Introduction

To study the geological structure of India, it is important to understand a number of points including the character and types of slopes and rocks, availability of minerals, the chemical and physical properties of soils, availability of water resources below the ground, the surface type, etc. Understanding about the geological time scale is also important.

The Geological Timescale

When did life begin? Dinosaurs did exist we know, but when did they become extinct? Birds appeared on the earth, but when? A numbers of questions like these and more are answered by geologists. ‘The Geological Time Scale’ is a special timeline used by geologists.  Geologic history of the Earth is recorded by the GTS. This system is based on evidence of early life conserved in layers of rocks.

The geological timescale is fragmented into smaller and larger subdivisions. This is helpful in getting an idea about how the different events in history fall in place together. Hours in a day are equal, but not the geological time intervals. In the Earth’s history, significant events are used to divide geologic time. Time intervals instead are variable in length.

The Geological History Of India

  • The geological history of India includes a number of major events.
  • It was from a section of an old landmass, that Peninsular India came into existence, right from the time the Earth’s Crust was formed.
  • Turbulence in the Himalayas during the tertiary period
  • Increase in the level of land during the Pleistocene period in the Indo-Gangetic plain. Sedimentation takes place in the lower region of the Gangetic plain and in the river floodplains till today.
  • On the basis of this varied and complex geological history, the rock systems in India have been classified by the Geological Survey of India into 4 main divisions.

India’s Geological Structure

1.Gondwanaland was an ancient continent divided into a number of plates. India is a part of Gondwanaland. One of the plates called the Indo-Australian plate advanced towards the north and crashed into the Euro-Asian plate, leading to the formation of the Himalayas, in the present day times.

  1. A basin was formed towards the southern part of the Himalayas. Flowing rivers brought alluvium, filling the basins to form plains in Northern India.
  2. The plateau was affected by 2 main events during the formation of the Himalayas. A huge volcanic eruption took place in the plateau’s north western part. While the Indian Ocean moved on to occupy the depression, the western part subsided. This led to the formation of the Arabian Sea and the Bay of Bengal.

India’s geology is diverse. In India, various regions contain some altered and some very deformed rocks that are part of various geologic periods which date back as far as the Eoarchean Era. Recently alluvium has been deposited which has undergone chemical and physical changes during the conversion process of sediment to sedimentary rock. Huge quantities of mineral deposits are found in the Indian continent. Included in the impressive fossil record of India are plant fossils, stromatolites, vertebrates and invertebrates. Classification of the geographical land area of India can be made into the Vindhyan, Gondwana and the Deccan Traps.

A major part of Maharashtra besides a marginal part of Andhra Pradesh, Gujarat, Madhya Pradesh and Karnataka is covered by the Deccan Traps.

2.The Archaean Formations (Pre-Cambrian)

In the history of the Earth, earliest part is the Precambrian, which was set before the current Phanerozoic Eon. Since the Precambrian anteceded the Cambrian, it has been named so. The Phanerozoic Eon’s first period is the Cambrian, named after Cambria. The Latinized name given to Wales was Cambria, from where rocks of that age were studied for the first time. Around 88 percent of the geologic time of the earth is accounted by the Precambrian. In the geologic time, the Precambrian is segregated into three eons of the geologic time scale called the Proterozoic, Archean, Hadean and Archean. The Earth was formed about 4.6 billion years ago. The geologic time ranges from this period up to the beginning of the Cambrian period which was about 541 million years ago. Creatures with hard shells appeared in huge numbers millions of years ago.

Though 7/8th of the history of the Earth is made up by the Precambrian Period, not much is known about it. However from the year 1960 onwards it has been largely discovered. As compared to the succeeding Phanerozoic, fossil record of the Precambrian is not very good. Besides this, fossils from the stromatolites or the Precambrian were used on limited scale. The reason behind this was heavy metamorphosis of the Precambrian rocks that concealed their origins. Erosion destroyed a number of these rocks besides which many remained buried deep inside the Phanerozoic strata.

