The hulking boulders, dubbed the Bull and the Cow, would not have needed a mega-storm to catapult them to the cliff tops. Charles Ommanney/The Washington Post/Getty

Climate sciences

Lofty stones reveal ancient storm secrets

Boulders’ rise to epic heights suggests that modest sea-level rise could batter shores.

Mega-storms need not be invoked to explain the presence of house-sized boulders high atop a cliff on the island of Eleuthera in the Bahamas.

Previous studies suggested that the boulders were deposited during storms more powerful than any in recorded history. Alessio Rovere at the University of Bremen in Germany, Maureen Raymo at Columbia University in New York City and their colleagues combined field surveys with fluid-dynamic and boulder-transport models to analyse two of the biggest boulders. These weigh hundreds of tonnes, and attained their current positions during the most recent interglacial period (116,000–128,000 years ago), when sea levels were 6–9 metres higher than they are at present. The team found that waves created by a storm similar to 2012’s Hurricane Sandy — a severe but not exceptional storm — could have pushed the boulders inland from the edge of the roughly 15-metre-high cliff, even if sea levels had been only 3.5 metres above current levels.

The results suggest that after only modest sea-level rise, cliffs and other coastal barriers could be more vulnerable to erosion by waves — even if storms do not grow more intense.