One of the most
common types of rock is Limestone. Limestone is also known as calcareous rock. It
is a sedimentary rock which is mainly made up of calcium carbonate. They were
formed in shallow seas and sometime fresh water under tropical conditions. Presently, new limestone is being formed in
calm waters off the coast of the Bahamas and in coral reefs throughout the
Caribbean. There are large areas of limestone in all the larger Caribbean
islands, also in Belize; the Bahamas are entirely made up of limestone. Limestone
can also be found in many of the smaller islands and Barbados. Most limestone
is formed from the remains of organic matter such as shells, corals and plants.
This accounts for the numerous fossils found in the stone.
Characteristics of Limestone
Limestone is a hard and permeable rock. It consists chiefly of calcium carbonate (CaCo3) which is
insoluble. Limestone areas are generally dry on the surface and for this reason they are known as karst areas. The water seep through (percolate) into it in the lines of weakness called joints or bedding
plane. Chemical weathering significantly affects limestone and as a result, many
distinctive landforms are formed. Rainfall combines with dissolved carbon
dioxide or organic acid creates a weak carbonic acid which percolates into the
rocks and dissolves it, this process is known as carbonation.
Limestone is well-joined rocks and its
joints and bedding planes become opened up by rain and water, and in time the
surface will consist of broken and rugged rocks. Limestone in tropical and
sub-tropical regions tends to show rounded landforms, known as cockpit karst, while
limestone in temperate regions is more angular and rugged. The permeability of limestone
permits rain to soak into it very easily. Joints rapidly become hollow out and
deepen resulting in limestone features.
Types of Limestone
There are several different types of limestone:
Carboniferous limestone: this grey
limestone forms upland areas in UK, such as the Pennine Hill. Is was formed
some 340 million years ago in tropical seas that were rich in shellfish and
corals.
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| Carboniferous Limestone |
Jurassic limestone: formed 120 and 150 million
years ago and can be widely found in Cuba, Jamaica and Puerto Rico.
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| Jurassic Limestone |
Oolitic limestone: these are balls of
limestone usually smaller than a pea. They are formed when small fragments of
shell or rocks are constantly rolled on the seabed and calcium carbonate dissolved
in the water may be precipitated to form limestone. This process occurs over
and over until the oolith grows to form an oolith. The oolith eventually grows
and become compressed to form oolith limestone. Oolith limestone can be found
in Turks and Caicos Island and is presently being formed on the Bahamas
Platform.
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| Oolitic Limestone |
Chalk: chalk is a white type of limestone
that was formed in the Cretaceous geological period (145-65 million years ago)
it is very rich in calcium carbonate and is commonly found in UK at Dover Cliff.
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| Chalk Limestone |
Limestone Features and Landforms
Limestone is physically tough and
resistant. It is permeable which result in a few surface rivers unless the
water table is particularly high. When rain falls, limestone absorbs the water
and the joints and bedding plane becomes enlarged. As a result, the surface in
time will become broken and rugged and most of the rain falling on the surface disappears
into the enlarged joints. Chemical weathering on limestone surfaces cause water
to dissolve leaving behind very little rock material to form soil causing the
area to be bare and rocky with little plant material.
Figure 1 below shows limestone features or
landforms that can be found on the surface and underground. Let’s look at the
surface features.
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| Figure 1: Limestone Features (Geography for CSEC; Guinness, et. al: Nelson Thornes pg75) |
Surface Landforms
One of the most noticeable features of
limestone landscape is that almost no surface drainage is visible, that is, almost
all of the water has gone underground because of the rock’s permeability. As the
rain soaks in easily, it changes the landscape forming different surface landforms
such as grikes (grykes), clints, limestone pavements, etc.
Limestone Pavements: Large areas of bare exposes limestone. Limestone pavements can be found in Pennies, UK. Long ago, advancing glaciers strip away the soil and vegetation exposing the limestone to weathering. They are also present in St Lucy, Barbados and Antigua. Different processes such as carbonation, freeze-thaw and ice action remove the horizontal bedding plane and overlaying soil exposing the limestone pavement.
In time joints in the pavement are often
enlarged by weathering to form deep cracks called grykes (grikes).
The blocks of rocks in-between the grikes are called clints.
In the past, when the water table was higher or when limestone was frozen during the ice age (UK) water would have been able to flow over the surface of the limestone. Valleys and gorges were eroded by fast flowing rivers or glacial melt water for examples, Fern Gully in Jamaica is a limestone gorge.
Since the water tables have fallen, the valleys and gorges have become dry, hence the term dry valley.
A swallow hole or sink is an enlarged joint down which water plunges or flows off an impermeable rock onto limestone.
Explore other surface features such as dolines and resurgent streams. Dolines are large depressions formed by solution or collapsed of limestone, while a resurgence stream is the emerging stream from underground.