It is a belief that the Earth mingled with material that was orbiting around the Sun at 4,543 Ma roughly. Shortly after its formation, a large planetesimal almost the size of Mars may have struck it. The Moon was formed out of material split off from the large planetesimal. By 4433 Ma, apparently a stable crust was in place. The International Commission on Stratigraphy recognizes the Precambrian term as the only ‘supereon’ in geologic time, as it includes the Proterozoic, Archean and Hadean. Phanerozoic, is the only other eon.

Many paleontologists and geologists still use the term Precambrian for general discussions.

Near the end of the Precambrian are a number of landmass positions including the well preserved microscopic fossils in Western Australia, carbon in rocks from islands off western Greenland, multi-cellular life forms in India and North America, soft bodied forms, hard shelled creatures, fauna, life forms etc in a diverse collection in different locations across the world. Cambrian explosion of life took place during the early Cambrian where diversity of life-forms increased. Land areas got devoid of cyanobacteria, animals, plants and other microbes thus forming prokaryotic mats to cover terrestrial regions. 551 million year old tracks of an animal having leg like appendages have been identified in mud as well.

The Oxygen And Environment Catastrophe On The Planet

A general observation indicates that before 4280 Ma, small proto-continents existed. Around 1130 Ma, many landmasses on the Earth gathered into one single supercontinent. Around 750 Ma, the Rodinia supercontinent, broke up. Many glacial periods, dating far back roughly 2400-2100 Ma, as the Huronian epoch have also been identified. The Sturtian-Varangian glaciations from 850-635 Ma have been one of the best studied landmass. It is believed that it is one main reason to bring glacial conditions towards the equator, giving rise to a “Snowball Earth”. Not much has been understood about the Earth’s atmosphere by most geologists. They believe that the Earth was deficient in free oxygen and comprised relatively inert gases, carbon dioxide and nitrogen. However, evidence is available that since the early Archean, there existed an atmosphere that was oxygen enriched.

It is a belief still that till the time photosynthetic forms developed and started producing molecular oxygen in huge quantities as their metabolism byproduct, it was not a significant fraction of the atmosphere of the Earth. An ecological crisis took place with the radical shift to an oxidizing atmosphere, to a chemically inert atmosphere, termed as the oxygen catastrophe, evidence of which is found in older rocks containing huge banded iron formations.

3.Dharwar System (Proterozoic Formations)

The geologic time of the Dharwar System expands from 2500 million years to 1800 million years ago. In the Geological Time Scale of India, the Dharwar System was one of the first systems of metamorphosed sedimentary rocks. It was in Karnataka’s Dharwar district that for the first time, these rocks in India composed of mainly igneous gneisses, schists and debris, were studied. Right from the Madurai and Nilgiris districts of Tamil Nadu and the Bellary and Dharwar districts of Karnataka, these Dharwar rocks have been found in scattered patches. Besides this, they are found in the Himalayan region, the Aravallis from Delhi to the south of Alwar, eastern and central parts of the Chotanagpur Plateau, the Mikir Hills and the Meghalaya Plateau. The highly metalliferous rocks are rich in precious stones, iron ore, nickel, manganese, tungsten, lead, copper, zinc, mica, gold, dolomite, silver and building materials. In the Dharwar System there are a number of important series including the Champion Series, the Champaner Series, Closepet Series, Chilpi Series, Iron-Ore Series and the Khondolite Series.

