In 1840, Agassiz introduced a radical element of contingency into geohistory, contrary to then widespread assumptions of uninterrupted gradual cooling. On the basis of investigations of glacial activity in Switzerland, Agassiz marshaled evidence not just for local glacier action but for global climate change, arguing that the extent of glacial debris required sheets of ice large enough to cover the continent in a relatively recent Ice Age. Rather than a gradually cooling Earth, Agassiz argued, past climates have fluctuated widely, and the Earth has actually warmed up since the last Ice Age.