This topic has generated a lot of controversy. Between Pamban, Southern tip of Tamil Nadu, India and Mannar, northern tip of Sri Lanka is the Palk Strait. Adam's Bridge is a chain of sand shoals and coral islands which appear as stepping stones along the Palk Strait . Everyone accepts that these are natural features. The controversy is whether there is a causeway linking these islands. Proponents of the causeway say that recent "studies" prove the presence of an artificial structure. This lends credence to the story in the epic Ramayan that Shri. Ram's army built a causeway in order to invade Sri Lanka and rescue Shri Ram's wife Sita. This causeway known as Ram Sethu, according to the proponents has since been submerged due to a sea level rise. Satellite pictures showing a milky white line connecting India and Sri Lanka have appeared in Wikepedia and also in the Indian blogosphere. The one above, of the Palk Strait is the NASA image that has contributed to the controversy. The perception of continuity is however an illusion caused by a combination of cloud cover and suspension of white calcium carbonate sediment along the length of the coral islands. The next image shows a close up of the Palk Strait. The discontinuous nature of the islands is clearly seen. The images are inconclusive on the issue of the causeway.
In the online magazine Rediff, there was an interview (July 31 2007) with Dr. Badrinarayanan, former director of Geological Survey of India and former coordinator of the survey division of the National Institute of Ocean Technology (NIOT), Ministry of Earth Science, in Chennai. NIOT recently drilled boreholes and recovered cores along the island chain. This has produced data more informative that satellite pictures. The cores are more than 10 metres thick and according to Dr.Badrinarayanan
"we found marine sands on top and below that was a mixed assemblage of corals, calcareous sand stones, and boulder like materials. Surprisingly below that up to 4-5 metres, again we found loose sand and after that, hard formations were there".
Dr. Badrinarayanan is clearly puzzled by this inter-layering of sand, boulder and coral and concludes that the boulders have to be artificially placed there, since there is no way they can appear on top on a marine sand layer. As a geologist who specializes in limestones, I can say that this sequence of sediment is exactly what you would expect from such a setting. The coral animal secretes a skeleton of calcium carbonate. Corals are colonial organisms, so hundreds of millions of such animals aggregate and secrete calcium carbonate skeletons. This, over time results in large structures which produce topographic highs on the sea bed. Such coral aggregates which show topography are called reefs. The shapes of these structures range from delicate branching types to more massive aggregates of carbonate material, some of which look like giant brains. As these structure grow upwards from the sea bed they encounter shallower water and start getting battered by waves. Particularly during storms, pieces of corals break off from the main reef.It is common for reefs to have an apron of debris composed of small and large boulders sitting on top of the sandy sea floor. In reef systems, sand is everywhere. The natural disintegration of coral skeletons upon death of the coral animal produces sand, erosion by waves produces sand, and additional sand can be transported from the surrounding continental shelf. In this case much of the sand is being brought by currents from coastal areas of India (Rameshwaram to Vedaranniyam coast) and Sri Lanka (Jaffna penninsula). This sand infiltrates the cracks and crevices of the coral structures and also blankets the sea floor.
Now, here is the crucial property of such very shallow water systems. They produce such vast amounts of sediment that they force a local change in sea level along with changes in currents. This means a certain type of sedimentary environment may in response to a shift in sea level encroach upon and bury an adjoining sedimentary environment. Corals can encroach and bury sand, just as sand can shift and bury adjacent coral and boulders. Over long periods of time such migrating and shifting environments will create a geological section, such as the one observed in Dr. Badrinarayanan's cores, a complex sequence of inter-layered sand, coral and boulders.
I suspect the sand shoals and the coral complexes represent a geologic history spanning the late Pleistocene to Holocene epochs (the last hundred thousand years). During the Pleistocene "ice-age" period, glacial buildup and melting forced fluctuations in sea-level by tens of meters , setting up the conditions for several episodes of coral reef and sand shoal formation. During periods of large sea-level drops in the Pleistocene, there would have been a land connection between India and Sri Lanka. But at the end of the last Wisconsin glaciation, the sea-level began to rise world-wide. This period of global sea level rise beginning around 10-12 thousand years ago marks the beginning of the Holocene epoch. The Palk strait became deeper as sea-levels rapidly rose by several ten's of meters in the early part of the Holocene. By mid-Holocene sea-level stabilized, and thereafter minor fluctuations in sea-level changes have been dictated by local geological processes more than any global control. It is important to understand where the Ramayan fits in within the context of all this history of sea-level changes. Mainstream historians place the Ramayan around 100-500 B.C, while the fringe historians place it as far back as 3000 B.C. Both parties though place it firmly within the mid-late Holocene, by which time the Palk Strait was certainly a few to ten's of meters deep. Dr. Badrinarayanan interprets the entire sequence in his cores as having formed in the mid Holocene, specifically from about 5800 to 4000 years ago. He bases this on coral reefs terraces on Rameshwaram island which give carbon dates ranging from 5400 to around 2600 years B.P. (before present). There is also recent coral in the low tide shorezone of Rameshwaram island. According to him, the causeway was built during a sea-level fall sometime in the last few thousand years, i.e in the mid Holocene, the boulders brought in by quarrying exposed coral rock in Rameshwaram. This may conjure up images of Pleistocene style sea-level falls of several ten's of metres, exposing much of the Palk strait sea-bed, leaving only few deeper patches to be filled up to form a causeway. But the coral reef terraces at Rameshwaram may be telling a different story. Reef growth track sea level change. So the successive reef terraces can be taken as an indicator of sea level change. The Rameshwaram reef terraces are about 1 to 1.5 meters above present sea level. This suggests that Holocene sea-level falls were probably not extensive enough to have exposed large parts of the basin. Plus there is the question of the timing of sea-level falls. I am not sure if there is an exposure surface between the older and younger terrace which would indicate that sea -level fell, exposed the sea bed and then rose again drowning the area and leading to the growth of the younger coral terrace. That would be an interesting find as it would give a solid date to the sea level fall and would provide more reliable evidence of the extent of Holocene sea level falls. At the moment the best we can say is that sea-level has been dropping sometime after the youngest dated exposed terrace around 2600 B.P. In any case I doubt if any sea level fall in the Holocene would have lead to any significant lowering of sea-level. Large stretches kilometers long would have remained under water metres deep, making causeway building an impossible exercise. In contrast, processes of coral reef formation in the Palk strait would have provided an in situ source for the natural formation of the debris and boulders found in the cores.
This is a controversial issue involving the religious sentiments of millions of Indians. I feel though that any detailed geological investigation of the Palk strait sediments will be able to explain Adam's bridge as a natural consequence of sand shoal formation and coral reef dynamics.
There are other natural processes by which boulders can appear on the sea floor. But that is a topic for another blog.