  • The Champion Series gets its name after the Champion reef in the Kolar Gold fields and belong to the Dharwarian System. This Series stretches from Karnataka’s Raichur and Kolar districts and lies to east of Bangalore and the north-east of Mysore City. Gold mines of this series are 3.5 km deep and one of the deepest across the globe. Per tonne of ore contains gold to about 5.5. grams.
  • An outlier of the Aravallis System is the Champaner Series found in the vicinity of Vadodra. It consists of marbles, quartzites, slates, phyllites and conglomerates. A marble variety in attractive green is obtained from this series.
  • The Closepet Series stretches over the Chhindwara and Balaghat districts of Madhya Pradesh. Magniferous, copper pyrite and quartzite rocks are contained in this Dharwarian formation. Ore required by the Malanjkhand Copper Plant is obtained from the Closepet series.
  • The Chilpi Series occupies parts of Chhindwara and Balaghat districts in Madhya Pradesh. This series consists of magniferous rocks, grit, green stones, quartzites and phyllite.
  • The Iron-Ore Series extends to about 65 kilometers in the form of a range in Keonjhar, Singhbhum, Mayurbhanj and Bonai. It reserves around 3000 million tons of iron-ore.
  • The Khondolite Series occupies a huge area from the northern extremiry in the Eastern Ghats upto the valley of Krishna. The gneisses, khondolites, charnockites and kodurites are the principal types of rocks in this series.
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Dharwar is a term obtained from Karnataka’s Dharwad District, where rocks were discovered for the first time. When denudation of the spotless Archaean schist and gneisses took place, the resulting sediments led to the formation of the rocks. The para-schists and para-gneisses rocks overlie the Achaean rocks. However in other areas they are interbedded on a large scale with gneisses. In certain cases, they are even older as compared to the schists. Tilting, faulting, folding and other tectonic activities have deformed these rocks greatly and hence they are not found in a horizontal manner which is their original form. The dolerite dykes and granitic bosses have intruded these rocks at an extensive scale. Besides losing traces of their sedimentary nature they have developed a schistose and crystalline structure making it difficult to differentiate them from the underlying schists and gneisses.

 

4.The Cuddapah System (The Purana Group)

An ancient sedimentary strata, is represented by the Cuddapah system with a thickness of approximately 6000 meters and many unconformities. It is after the Cuddapah District of Andhra Pradesh that it derives its name. The rocks occur in the Kumool and Cuddapah district of the state and cover an area of about 35,100 square km, in a semi-circular basin. These rocks include unfossiliferous limestone, sandstone, quartzite, slate and shale.

Before the rock system next to the Peninsular gneisses and the Dharwars started being deposited, a long interval of time had passed.These rocks have been deposited in the synclinal basins, Cuddapah depression, major parts of the (N-S-D) Narmada-Son-Damodar graven, Godavari graven, from the Cambrian period to the pre-Algonikan period. The rocks have been deficient in fossils and have been metamorphosed greatly. The Upper Cuddapah and the lower Cuddapah are the two rock formations in the Cuddapah system.

Rocks of the Cuddapah age include 3 major outcrops, including the Cuddapah basin of Madhya Pradesh, the Cuddapah basin of Andhra Pradesh and the Delhi System.

In the Cuddapah district of Andhra Pradesh a number of outcrops are best observed. Ores of nickel, iron, cobalt, copper, manganese etc are present in these rocks. They contain cement grade limestones in huge deposits. An outcrop in irregular concave crescent shape covering approximately 45000 sq.km area is seen in the Kurnool district of Andhra Pradesh. Besides this, outcrops like these are also seen in Karnataka and the southern parts of Chhattisgarh covering districts of Raipur, Dantewara, Rajnandgaon, Bastar, Durg, Dhamtari and Kanker. Isolated exposures in few numbers occur in Keonjihar and Kalahandi districts of Orissa, in Singhbhum district of Jharkhand and main axis of the Aravalli range from Gujarat to Delhi aggregating to a thickness of 5500 meters also at some places. Economic significance of this system lies in one fact that the rocks contain large deposits of cherts, iron, stealite, copper, asbestos, copper, jasper, cobalt, barites and nickel. Besides this, cement grade limestones and building purpose quartzites.

In the Delhi System a belt is formed by the outcrops of rocks. Starting from Delhi, the belt has developed through Mewar and Ajmer in the north to Palanpur in the South and well developed along the main axis of folding of the Aravali Mountains.  At many places the rocks are present in an unconformable manner over the Archean rocks. At some places, the younger in age rocks of the Vindhyan system, overlay the rocks of the Delhi System.

Rich in metamorphic rocks, the Cudappah System rocks also contain iron and minerals and ores in inferior quality. The Papaghani series is one important series named after a river in Andhra Pradesh, having a similar name.

Known as Cuddapah formerly, Kadapa is a city in the south-central part of Andhra Pradesh. Cuddapah is well known as the ‘Heart of Rayalaseema’.  In its history, this city had a range of rulers including the Cholas, Nizams, the Kingdom of Mysore and the Vijayanagara Empire.

