KEYS GEOLOGY

      These islands were referred to as "cayos" by the early Spanish travelers and map makers. The English generally used the word "kay" or "cay" for small islands. Somehow this became "Key" for us Americans. The first specific written use of "key" being used for our islands that I have is lawsuit for a shipwreck in 1744 which was before the Revolution. There is probably more to the derivation than this.

     The Florida Keys can be categorized based on their geology. The upper-Upper Keys or northern Keys (Miami-Dade County) are sedimentary in nature. These begin at the northern boundary with Virginia Key and pass southwestward to about Soldier Key and are generally sediments on top of a foundation of a more solid layer of rock. The surfaces of these islands are not rock-hard as the remainder of the Keys.

     From Soldier Key southward are the non sedimentary Keys classified as High Coral Keys, Low Coral Keys and the Oolite Keys. There is not a clear dividing line but the Upper and Middle Keys are of Key Largo Limestone which are the peaks of once live coral forests. These were once live, thriving and dense forests of many corals and other marine organisms - flora and fauna. As one would expect the coral forests were of various densities and heights; however, they are of less height the farther south one goes except for Key West. These have heights from 10 to 18 feet. The Low Coral Keys are from five to 10 feet. As the glaciers reformed taking water from the ocean, sea level dropped, the coral forests died and collapsed into islands we now live on.

     The Lower Keys was a huge shallow sandy and oolite shoal traversed with many channels. Oolite is compacted small egg shaped deposits of calcium carbonate and in this area lays on top of Key Largo Limestone. Big Pine Key signals the beginning of the Oolite Keys.

     Human history begins with geography and migration. Geography defines possible physical places to live, and we migrate to the geographic locations that serve our needs. Many geographic locations are the result of internal upheavals of the earth's crust, or lack thereof. The newer geological structures of the Florida Keys, however, were created by marine life. 

     I do not believe we know the complete process, but it is said that there was an ancient reef about 500 million years ago. During the Pleistocene Epoch (2-3 million years ago) shifting sands formed massive shoals over the ancient geologic ocean bottom. 

     The tops of these sand shoals were shallow enough for photosynthesis from sunlight to allow corals to grow. Corals do not significant grow is waters deeper than 40 feet - there is not enough sun light. The "corals" are carnivorous animals with hard skeletons that eat other small organisms. Actually, the coral animal is called the "polyp" and its outer skeleton is "coral." It is of the phylum Coelenterata (hollow sac) and the class Anthozoa (flower animal). These coral polyp growths formed on top of the sandy shoals. If conditions are right, corals can grow forests under water just as trees do on land. As they die the calcium carbonate skeletons accumulate as do leaves and branches of forests. Unlike trees whose leaves and branches are organic and decay (rot), the skeletons of the corals remain and form piles which when large enough become an island. At that time the water level is estimated to have been about 20 or more feet higher than it is now. This was the beginning of the not-so-ancient (today's) coral reef; albeit, under water.

     Principal reef building corals were the elk horn, stag horn, brain and star corals. They were the architects and builders. The spaces in between these large corals were filled in by many other calcium carbonate organisms and plants. These include sea fans, mollusks, corals debris, et cetera; however, the most prolific is a relatively unnoticeable green algae called "Halimeda." 

     The entire mass is solidified and cemented together by the many calcium carbonate precipitates. There are a host of other organic and inorganic calcium carbonate sources, including the sea water itself. Therefore, the deepest layer was the ancient ocean bottom with layers of sand on top of it and a new coral reef building layer by layer on top of that. 

     For some unexplained reason, about 100,000 years ago the earth's temperature began cooling and the two polar ice caps began to grow. The increased ice in the polar caps very slowly removed fresh water from the oceans. As a result, the ocean level dropped, the living reefs began to die, and, with time, became completely exposed - Voilia, the Keys. As the sea waters fell, the shoreline moved outward and more land mass was exposed. 

     The ocean level is thought to have fallen as much as 250 to 300 feet at its lowest levels. Florida's coastline, and especially the gulf coastline, would have extended outward many miles from the present coastline. One could have walked to the outer reefs and even to the Dry Tortugas area. Most of the Keys may even have been connected together. The Everglades became a vast savanna with animals such as the camel, bison and mammoth. 

     As nature's pendulum swung, about 30,000 years ago the polar caps slowly began to melt and the sea water began to rise. With the passing of time, the coral crust was again covered with water. As the sea level rose, it spilled over the outer reef from the east and filled today's Hawk Channel. Florida Bay was flooded by the Gulf from the west. New coral began to grow where conditions were right. 

     Strong water currents of tidal and storm actions washed back and forth cutting channels at lower passages. The colder, saltier and nutrient-richer Florida Bay water flowed through and impeded new coral growth wherever it touched the live corals. 

     Geologists and oceanolgists that I have spoken with estimate that the higher points of the Keys (Solares Hill, Lignum vitae Key, and Windley Key) probably were exposed to air about 50,000 years ago.

     Because the Upper Keys were higher and generally longer than the Lower Keys, there were fewer channels for water to flow through and impede coral growth. That is the reason there are more outer reefs in the Upper Keys. This effect can be easily seen on a map. The flow was so significant that the Lower Keys are aligned more north and south than the Upper Keys. It was so killing that very little coral has grown west of Key West in the last 6,000 years. 

