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Paul D. Ryan

How did these giant boulders get here?

Scientists have long wondered how giant boulders—like the multiton, automobile-size rocks that dot this shore in northwestern Ireland—got here. Were they carried by a tsunami triggered by an asteroid impact at sea? It turns out that large, nontsunami waves are enough to do the trick, according to a new study. Computer simulations suggest that breaking waves of the size that occasionally strike this part of the world could pluck boulders weighing up to 100 tons out of fractured bedrock and carry them more than 100 meters inland, researchers report today in the Proceedings of the National Academy of Sciences. Field data back this up: The closest boulder to the shoreline at this peninsular shore, known as Annagh Head, is a 4.3-meter-long behemoth weighing about 48 tons that lies 115 meters inland from the high tide line. Other evidence bolsters the notion that the Annagh Head boulder field wasn’t created by a tsunami caused by an asteroid strike at sea or an underwater landslide, the researchers note. Although a tsunami typically creates such deposits over a large span of shoreline, Annagh Head hosts one of only two boulder fields in the area. Such simulations could help explain the origin of mysterious boulder deposits along shorelines worldwide.