In the Cuddapah rock system there are some important economic minerals. At Kurnool, Anantpur and Cuddapah are found the Asbestos and Barite of Papaghani series. Extensive use of road-stones and building stones is made of quartzitiers of the Delhi system.

5.The Vindhyan System

The Vindhyan System forms a dividing line between the Deccan and the Ganga Plain. Its name is derived from the Vindhya Hills. The rocks here are very noticeably exposed. Right from Chittaurgarh in Rajasthan to Sasaram in Bihar, a huge area of over 103,600 square km is covered by the Vindhyan System. Limestones, shales, sandstones, shallow-water sediments and other sedimentary rocks, 4270 m. thick cover these regions in huge deposits. The rocks are buried below the Deccan lava at a number of places. The epeirogenic upheaval of shallow-water sediments deposited over the uneven and denuded surface of the Cuddapah rocks are represented by the Vindhyan System. The period of formation took a long time with a marked interval between the upper and lower Vindhyans. Rise of the Aravallis created unconformity in a distinct line, in addition.

The fine textured, medium and coarse limestones, shales and sandstones are the main rocks of the Vindhyan system. While the upper rocks contain sandstones of estuarine and fluviatile origin, the lower Vindhyan rocks mainly of marine origin, are argillaceous and calcareous. Igneous matter is depicted in extrusions in the form of tuff and hyalites in the lower Vindhyan, while in the upper Vindhyan, the sandstones contain two layers of diamondiferous conglomerates. The original horizontality is retained by rocks of the Vindhyan system excluding narrow areas of south-east Aravallis where some folding exists. No signs of metamorphism are seen due to which original characteristics of sedimentary rocks like sandstones, limestones and shales remains retained.

Shallow water formation is proved by the current-bedding, sun-cracked surfaces and ripple marks exhibited by the rocks. A 800 km major fault called the Great Boundary Fault, is seen in the junction-region of the Vindhyan rocks with the older Aravallis extending from Khed-Brahma in Maharashtra to Bewar in Rajasthan.

In general, the Vindhyan System is unfossiliferous. At the base of the Kaimur series, carbonaceous shales are present which indicates that during the Vindhyan times, plant life did exist. Here 1-3 mm of certain fossil organisms, horny discs, fucoid markings, etc have been found in the sun-cracked and ripple marked shales and sandstones and black Sucket shales.

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Rocks in the Vindhyan system bear out diamonds, precious stones, materials for building purposes, ornamental stones and raw materials used in chemical, glass, lime and cement industries. Manganese and iron ore in inferior quality is yielded by these rocks at certain places. Vindhyan conglomerates are popularly associated with diamond mining areas in Panna and Golconda in India. Vindhyan sandstones are used in building a number of historic buildings like the Jama Masjid, the Buddhist Stupa at Sarnath and the Red Fort of Delhi. Extensive use of limestone, from this system, is made in lime making and Portland cement manufacture in the region. Coarser sandstones are utilized as millstones and grindstones.

6.The Palaeozoic Group (Cambrian to Carboniferous Period)

The Paleozoic Era is also spelled as the Palaeozoic Era.  The word Paleozoic comes from a Greek word meaning, ancient life. It has been the longest of the Phanerozoic eras, a time of dramatic evolutionary, climatic and geological change. It lasted 541 to 251.902 million years ago and divided into 6 subdivisions or geologic periods, with the oldest starting from the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and the youngest being the Permian. This era began with marine animals in an extraordinary diversification and ended with the end-Permian extinction, around 252 million years back. In the history of the Earth, the end-Permian extinction was the greatest one.

The Paleozoic Era

Earliest Paleozoic animals like simple fungi and related fungi lived in the sea and freshwater environments. However no evidence is in fossil records about these life modes. Life forms existed in their simplest forms in barren terrestrial environment.

The evolution rate increased suddenly and sharply with the Cambrian explosion. At the onset of the Cambrian Period, around 541 million years ago, more than 35 new animal phyla resulted due to the intense diversification. New discoveries however indicate that the explosion began with the Ediacara fauna, approximately 574 million years ago towards the end of the Proterozoic Eon. All throughout the Ordovician and Cambrian periods the life forms started adapting to all kinds of marine environments, diversifying the biota rapidly. Cambrian rocks have been dominated by fossils of trilobites while lampshells (brachiopods) are predominant in strata right from the Ordovician through the Permian Period.

