The Ariyalur district located on the busy highway between Chennai and Trichy unknown to many is a geologist’s treasure trove. In 1843, ‘strange stone objects’ were collected by the British in the Ariyalur region. Soon, geologists from all over the world descended upon this small town to discover much to their amazement a rich fossil presence and diversity.
Consequent findings of palaeontologists have been very conclusive that more than 5000 species of animals that walked this earth over 65 million years ago are found pretty much anywhere and everywhere near Ariyalur. But the question which all of us are asking is why here?
Geologists have been very enthusiastic about the sedimentary sequences of Ariyalur and Perambalur. They concluded that the sedimentary rocks of Cretaceous – Palaeocene age are well-defined in this area consisting of both clastic as well as carbonate facies. Research has shown that the limestone sediments at Ariyalur and Perambalur are of marine origin and were deposited by a rare geological phenomenon called as marine transgression.
It would appear that the sea had invaded the land between Puducherry in the north to Karaikal in the south millions of years ago and remained in that state for about 81 million years and then for reasons unknown, regressed to its present location. As the sea had covered more than 1 lakh hectares of land, it has left behind fossilized traces of marine life. The 25 km stretch with a depth of about 20 metres between Kadur and Yellakurichi villages in Ariyalur district are believed to be the deepest part of the sea.
The rock structure of Uttathur, Trichy, Ariyalur and Ninniyur were studied in depth and it was found that Uttathur had the oldest structure which meant that every era of 1000 years could be seen on the rock formations – that is when it rains, the top soil gets eroded and only the soft rock is left behind. It has also been concluded that the age and colour of the rock formations here are of the same age as those of the Himalayas.
This vast marine fossil site dating back to the Cretaceous era is spread across at least four districts of Ariyalur, Samayapuram and Kallakudi in Trichy district, Sathanur in Perambalur district and the coastal region close to Marakkanam in Cuddalore district. Ammonites (an extinct group of marine mollusk animals), big teeth of the Megalodon (a species of shark that lived approximately 23 million to 3.6 million years ago) and belemnoids (cephalopods that existed during the Jurassic and Cretaceous periods) are widely found in Ariyalur district. Even fossilized remains of shellfish, corals, clams, gastropods and branchiopods are found here.
There is also evidence of Deccan volcanic eruptions as volcanic ashes have capped many of the eggs found in the sand beds at the stream near Sendurai. This magnificent find of fossilized eggs of 65 million years ago included those of a Carnosaur (large predatory dinosaur) and a Sauropod (long-necked herbivore of enormous height and size).
There have been many reports where fossils have been found by the side of the road, in one’s backyard, during mining operations in cement factories and other manufacturing units that have been set up. There are nearly 320 types of fossils found here which are high grade limestone. The limestone, gypsum and phosphatic repositories form the primary source of raw material for a thriving cement industry. Sadly though, the industries sometimes destroy these fossils during mining operations.
It is very heartwarming to see the Government and Geological Survey of India (GSI) step up to protect the antiquity of Ariyalur and the adjoining archaeological areas of Adichanallur, Korkai, Alagankulam and Keezhadi. Museums have been commissioned to showcase the fossil wealth of Ariyalur with natural samples as well as plaster replicas and to help one understand and learn more about minerals, rocks, general geology, river geomorphology and the solar system. One doesn’t leave this ancient site without seeing the famous petrified tree that is thought to be over 120 million years having terrestrial and marine fossils embedded in it.
Written by Lakshmi Subramanian
* Photos are only symbolic (Taken from public domain/internet and any copyright infringement is unintentional and regrettable)