On the southern coast of Madagascar, chevron deposits record a wave that made landfall at least 600 feet high. How can anyone know that? The chevrons run up to a 500-foot escarpment that starts about 100 feet above sea level, and the same deposits appear on top of the cliff.
The wave washed over the top, so it had to be high enough to overtop the escarpment. In Randall's view, that satisfies the definition of a cataclysm.