Effect and Cause

into the river, and a great mass of water overwhelmed with a sudden and violent flood all that was in its path, demolishing homes, and several people inside the city walls of Geneva were killed. Historians and scientists have long believed that Gregory and another chronicler, Marius of Avenches, who told a similar tale, were describing a tsunami that raced across the lake. But there has not been any direct evidence of it until now. Researchers at the University of Geneva now say they have found that evidence, in the form of a large deposit of sediment in the middle of the lake. In a study published in the journal Nature Geoscience, they also propose the sequence of events that caused the deadly surge. The researchers think that a large boulder crashed down onto soft sediments which had accumulated at the river mouth because of the slowing of the river's flow when it enters the lake. These sediments formed an underwater delta that had several canyon-like channels. When the falling rocks hit the delta, they destabilized the sediments and caused the canyons to collapse. It was this collapse that created the tsunami. The sediments from this collapse would have been propelled towards the lake's centre.