| Adam's Bridge consists of a series of parallel ledges of sandstone and conglomeration, that is hard at the surface and grows coarse and soft as it descends to sandy banks. In the 19th century, there were two prevalent theories explaining the structure. One considered it to be formed by a process of accretion and rising of the land, while the other surmised that it was formed by the breaking away of Sri Lanka from the Indian subcontinent. The friable calcerous ridges are broken into large rectangular blocks, which perhaps gave rise to the belief that the causeway is an artificial construction.
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, which according to famous ornithologists Sidney Dillon Ripley and Bruce Beehler supports the vicariance model for speciation in some birds of the Indian sub-continent.
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