     The fossilized layer of this coral growth is known as Key Largo Limestone, a name given by geologists for this type of limestone formation. The Upper and Middle Keys are primarily of Key Largo Limestone although it lays below the oolitic formations to the north and south. 

     Coral rock is a generalized word to define the aggregate of the "corals" and all of the other calcium carbonate-producing organisms. Splendid examples can be seen at the walls of the Marvin D. Adams Waterway, or the Windley Key Quarry (see photo). The Lower Keys and Miami-Dade area are of oolite (both are called Miami Oolite). Below the oolite is Key Largo Limestone. 

     As time passed, well defined islands evolved and mature botanical growth flourished. The water levels continued to rise and formed and reformed channels. Wind, birds and water carried in botanical life that slowly took root and grew. These plants and newly arrived animal life lived their life cycles on the islands. Rich soil was formed from the decayed organic material. 

     Hurricanes, fires (organic soil will burn) and clearing by settlers, including us, have considerably thinned the once rich soil. Some original soil has been covered by fill obtained by dredging of canals that makes it appear even less evident. 

     North American Indians, and possibly other native groups, migrated to these fertile islands. The islands are peppered with habitation midden sites waiting to be scientifically studied. A huge mound of coral rock, known locally as the Key Largo Temple Mound, and a burial mound on Lignumvitae Key beckon credentialed archaeologists to reveal their history. 

     Geologists are certain that as the polar ice melted, Hawk Channel changed from dry or damp land to swamp land and finally as it is today. Florida Bay was the Everglades of yesteryear and the Everglades area was a savanna. This means that early Paleo-Indians could have walked or waded out to the present reef some six to seven thousand years ago. If so, these early Paleo, or Archaic, Indian village sites that might have existed are now under water. In November 2002, Mel Fisher under-water archaeologists found an underwater forest near the site of of the Henrietta Maria slave ship about 35 miles off of Key West. It appears to be a burned forest of prehistoric pine land and radiocarbon cones and wood were dated as 8,400 years old. Albeit no human remains have been found, it does indicate that a habitat was available had humans been there. I predict more evidence will be discovered where there is sand to cover the artifacts.

     As a comparison date, this would have been some 3,000 years before the earliest Mesopotamian Cities that have been carbon test dated. 

     Going back to the formation of soil, it was evidently very good. The type of vegetation also had to be of the type that could anchor itself to withstand the wind and water. So prolific and mature were the trees that the Keys served as an important source for lumber in the early times. James Grant, the first English governor of East Florida, sent Captain Benjamin Barton to the Keys in 1763 to stop Bahamians from cutting large quantities of mahogany and other valuable timber.

     As late as 1849, U.S. Coast and Geodetic Surveyor F. H. Gerdes stated in his report on Key Largo: "At the lower pt. of Whalton's Cove or about 7 miles from the upper pt. of the Id. [island] is an elevation, named Basin Hill, 90 ft. high, according to Capt. Barnet and Blunt's chart. " We know that the surface elevation is about 15 feet there today. Gerdes went on to explain that he also thought the 90 feet included the trees as "the elevation does not appear to be near 90 ft." One of the Keys north of Ocean Reef is named "Palo Alto" which could be translated as "tall wood." 

     In historical times the outer and patch reefs have been growing upward, almost keeping pace with the rise in sea water. As previously stated, outer reef growth is especially evident in front of the longer Upper Keys, which have held back the coral-killing currents of Florida Bay. 

     None of these changes in sea level happened rapidly. In geologic time, 1,000 years is just a blink of the eye. Sea level is not static; it has been rising and falling since the beginning of time. For example, three recent sea level peak heights were about 120,000, 80,000 and 30,000 years ago. The one that completely covered the Keys sand shoals well enough for significant coral growth on their highest points was 120,000 years ago.

     The navy had been recording specific tide and sea levels at Key West for decades. Using that data as a reference, some experts believe that the sea level has been rising an average of about 2 feet every 1,000 years. Almost all agree that it has been rising, but others believe that the rate of rise has been less. This generally is thought to be caused by global warming due to a decrease in the ozone layer. Instead of the 8,000-year average of 3.5 inches per 100 years, in 1989 it was reported to be 15 inches per 100 years at Key West. Will the future ocean begin to fall again, or will the pendulum keep swinging and raise the water another 20 feet to inundate south Florida up to about Sebring in central Florida?

     At this rate and without other changes, in about 1,300 years the water will have risen about 16 feet (5 meters). Only one small point in Key West, Lignumvitae Key, Windley Key and a ridge on Key Largo and Plantation Key will remain dry at high tides. An argument against this extreme is that there have been smaller undulations of sea level since the large one about 30,000 years ago. These are estimated at 8,000 to 6,700; 4,300 to 3,400; 2,800 to 2,000 and 1,600 to 1,200 years before present. The argument for an extreme increase is the effect of man and the ozone layer called global warming.

  Sources: Land from the Sea by John Edward Hoffmeister, 1974; Geology of the Florida Keys by Eugene A. Shinn, Oceanus, 1988 and Paleoshorelines, Reefs, and a Rising Sea: South Florida, U.S.A. by Barbara H. Lidz and Eugene A. Shinn, 1990.