'Curious formed valleys, petrified shells, volcanoes and strange scenery'

The explorer Alexander Caldcleugh drew this sketch map of a route through the Cordillera of the Andes for Darwin. CUL DAR 35.2, f. 405r

The miserable tedium was violently broken on 19 January 1835 when the voyagers witnessed the volcanic eruption of Mount Osorno, and were confronted with the effects of an earthquake and tidal wave. Darwin was in Valdivia where the damage was relatively slight – ‘I have had ill luck however in only one little earthquake having happened’. Both he and FitzRoy were struck by the measurable uplift of the land. Darwin, pondering a possible connection between these apparently separate phenomena, began to conceive a grand geological theory. He had explored the Cordilleras from the east the year before. Now he crossed them from the west, where, high up in the Uspallata pass, he encountered fossil trees that had clearly once been submerged in sea water – further evidence of dramatic changes in the landscape. In a letter to Henslow he confided ‘the picture so plainly drawn of the great epochs of violence…causes in the mind a most strange assemblage of ideas’.