New Delhi, Sept. 12: Studies suggest that Adam’s Bridge is not man-made, but a natural structure fashioned over thousands of years by geological processes that occur worldwide in marine environments.

Geologists believe the chain of limestone shoals between Rameshwaram in India and Mannar in Sri Lanka took shape over thousands of years, beginning at a time when the sea level was so low that people could walk from India to Sri Lanka.

A three-year study by the marine wing of the Geological Survey of India around Rameshwaram and Adam’s Bridge that involved scanning the seabed and drilling through submerged rock has provided “no evidence” of man-made structures.

Four holes drilled between the island of Dhanushkodi and the third island of Adam’s Bridge revealed three distinct sedimentation cycles dominated by clay, limestone and sandstone.

“Such sedimentation cycles take thousands of years,” said Tirumalai N. Prakash, a marine geologist at the Centre for Earth Sciences, Thiruvananthapuram. “A man-made process cannot put clay, limestone and sandstone in sequence,” he said.

Studies of sea-level changes along the east coast of India have revealed that the area between Rameshwaram and Sri Lanka was exposed — not submerged under the sea — between 18,000 years and 7,000 years ago.

About 6,000 years ago, the sea level was 17 metres below its present level, and the seabed between India and Sri Lanka was partially exposed.

Scientists at the Space Applications Centre, Ahmedabad, who used a satellite to peer at Adam’s Bridge, have also concluded that it is not man-made. Satellite images of the structure show features typical of small-patch coral reefs.

The scientists’ team identified about 103 small-patch reefs lying in a linear pattern. The researchers said the linearity — the sequential structure — of Adam’s Bridge probably shows that it was associated with a previous shoreline.

Coral reefs are linearly distributed, parallel to the mainland. The edge of the reef has signatures very similar to those observed in the Great Barrier Reef of Australia, the scientists had said. Similar edges have also been mapped by satellite on other Indian reefs such as Kadmat, Aggati and Bangaram of the Lakshadweep islands, and the Musal-Manoli in the Gulf of Mannar.

Some geologists say ocean currents also contributed to the formation of the bridge.

The wind-driven ocean currents are circular and sediment brought from India and Sri Lanka may have possibly converged in the sea to create such a structure, said Varadan Ramomohan, a geologist at the University of Madras.