Bottaccione Gorge, Gubbio

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A gorge cut through pelagic limestones just outside the town of Gubbio in Umbria, where a continuous section of deep-water Scaglia limestone spans the Cretaceous–Palaeogene boundary. Walter Alvarez was working on the palaeomagnetic stratigraphy of this section in the 1970s when he became interested in the two-centimetre clay layer at the boundary and in how much time it represented. The attempt to answer that question — using iridium delivered by cosmic dust as a clock for the deposition rate — produced instead an iridium concentration far too high to be explained by dust, and turned a stratigraphic side question into the impact hypothesis. The site is therefore an unusually clean example of a discovery arriving as the failure of a measurement rather than as the confirmation of a prediction. The boundary clay at Gubbio is still visible in outcrop and the section remains a reference locality.