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1. Jamaican geology for cavers
Two thirds of the area of Jamaica are covered by cave riddled limestone and are deposited on top of the older igneous, metamorphic and clastic sedimentary rocks. The age of the limestone ranges from Paleocene to Middle Miocene (54 - 14 millions of years) and can be divided in two main stratigraphic units: the Yellow Limestone Group and the White Limestone Supergroup in which nearly all caverns are formed. Jamaica's limestone is very young compared to the situation in Belgium where the caves are formed in limestone of Devonian and Carboniferous age (408 - 320 millions of years).
Since Jamaica is located in a tectonically active zone (the Northern Caribbean Plate Boundary Zone) many landforms and also cave formation will be influenced by faulting. The island is now being gradually tilted to the S. The N coast is being pushed up above sea level, and the S drowned.
With a bit of effort cave formation on Jamaica can be split up in three distinct mechanisms:
The big river caves
In the centre of the island complete erosion of the limestone revealed the underlying cretaceous rocks as a series of stratigraphic windows (inliers). Surface drainage from these impermeable rocks sinks in the surrounding limestone and has produced most of the large river caves of Jamaica like Quashies River Cave, Coffee River Cave,...
High temperatures, plenty rain and tick tropical forests made the dissolution of limestone much faster than in our colder regions. This excessive erosion created such spectacular karst phenomena like cockpit karst and tower karst. In these regions numerous vertical shafts (sinkholes) can be found. They all end on narrow joints or chokes of mud and debris.
The horizontal caves
Large horizontal caves showing ancient solutional features suggesting the influence of sea level changes, e.g. Jackson's Bay Cave at the Portland Ridge. Off the coast of southern Jamaica, there are large areas of relatively shallow sea which were land when sea levels were about 30m lower than they are now , during the ice ages of the past million years.
Soon come: The Carbonate Island Karst Model
2. A GEOLOGICAL MONUMENT IN JAMAICA
There is a special monument devoted to the geology of Jamaica. You'll find it at the UWI Mona campus in front of the geological faculty (coordinates 18"0'22.14"N 76°44'55.38"W, here is a KMZ file which you can use in Google Earth and bring you directly to the monument.)
The monument consists of the Jamaica's main rocks and are placed in the right stratigraphic position. This means the oldest rocks form the base of the monument and are one hundered and twenty millions year old. The higher you get, the younger the age of the rocks. The scale they used to design the monument is 1 foot (or 0.3048 m) equals 12 million years. This makes the total hight of the monument 9.8 ft (or 3 meter), spanning the 120 million years.
The sides of the obelisk are made from eight different sections. Each section representing a major geological time period. The obelisk is caped with an aluminium pyramid representing the bauxite which is found in large quantities in the red earth of the island. The oldest rock are dark coloured metamorphic rocks like serpentinite and marble. The section higher contains igneous rocks like basalt, andesite, tuff, keratophyre and sedimentary rocks like conglomerate, dark limestone and sandstone. The light coloured upper parts are limestones.
3. THE RUDISTS OF JAMAICA.
Picture 1: Rudist graveyard (Titanosarcolite gorgoni) in Rudist Rock Cave.
Rudist are strange bivalves which lived in shallow tropical seas from the late Jurassic till the end of the Cretaceous. They had massive shells and where filter feeders. They are thought of as reef builders and played an important roll in the creation of carbonate sediment.
Rudist are large and attractive fossils which can be found in Jamaica. One Rudist species, the Barrettia, was discovered in 1860 by the englishman Lucas Barrett in the Back Rio Grande Valley.
Go to the links page if you want to find more information about these interesting rudists.
picture 2: this very nice section through a Barretia fossil can be found at the entrance of the geological departement on Mona Campus.
picture 3: A section through a nice Titanosarcolite gorgoni fossil extracted from Bottom Pasture Cave 2.
4. PRACTICAL GEOLOGY4.1. THE RED STRIP TEST
During the 2004 expedition we saw in the Roehampton School Cave these strong bedded layers of brown rock. This rock seemed to be more resistant to erosion than the other limestone. So what was it ? As a caver you don't cary a laboratory with you to analyse rocks so we have to be inventive.
But first let me tell you something about the hardness of rocks and minerals. What is hardness? It is the resistance of the surface of a mineral or rock to scratching. All right, and what can we do with it ? Now depending on the chemical composition, each mineral has a well determined hardness. Early in the nineteenth century there was that clever man Mr F. Mohs from Austria who made a hardness scale based on a series of ten fairly common minerals. The softest mineral got a number one and the hardest a number ten. So here we have as an amateur mineralogist a very nice tool to check what mineral or rock we have in our hands.
If for instance our unknown species of mineral can scratch calcite but can't scratch fluorite then the hardness of this specimen must be between 3 and 4. But we normally don't carry those minerals with us and do we need all those different hardnesses? No, because as a caver you will mostly see rocks based on calcite or on quartz. So we need a reference material somewhere between hardness 3 and 7. And what is very abundant throughout the island? Yes The Red Strip Bottle!! These multifunctional bottles have a hardness of 5.5!!
The different steps of the RED STRIPE TEST!!!
We took some specimens of the brown rock in Roehampton School Cave. When you break this rocks the inside is white. The brown colour is ony a very thin patina on the surface. The Red Stripe Test showed us that the mineral was harder than the bottle. Also the typical resinous luster and conchoidal fracture gives a certainty of 99.99 % that this rock is indeed flint. This mineral is a subvitreous form of quartz (silica) which has a hardness of 6.5.
Limestone which is almost pure microcristaline calcite has a hardness of 3. So doing the Red Stripe Test with a piece of limestone would give a powdry line on the bottle which we can wipe of without any scratch.
4.2. HOW DEEP IS THAT HOLE?
Lately we were confronted with a newly discovered deep pit in Jamaica with the ringing name: Smokey Hole. Big question was of course how deep it was. Eventualy we knew it by descending but it would have been handy to know it before we went in... There's an easy way: drop a stone into the abyss, count the seconds and use the next graph:
graph: depth of a pit versus the time of a falling stone (based on the formula of Hoffman J. (1985, Lumiere et Noire))
IMPORTANT REMARK: Count the seconds with your stopwatch. It's very important to count the seconds as precise as possible. Don't use twohunderdandone, twohunderedandtwo,... You will be way of! A miscount of two seconds can make a difference of 100 meters and more...
To proof that it works, look at this little movie.
Smokey Hole is now the deepest known cave of the island with a total depth of 195 m and a pit of 135 m, pushing Morgans Pond Hole (186 m) from the first place. But Morgans Pond has a pit of 150 m!