The age
of the earth is about 4 billion years according to modern geologists and
astronomers. Much information about the earth has been recovered from the
geological records enshrined in the ancient rocks and stratified formations
laid down in successive
ages as mud, sand, gravel, and shells in the beds of oceans, lakes, and rivers.
During the greater part of this incalculably long period of the Earth's
History, that small portion of the earths' land surface
constituting the Island of Sri Lanka, at the tip of the Indian Sub-Continent,
did
not form a separate geographical entity.
Towards the end of the Paleozoic Era, when plant and animal life had
undergone a long period of Evolution, the land now
comprising Sri Lanka formed part of a vast Southern Continent, to which the
name Gondwanaland has been given, and which
included the modern Indian Sub-continent, the greater part of Africa, South
America and Australia. The area of the earth now
comprising the North Indian Plain, together with the mighty Himalayan range
formed part of an ocean known as the Tethyss sea,
bordering Gondwanaland on the North, and almost encircling the earth. There
were lots of climatic changes ,with long periods of
Arctic cold temperatures followed by warmer periods with luxuriant
vegetation dominated by Pteridophyta-ferns and club mosses to
which is due the petroleum and coal deposits found in many parts of what was
once Gondwanaland. Certain forms of fauna and
flora being peculiar to Ceylon and Madagascar, and a certain degree of
similarity which Sri Lanka has with parts of Africa, the
Deccan and Western Australia in lithology, Structure etc, are to be explained
by these regions having been part of one continent
in remote geological times, before continental drift took place. The recent
discovery of large deposits of precious and semi-precious stones in
Madagascar like sapphires, rubies, tourmalines, etc. , is
additional evidence for the above phenomenon, and
clearly establishes the fact that Sri Lanka which has been famous for its
gemstones for thousands of years and Madagascar
were closely associated together in the southern continent of
Gondwanaland. Almost all the minerals and gem stones discovered
in Sri Lanka over the centuries are now gradually being discovered in
Madagascar too.
The dismemberment of the Gondwanaland continent by continental drift into
several units including the Indian Sub-continent
Southern Africa and Australia is believed to have taken place towards the
close of the Mesozoic Era, but Sri Lanka still
continued to be a part of the Indian Sub-Continent, and many more millions of
years had to elapse before it became a separate
geographical entity in the Miocene period of the Tertiary Era.
During the Miocene period, a belt of sea much wider than the Palk Strait of
today flooded the land between Tamil Nadu and
the Puttalam-Jaffna coast, thus creating the Island of Sri lanka. At the bed
of this sea, shells and other remains of marine
organisms accumulated. Subsequently fringes of this sea were brought up above
the sea-level on both sides and this led to the
formation of Miocene limestone outcropping along the north-west coast of Sri
lanka and the Jaffna Peninsula on the one side
and the Karikal region on the other.
Further evidence that Sri Lanka and Peninsular India had been one
geographical unit until geologically recent times,is
that they both stand on the same platform or shelf. The average width of this
continental shelf is about 12 miles (20 km)
around the Island, where the mean depth of water is only about 36 fathoms
(216 ft or 65 metres), and beyond which there is a
drop abruptly to 500 fathoms (3,000 ft.or 900 metres) in two miles (3.2
km.), and 1,000 fathoms (6,000 ft. or 1,800 metres) in
about 10 miles (16 km),plunging eventually to a steep descent of 3,000
fathoms (18,000 ft. or 5,400 metres) and over.In the
Palk Strait the sea is barely 15 fathoms (90 ft.or 27 metres) deep.
An event of great importance in the geological history of Sri Lanka was the
unwrap of the central massif that took place
in the post-Jurassic period, and had an immense bearing on the landscape and
the climate of the Island.
Almost nine-tenths of the Island is composed of igneous and metamorphic
rocks of Archaen or pre-Cambrian age. The igneous
rocks were formed at great depth through the solidification of molten rock
material (magma) beneath the outer shell or crust
of the Earth. The source of the present day gems both precious and
semi-precious are the igneous rocks of the pre-cambrian age.Thus almost 90% of the land surface of Sri Lanka is gem-bearing.
The igneous
and sedimentary rocks were gradually transformed
to metamorphic rocks by the action of heat, pressure, magmatic intrusions
etc. The metamorphic rocks formed were gneiss and
schists, leptynites and granulites, khondalites, quartzites and quartz-schists, crystalline
limestones, dolomite, etc.
In the Jaffna Peninsula, the adjoining islands, and the north-western coastal
strip from Kalpitiya to Mullaitivu,the
Archaen or pre-Cambrian rocks lie buried under a layer of sedimentary
limestone of the Miocene period. The surface of this
limestone region is generally flat and not much above sea-level except in
certain areas like Keerimalai where sea-cliffs of
about 50 ft.in height are formed.
