises about 10% of the whole structure. It

10296-404: Was completed by 1771. By 1776, the population totaled 6000, half of which were soldiers. The garrison town of San Juan included 250 acres of military installations and 62 acres of public and private use. By 1781, the city's fortifications included 376 cannon. By 1876, 24,000 lived inside the walls of San Juan, encompassing 62 acres, and 926 buildings. Prior to the 19th century, the area outside

10400-479: Was established in 1949 to preserve historic fortifications in Old San Juan and the National Register of Historic Places listed it in 1966. Both that site and La Fortaleza ( English : The Fortress ) were then declared a single World Heritage Site by UNESCO in 1983. In 1951, the broader area within the old city walls became the first listing of the Puerto Rico Register of Historic Sites and Zones as

10504-731: Was given incentives to set up shop in the Calle del Cristo (the city's red light district during the early 1940s) in order to persuade other retailers to follow suit and somehow "clean up" the district. Potential developers were offered sketches of their properties after a remodeling, as to suggest which architectural elements to improve or change. The paradigm to reconstruct and renovate the old city and revitalize it has been followed by other cities in Latin America, particularly Havana , Lima and Cartagena de Indias . The San Juan National Historic Site ( Spanish : Sitio Histórico Nacional de San Juan )

10608-416: Was likely deposited in pore space between grains, suggesting a high-energy depositional environment that removed carbonate mud. Recrystallized sparite is not diagnostic of depositional environment. Limestone outcrops are recognized in the field by their softness (calcite and aragonite both have a Mohs hardness of less than 4, well below common silicate minerals) and because limestone bubbles vigorously when

10712-530: Was not till 1587, however, that Field Marshal Juan de Tejeda and the Italian engineer-architect Juan Bautista Antonelli drew the fort's final design. The plan, which was based on the then firmly established Spanish military fortification principles of the time, included fortifying nine other sites in the Spanish Main and Spanish West Indies : Santo Domingo , Santa Marta , Cartagena , Nombre de Dios , Portobelo ,

10816-409: Was then designated a National Historic Landmark District in 2013. La Perla (Spanish for the pearl ) is a former shanty town or informal settlement located along the northern section of the historic San Juan city walls. Originally the site of slaughterhouses, the area was later settled by freed slaves and non-white servants in the 18th century. In the 20th century the community grew to become

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