During the mid-Paleozoic Period, many organisms of different type adapted to life on land, independently. By Silurian time, invertebrate animals and leafless vascular plants established themselves on land. During Devonian times, transition was made by vertebrate animals, to land via the amphibian evolution from crossopterygian fish that breathed air. During the Carboniferous Period evolution of animals and plants took place, gradually overcoming their dependency for reproduction on moist environments. Seeds in plants with origin of seed-fern got replaced by waterborne spores and emniote eggs replaced shell-less eggs. Insects developed wings in the Carboniferous Period.

When seen on a global level the Paleozoic period was a time when the continents assembled. A supercontinent, Gondwana was formed with the gathering of a majority of Cambrian landmasses. This present day supercontinent is made up of continents of Antarctica, Africa, Australia, South America and the Indian subcontinent. It extended from the southern polar region to the northern tropics. The global Panthalassic Ocean covered the remaining part of the Earth, excluding 3 main cratons (landmasses that formed the sturdy and solid interiors of the continents)

7.The Mesozoic Era (The Gondwana System)

The oldest geological features and bodies are seen in India. As far as the geological time scale is concerned, this country has unique structural and geological conditions of almost every age. Mineral fuels, masses of rocks, mineral deposits, oil and coal resources are seen in India. India’s geology on different materials on Earth, has been understood, thanks to geological exploration and investigations that took place since the past 100 years.

The Archean System, Dharwar System, Cudapah System, Vindhyan System, Paleozoic Period, Mesozoic Period, Gondwana Period, Deccan Trap, Tertiary and Alluvial are the several divisions into which India’s stratigraphy is divided into. One of the Eras is the Mesozoic Era which includes systems between 245 and 66 million years.

The supercontinent of the ancient times, Gondwanaland also called Gondwana is incorporated by Antarctica, South America, Australia, Africa, India, Madagascar and Arabia of present day times. This huge supercontinent centered over the South Pole, including smaller pieces of the Malay Peninsula, Florida, Tibet, Southern Europe, Afghanistan, Turkey and Iran also.

Around 600 million years ago, the Late Precambrian time, fully assembled it. About 180 million years back, in the Early Jurassic Period, began the first phase of its breakup. Eduard Suess, a geologist from Austrian coined the name Gondwanaland. The name had reference to the Upper Mesozoic and Paleozoic formations in region of Gondwana in India which had great similarity to formations of the same on continents in the Southern Hemisphere.

When the maps of the New World and Africa became available in 1620, it was Francis Bacon who first noted that the coastline shapes of eastern South America and Western Africa, matched.  In 1912, a German meteorologist, Alfred Wegener set forth a concept that at one point of time the Southern Hemisphere continents were all joined. This supercontinent’s southern half comprised Gondwana, a part of Pangaea, a great solo landmass, which he had envisioned.

In 1937, a South African geologist, Alexander Du Toit expanded the Gondwana concept in his book ‘Our Wandering Continents’, in which many paleontological and geologic lines of evidence were documented carefully. Occurrence of fauna and flora not found in the Northern Hemisphere, the seed fern (widely distributed) and occurrence of tillites – glacial deposits of the Permo-Carboniferous age were included in the evidence. This evidence is contained in the rock strata known as the Santa Catharina System in South America, the Karoo System in South Africa and the Gondwana System in India. Besides this, evidence also occurs in Antarctica’s Polarstar and Whiteout conglomerate formations and in the Eastern Australia’s Maitland Group.

Though scientists from the Southern Hemisphere widely accepted the Gondwana concept, until the 1960s the intention of continental mobility was resisted by scientists in the Northern Hemisphere. It was during this time when the plate tectonics theory indicated that ocean basins are not global features of the permanent kind, thus absolving the continental drift hypothesis of Wegener.

