The total time taken for the formation of whole of the sedimentary rock formation from the Achaean

Among various methods employed for determining the age of the earth, the help of fossils preserved in sedimentary rocks is very important.

The simple observation that younger layers formed on top of older ones became the first key to the long geological time scale.

Considering the time taken for the formation of the present day sedimentary rocks as ‘standard time’, the age of first formed sedimentary formations, which were formed during a period when neither any animal or plants were leaving on the surface of the earth, named as ‘Achaean’ formation is fixed at 3000 to 4500 million years.

The rock layers formed before the Achaean formation might have been eroded but most of the layered rocks formed afterwards are fossilferous. The total time taken for the formation of whole of the sedimentary rock formation from the Achaean till the present day has been divided into four time blocks known as era.

Azoic and Proterozoic era:

Out of four time blocks mentioned above, the rocks formed during the first part of the earliest era does not preserve any fossil indicating that during that period of time life was not existing on the surface of the earth though rocks formed during the later period are found to contain fossils. The first part is known as Azoic, while the later part is known as Proterozoic, which is sometimes called as Precambrian period. The combined age of Azoic and Proterozoic is 3000 to 4000 my (million years).

Paleozoic era:

This is the second time block. It has been divided into five periods.

Cambrian:

The rocks formed during this period were studied around Wales and named after its Latin name Cambria. A variety of marine invertebrate fossils dominated by trilobites are preserved in the rocks formed during this period that experienced a temperate climate. The duration of this period is about 100 my.

Ordovician:

This period is named after a tribal community of Europe. During this period transgression of sea took place. There was an expansion in the verity of marine invertebrates which includes brachiopods, trilobites, gastropods, corals, echinoids, foraminifer, and sponges. The cephalopods and many types of mosses and algae flourished during this time period. This period continued for about 60 my.

Silurian:

This period has been named after an ancient tribe of Welsh. During this period volcanic activity occurred in many parts of the globe. Dry climate prevailed, with the development of deserts. The end of this period witnessed mountain building activities. Abundance of bony fish, coral, trilobite, mollusk, marine moss and algae are observed during this period. The time span of this period is 250 to 230 my BP (before present).

Devonian:

This period commenced about 405 my ago and lasted for 60 my. This period witnessed expansion of fishes, land plants, emergence of first land animals and primitive amphibians. Great forests of seed ferns and appearance of land plants marked the Late Devonian period.

Lower Carboniferous:

This period started some 345 my ago and lasted for about 35 my. It was a period of shallow warm seas with the abundance of corals, brachiopods and crinoids while on land amphibians continued to develop with the forests covered by land plants.

Upper Carboniferous:

This period lasted for about 30 my and witnessed large scale sea transgression, landslides, development of lowlands and great swamps. The land area was covered with thick forests crowded with very tall plants. The giant ‘dragonflies’, many types’ amphibians and primitive reptiles roamed the forests while foraminifera, coral, brachiopod, mollusk and a few trilobites are among the marine invertebrates found during this period.

Permian:

This period is named after a state in Soviet Russia. During the early part of this period, large scale natural calamity and devastation took place throughout the surface of the earth. Volcanic and mountain building activities were wide spread accompanied with change in the course of rivers and shrinking of seas. Large reptiles were dominating the land as well as small and big water bodies. The ferns of the earlier period were replaced by pine type plants. The sea was crowded with giant cephalopods along with other invertebrates. The Paleozoic era ended at the end of this period.

Mesozoic era:

The animals and plants of this era were much more evolved than those of earlier times and hence the name Mesozoic. Because of domination of reptiles, some name it as ‘era of reptiles’. This era has been divided into three period’s viz. Triassic, Jurassic and Cretaceous.

Triassic:

During this period, major part of the earth surface experienced dry climate while active volcanoes poured out hot lava in many places. Many types of new plants and animals appeared on the land and water. Reptiles, in particular, developed and established their mastery with the incoming of flying reptiles for the first time. This period began some 230 million years ago and lasted for about 49 my.

Jurassic:

Named after the Jura Mountains, this period, began 180 million years ago and lasted for about 46 my. Dinosaurs dominated land and swamps while flying reptiles were seen gliding through the air. Large ammonites together with gastropods, pelecypods and squids crowded the shallow seas. The oldest known bird and mammals were seen for the first time during this period.

Cretaceous:

This period began about 135 my ago and lasted for 70 my. It witnessed wide spread sea transgression and experienced cold and dry climate. With the appearance and spread of flowering plants, abundant food was available for the growth of mammal, bird and reptile population though dinosaurs were still dominating the surface of the earth. However, the later part of this period witnessed large scale devastation due to which most of the dinosaurs and the gigantic ammonias faced death.