Louis Agassiz (1807 - 1873) revolutionised geological theory by his study of glaciers, which led him to put forward the idea that most of Eurasia had once been covered in ice. He had observed that many features far from existing glaciers could be explained by the action of ice: the formation of great valleys, the scratching and smoothing of rocks, the mounds of debris - moraines - pushed up when glaciers advanced, and the huge boulders - erratic blocks - carried long distances by the ice. Scientists at the time believed that these features had been caused by water (the Biblical flood), and Agassiz' theory was by no means universally accepted.

In 1846 Agassiz moved to the US, where among other things he worked for the creation of the Museum of Comparative Zoology and the National Academy of Sciences, and did much to bring scientific knowledge to ordinary people.