8.The Cretaceous System (The Deccan Trap)

The Cretaceous System is made up of rocks that were either formed or deposited in the Cretaceous Period. The two rock series of this system include the Lower and Upper series. The Early Cretaceous Epoch corresponds to time units (145 million to 100.5 million years ago) while the Late Cretaceous Epoch corresponds to units of time (100.5 million to 66 million years ago).

In turn, both the Early and the Late Cretaceous epoch are divided into 6 ages of invariable length. Geologists while working in Switzerland, Netherlands, Belgium and France, during the mid-to-late 1800s initiated their definition. Twelve corresponding rock stages were first recognized and then named. Depending upon fossils, sediments and rocks found in specific locality, each stage was accordingly defined. For example for the Lower Cretaceous Series the stages include the Berriasian Stage, Valanginian Stage, Hauterivian Stage, Barremian Stage, Aptian Stage and the Albian Stage. For the Upper Cretaceous Series the stages include the Cenomanian Stage, Turonian Stage, Coniacian Stage, Santonian Stage, Campanian Stage and Maastrichtian Stage. While the shortest stage was the Santonian lasting below 3 million years, the Aptian was the longest one lasting approximately 12 million years.

Many forces interplayed resulting in the existence and distribution of Cretaceous rocks. Some of the forces include sea level relative to the continental landmasses, position of the continental landmasses, local orogenic and tectonic activity, source material availability, volcanic activity, climatic conditions, the history of sediments and rocks after deposition or intrusion and remains of marine plants and animals. During the Cretaceous, the plate tectonics of particular regions were active especially in the Pacific margin of Canada and Japan. For example in Japan from north to south, variations in sedimentary record are seen from island to island. Much of the region in the Pacific margin of Canada has been uplifted to approximately 2600 to 6600 feet. On the other hand, lime-stones and chalks in North America’s western interior were deposited under the water when levels of the sea were at their highest.  Since their deposition, erosion of many Cretaceous sedimentary rocks has taken place, while others are under water and covered by younger sediments, or both.

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As far as the western interior of North America is concerned, the rock record compared reveals deposition of chalk in eastern England right from Cenomanian time to Maastrichtian time. However, marine limestone and chalks are restricted to late Cenomanian through early Santonian time in North America. Yet, inundation histories of both areas are almost the same. During the Late Cretaceous, land regions of western Europe had few regions that were stable and represented low lying islands within a chalk sea. An arid climate indicated, evidence of sedimentary rocks. The climate would have reduced clay and sand deposits in the basic and limited erosion of these islands. In comparison the interior sea of North America received clastic sediments in plenty which eroded from new mountains along the western margin.

Cretaceous rocks have cropped out in the Atlantic coastal plains of the United States, the Arctic, the Gulf, central California, the Atlantic and Gulf plains, Greenland, Arctic, southern and central Argentina, northeastern and eastern Brazil, Caribbean islands and a number of European nations.

9.The Tertiary System (The Cenozoic Era)

The 3 main subdivisions of animal history are the Cenozoic, Paleozoic and Mesozoic Eras, out of which the Cenozoic Era or the Tertiary System, is the most recent one. It spans from the end of the Cretaceous Period, the extinction of the non-avian dinosaurs right up to the present day times, for about 65 million years. Another name for the Tertiary System is the Age of Mammals as during that time, the biggest animals on land, have been mammals. For many reasons, this is an inaccurate use of name.  First of all, long before the Cenozoic started, the history of mammals began. Besides this, during the Cenozoic, the diversity of life is far wider as compared to mammals.

In terms of accuracy, the Cenozoic Era could have been termed as the ‘Age of Birds’, ‘Age of Insects’, or ‘Age of Teleost Fish’ or the ‘Age of Flowering Plants’ with as much accuracy.

Right from 65.5 million years ago upto the present, the Cenozoic Era is divided into 3 periods including the Paleogene Period (65.5 to 23.03 million years ago), the Neogene Period (23.3 to 2.6 million years ago) and lastly the Quaternary Period (2.6 million years ago till the present times). Relatively, the 2 new terms are Neogene and Paleogene. This disapproved term is replaced now by the Tertiary Era. Subdivision of the Paleogene is made into 3 epochs: including the Paleocene (65.5 to 55.8 million years ago), the Eocene (55.8 to 33.9 million years ago) and the Oligocene (33.9 to 23.03 million years ago). Subdivision of the Neogene is made into two epochs including the Miocene (23.03 to 5.332 million years ago) and lastly the Pliocene (5.332 to 2.588 million years ago).

