EXPLAIN THE FORMATION OF LIMESTONE FEATURES

IN THE CARIBBEAN

Over 70% of Jamaica’s total land surface is made up of limestone rock. Limestone is mainly

composed of the mineral calcium carbonate. The island experiences a tropical marine climate

with rain falling most of the year, and high temperatures for most of the year. This abundance of

water, heat and calcium carbonate are the exact requirements for the chemical weathering process

of carbonation. In carbonation, rain mixes with carbon dioxide gas in the atmosphere to

produce a weak acid called carbonic acid. Limestone is a porous rock, this means it allows water

to pass through it. This acid enters the structure of limestone through its cracks, pores, joints and

bedding planes where it chemically changes the insoluble calcium carbonate minerals in

limestone to soluble calcium bicarbonate.

Surface features are those visible above ground, they include:Clints and GrikesSink holesSwallow holesPoljesResurgent streamsCockpitsTower Karst

Underground features are those found under the Earth’s surface. They include:Underground streamCavesDrip Features

The Cockpit country which stretches through the parishes of St. Ann and Trelawny, and Lluidas Vale in St. Catherine, Jamaica are known for their karst landscape. Lets look at how these landforms are created and observe some examples from the sites mentioned.

SURFACE FEATURES CLINTS AND GRIKES

Carbonic acid enters areas of weakness in

limestone (joints, cracks, pores, bedding planes)

wearing these areas down to form narrow

grooves in between the limestone pavement.

These narrow grooves are called grikes. The flat

limestone surfaces are called clints. Can you

identify the clints and grikes from the picture?

SINK HOLEClick icon to add picture

Watch the video to see how sinkholes are formed.

Question: How is water able to form a hole or void in limestone?

SURFACE FEATURESSWALLOW HOLE

A swallow hole like the one seen in

the photograph (at right) of the One

Eye River, Wallingford,Cockpit

country, Jamaica, marks the area

where a surface stream disappears in

the limestone to flow underground.

Swallow hole

POLJEThe polje such as the one seen in the photograph at Lluidas Vale, St. Catherine, Jamaica. The picture shows that a polje is a long basin with a flat floor and steep walls. Rivers brought down alluvial soil and deposited it inside the basin to form the floor of the polje. Farming is therefpre practiced on this polje since the soil is fertile.

Polje Floor

RESURGENT STREAM

Rivers which flow underground through limestone will be forced to come back up on the surface when they meet upon impermeable rock such as granite as shown in the picture of the Dornock river head in Stewart Town, Trelawny, (Cockpit country) This is the largest river rising in Jamaica and is the resurgence for two rivers which flow underground – Quashie’s river and the Cane River. It forms the source of the Rio Bueno river.

Resurgence Point

COCKPITS AND TOWER KARST

Cockpits are small rounded hills interspersed with depression (Ottley, Gentles, 2006). They are formed wherever limestone joints from a criss cross or star shaped pattern Carbonic acid then enters the limestone where the joints intersect. These areas are eroded to form solution basins. The unweathered portion of the limestone will stand out as a cockpit hill. Go to www.cockpitcountry.com/landscapeimgs/film.cc.swf to watch a movie on cockpit formation.

Overtime, continuous chemical weathering on limestone will wear away the sides so that they look almost vertical. At this stage, the cockpit is called a tower karst. Example, in Warsop, Trelawny (Cockpit country) as shown in photograph.

Tower Karst

UNDERGROUND FEATURESUNDERGROUND STREAM

Observe the map of the One Eye River on the next page. Observe where it breaks at

Wallingford, indicated by the brown arrow. That area shows where it sinks underground in the

limestone rock (also shown in the picture) to become an underground stream. It also shows the

extent of its underground journey (Source of map: UK D0OS map sheet 19959).

CAVES AND DRIP FEATURES

According to Rahil (1994) calcium carbonate in underground water flowing along the bedding planes and joints of limestone rock will dissolve the rock to form large underground passages called caves. When the roof of a cave collapses, a gorge is formed.

Carbonated water passing through cracks and pores in the roof of a cave will form drip features as shown in the photograph taken in the Swansea Caves, Lluidas Vale, St. Catherine, Jamaica.

Stalactite

Stalagmite

Carbonated water passing through cracks in the roof of a cave will dissolve the calcium carbonate

in the roof. Water droplets mixed with calcite will settle on the roof . Each droplet which hangs

from the ceiling will lose water and carbon dioxide by evaporation causing a thin layer of calcite

to be left behind. “Successive drops add a layer after layer building up the calcite (Rahil, 1999).

These deposits on the roof will build up into a long column of calcium bicarbonate which hangs

downwards called stalactites (see picture in Swansea cave).

Some of the water mixed with calcite will drip slowly from the roof to the floor. As the

water and carbon dioxide exaporates, these will build up and grow upwards forming stalagmites

as shown in the photograph in Swansea caves. Overtime, a stalactite and stalagmite will join

together since they are directly aligned from top to bottom. This will form a pillar. The picture of

the drip features show that the stalactite and stalagmite may become a pillar in the near future.

REFERENCES

Ottley, Gentles “Longman Geography for CSEC, Hodder Education, 2015 Rahil V. “New Caribbean Geography with Map Reading and CXC Questions, Eniath’s Printing Company Ltd, 1994 The One Eye River, UK D0OS map, sheet 1959