Explaining the stratum

Going down a steep slope into the site, we reach eight metres below the surface. The excavation will go to ten meters, but it has been temporarily suspended during monsoon. It is planned to be finished by March. 

“The excavation of the Matrimandir Lake gives us a beautiful opportunity to study the geological formations beneath our feet. You see different layers. They are a testament to the forces of wind and water that shaped the Auroville plateau over millions of years,” says Giulio. Pointing at the reddish walls around us, he explains that we are looking at a stratum called Cuddalore Sandstone, after the city of Cuddalore where this geological formation was first described. “This looks like red soil. If you go further westward in Auroville, you’ll find different soil types: yellow soil at the Visitors’ Centre, gravel at Pebble Garden, blackish clay at Annapurna, and then again reddish soil. These are different strata that formed over millions of years on top of each other, like a multilayered cake. Due to earth movements, these strata tilted, so that lower strata also came to the surface; that’s the reason why you can see different soil types next to each other, as for example, near the Visitors’ Centre and Buddha Garden. There you see the border between the Cuddalore Sandstone formation and that of the next formation, which is called Manaveli clay.”

Giulio explains the particulars of Cuddalore Sandstone by taking a bit of the red soil between his fingers; he spits on it, rubs his fingers together, and invites his audience to do the same. “What do you feel? Smooth? Grainy? Sticky? Rocky? Does it feel scratchy, like sandpaper? If so, what you have is sand. If the feeling is soapy, it’s silt. And when it’s sticky, it’s clay. This is the simple way for geologists to determine the texture of a geological formation. The difference between the three is measured in the size of the grains: sand goes between two and 0.0068 millimeter, silt is smaller, and clay even smaller.  But what we have here is mainly sand.”

The Cuddalore Sandstone formation stretches from Marakkanam north of Auroville to Kanyakumari, the southern tip of India. In Auroville, the formation has a depth of about 30 to 35 meters - we know this from the many bore wells that Auroville has dug - but in other places the stratum goes to a depth there of about 440 meters. “This is a relatively young geological formation, it was formed between 1.8 and 23 million years ago. Below it are older formations, with the oldest cropping to the surface 25 kilometres west of Auroville. That one is between 2.5 to 3.8 billion years old,” says Giulio. 

We are impressed by an ancientness none can imagine. “Over these millions of years, many things have happened. As is well-known, some 160 million years ago the Indian tectonic plate broke off from Africa and started moving towards the Asian plate. The collision that caused the Himalayas happened some 55 million years ago - that is 10 million years after the dinosaurs had disappeared. At that time, the place we are standing now was a reef.” He shows a sample of limestone, which has small shells embedded in it. “Those reefs were not made of corals as we know them today, but of shells. Those shells died out at the Cretaceous-Tertiary extinction event, when all the dinosaurs and a good 75% of all organisms on the planet died, some 65.5 million years ago. That’s how we know how old this piece of limestone is.”

But what happened in all these millions of years, we’ll never know, says Giulio. “From the boulders and gravel we find we can deduce that there once was a river here - but maybe there were many rivers in those millions of years that wandered over this area.” He points at smoothly rounded boulders and lots of small pebbles, and explains that they all were brought here by water. “When water flows, its energy can move objects. The large boulders must have been moved here from many kilometres away, perhaps from Gingee, otherwise they wouldn’t have become so smooth. They must have been carried by large fast-flowing rivers. The smaller pebbles came when there was less energy in the water - when a river was wide or slow moving. If an area is flooded and quiet, the receding waters leave sand, silt or clay behind.”

There is a neat black line about midway in the wall of the excavation which looks like a thin layer of small blackish rocks. Asked what they are, Giulio replies “they are not rocks.” To understand what they are, he says, you have to look at soil first. “Soil is the result of weathering and modification of bedrock due to climate, vegetation and micro organisms. Soil also contains minerals, including iron, and over millions of years these minerals dissolve and seep down till they reach a place where they cannot go down any further. This process happens because there is a certain balance in the chemistry of the ground. When that balance is no longer there, the seeping process stops. When it stops, the minerals cannot go further down and then, over millions of years, they coagulate into iron nodules.” He breaks a nodule with a hammer. Inside we see orange, yellow, and red, outside the nodule is red and black. “Orange, yellow and red indicate iron, black is manganese which has coated the nodule,” explains Giulio.

He takes up another piece of rock, reddish and interspersed with white lines. “We call these ‘bio-galleries’, he says. “A bio-gallery is a channel made by either a living a micro-organism like an earthworm, or by the root of a tree. The weathering of the bedrock and erosion led to calcium leaching downward these channels. That’s why they became whitish. This white is calcium carbonate. The bio-galleries prove that once upon a time, there was a rich forest area here.”

While the sun slowly sets, Giulio speaks about the chemical processes that shaped this area over millions of years. When he concludes his talk, he has just scratched the surface of the depths.