History related to the Tertiary is quite interesting. Giovanni Arduino, a geologist studied minerals and rock in the 1760s and 1770s in Tuscany.   He gave the term ‘Primitive’ to all volcanic rocks, including granites, unfossiliferous schists and basalts. Clay and limestone rocks rich in fossils were found on mountain flanks over the Primitive rocks, which he called as Secondary.  Lastly, sandstones and lime-stones lying over the Secondary rocks and forming foothills of mountains were termed as Tertiary by, Arduino.  This way rocks of certain types found in Tuscany were referred to as Tertiary. However later on, fossils found in the Tertiary rocks were used by geologists to recognize rocks of the same age, at other places.

In France, at Paris Basin, extensive Tertiary age rocks were found. These rocks were subdivided on their fossils, by a well known English geologist, Charles Lyell in the 1820s and 1830s. He found that rocks at the top section had a high percentage of fossils containing mollusc species while few living forms were found at the bottom section.  Accordingly he segregated the Tertiary rocks into Pliocene, the Miocene and the Eocene, the 3 sub-ages. As of today, the Pliocene rocks contain 90% of the fossil molluscs. Only 18% of the molluscs were of living species in the Miocene rocks and 9.5% in the Eocene rocks. Thus, fossil species contained within the rocks have been used to correlate subdivisions of the Tertiary, all across the world.

10.The Quaternary Period (The Pleistocene and Recent Formations)

In the geologic time scale, out of the 3 periods of the Cenozoic Era, the most recent and current is the Quaternary Period. Spanning from 2.588 ± 0.005 million years ago till the present time, this period follows the Neogene Period. The 2 epochs of the Quaternary Period are the Pleistocene (2.588 million years ago to 11.7 thousand years ago) and the Holocene (11.7 thousand years ago till date). The past 0.5-1.0 million years are informally termed as ‘Late Quaternary’.

Decay of ice sheets in the continents, cyclic growth related to the Milankovitch cycles and associated environmental and climatic changes that occurred define the Quaternary Period. The time span of glaciations classified as Pleistocene is the Quaternary and includes the Holocene, which is the current interglacial time-period. With the onset of the Northern Hemisphere glaciations, approximately 2.6 million years ago, the Quaternary period started.  gtge

As of 2018, according to the ICS – International Commission on Stratigraphy – the Quaternary period is subdivided into ‘Chibanian’, ‘Tarantian’, ‘Calabrian’ and ‘Gelasian’. The Quaternary period of 2.6 million years is represented during the time when existence of human beings was recognized. Plate tectonics caused little relative change in distributing the continents over this short time period.  As compared to earlier periods, geological records of this particular period have been persevered with lots of detailed records.

During this time period a number of major geographical changes took place. These include the appearance of the Strait of Skagerrak and Bosphorus during the glacial epochs after which the Baltic Sea and Black Sea turned into fresh water, respectively.  Flooding followed, sea levels rose, the English Channel filled up periodically, forming a land bridge between the European and Britain mainland, the Bering Strait closed forming a land bridge between North America and Asia and lastly flash-flooding of the American Northwest Scablands by glacial waters.

Since the last ice age, readjustment of the Canadian Shield has resulted to a number of major lakes in North America, the Great Lakes and the current Hudson Bay. During the course of the Quaternary time, a range of shorelines came into existence.

One of the periodic glaciations was the climate in which continental glaciers moved as far as 40 degrees latitude from the poles. At the end of the Pleistocene Epoch, large mammals became extinct in huge numbers in the Northern areas. Glyptodonts, saber-toothed cats, mastodons, mammoths and many more mammal forms became extinct across the globe. In North America, camels, horses and American cheetahs became extinct.

In 1839, Schimper coined a term ‘Quaternary Ice Age’ for this period in which glaciation took place. Glaciations began at the start of the Quaternary about 2.58 Mya and till the present day, it continues.