The three tiny Aran Islands are just off the western coast of Ireland. The elongated rock ridges form a collar along extended stretches of the islands’ Atlantic coasts. The sizes of the boulders in the formations range “from merely impressive to mind-bogglingly stupendous,” writes Dr. Rónadh Cox, who led the research with her Williams College students. One block the team studied weighs an estimated 78 tons, yet was still cut free from its position 36 feet above sea level and shoved further inland.
Armed with equations that model the forces generated by waves, some researchers have concluded that no ordinary ocean waves could muster the force necessary to move the largest of the boulders this high above the ocean surface and so far inland. The math suggests the rocks in the ridges could only have been put there by a tsunami.
The equations tell one story. The islands’ residents tell another. According to some locals, enormous rocks have moved in their lifetimes, despite the fact that there hasn’t been a tsunami to hit the islands since 1755.
“Unless you have little green men from mars doing this on the quiet, it must be storm waves,” Cox said.
The residents had their old anecdotes. The anecdotes are poor evidence on their own but they can LEAD you to gathering better evidence. And, in this case, it worked. The researchers used old maps meticulously produced by hand long ago. The maps were accurate to less than a meter. But then, other lines of evidence corroborated the accounts of recent movement of boulders such as radiocarbon dating and photographic records. All lead them to conclude that no tsunami was necessary. An occasional huge storm wave would do it.
What a lovely example of local knowledge and science working in concert to get to an answer.