Adam's Bridge, also known as Rama's Bridge or Rama Setu, is a chain of natural limestone shoals between Pamban Island, also known as Rameswaram Island, off the south-eastern coast of Tamil Nadu, India, and Mannar Island, off the north-western coast of Sri Lanka. Geological evidence suggests that the bridge was formerly a land connection between India and Sri Lanka.
The feature is long and separates the Gulf of Mannar (southwest) from the Palk Strait (northeast). Some regions of the bridge are dry, and the sea in the area rarely exceeds in depth, making it quite difficult for boats to pass over it.
Etymology
Ibn Khordadbeh's Kitāb al-Masālik wa-l-Mamālik () refers to the structure as Set Bandhai (lit. Bridge of the Sea). Al-Biruni's Tārīkh al-Hind () was probably the first to use the name Adam's Bridge. This appears to have been premised on the Islamic belief that Adam's Peak — where the biblical Adam fell to earth — is located in Sri Lanka, and that Adam crossed over to peninsular India via the bridge after his expulsion from the Garden of Eden.
The ancient Sanskrit epic Ramayana (8th century BCE–3rd century CE) mentions a bridge constructed by the god Rama to reach the island Lanka and rescue his wife Sita from Ravana. In popular belief, Lanka is equated to present-day Sri Lanka and the bridge is described as "Rama's Setu".
Geological evolution
The bridge starts as a chain of shoals from the Dhanushkodi tip of India's Pamban Island. It ends at Sri Lanka's Mannar Island. Pamban Island is accessed from the Indian mainland by the 2-km-long Pamban Bridge. Mannar Island is connected to mainland Sri Lanka by a causeway.
The lack of comprehensive field studies explains many of the uncertainties regarding the nature and origin of Adam's Bridge. It mostly consists of a series of parallel ledges of sandstone and conglomerates that are hard at the surface and grow coarse and soft as they descend to sandy banks. The Marine and Water Resources Group of the Space Applications Centre (SAC) of the Indian Space Research Organisation (ISRO) concludes that Adam's Bridge comprises 103 small patch reefs. One study tentatively concludes that there is insufficient evidence to indicate eustatic emergence and that the raised reef in southern India probably results from a local uplift.
Transport corridor
During periods of lowered sea level over the last 100,000 years, Adam's Bridge has provided an intermittent land connection between India and Sri Lanka. According to famous ornithologists Sidney Dillon Ripley and Bruce Beehler, this supports the vicariance model for speciation in some birds of the Indian Subcontinent.
Due to shallow waters, Adam's Bridge presents a formidable hindrance to navigation through the Palk Strait. Though trade across the India–Sri Lanka divide has been active since at least the first millennium BC, it was limited to small boats and dinghies. Larger ocean-going vessels from the west have had to navigate around Sri Lanka to reach India's eastern coast. Eminent British geographer Major James Rennell, who surveyed the region as a young officer in the late 18th century, suggested that a "navigable passage could be maintained by dredging the strait of Ramisseram ". However, little notice was given to his proposal, perhaps because it came from "so young and unknown an officer", and the idea was only revived 60 years later.
In 1823, Sir Arthur Cotton (then an ensign) was assigned to survey the Pamban channel, which separates the Indian mainland from the island of Rameswaram and forms the first link of Adam's Bridge. Geological evidence indicates that a land connection bridged this in the past, and some Ramanathaswamy Temple records suggest that violent storms broke the link in 1480. Cotton suggested that the channel could be dredged to enable passage of ships, but nothing was done until 1828, when Major Sim directed the blasting and removal of some rocks.
A more detailed marine survey of Adam's Bridge was undertaken in 1837 by lieutenants F. T. Powell, Ethersey, Grieve, and Christopher, along with draughtsman Felix Jones, and operations to dredge the channel were recommenced the next year. However, these and subsequent efforts in the 19th century did not succeed in keeping the passage navigable for any vessels except those with a light draft.