pretty impressive piece of rock on the island of Santiago in the Cape Verde Islands. It is the result of a tsunami wave caused by the side of a volcano blowing out on the nearby island of Fogo. It took place 70,000 years ago – it is thought. See www.theatlantic.com/science/archive/2015/10/traces-of-an-ancient-mega-ts…
However, for the full story you need to go to an article in Science – www.advances.sciencemag.org/content/advances/1/9/e1500456.full.php