The Bottaccione Gorge at Gubbio, Italy, a source of many discoveries in Earth history, was first recognized as an outstanding geological section by Guido Bonarelli (1871–1951). Bonarelli is remembered today mainly for the meter-thick Bonarelli Level, the local manifestation of oceanic anoxic event 2 (OAE 2), which he first recognized and described. Setting aside Bonarelli’s long and distinguished career as a petroleum geologist in Borneo and Argentina, this paper concentrates on his role in the long and difficult effort to date the Scaglia rossa pelagic limestone of the Bottaccione Gorge and the surrounding Umbria-Marche Apennines. Old photographs show a barren Bottaccione Gorge a century ago; Bonarelli apparently had much better outcrops than we do today, after reforestation shortly before the middle of the twentieth century. In the absence of macrofossils, and with the inability to extract isolated foraminifera from these hard limestones, the Scaglia was dated indirectly in the late nineteenth century, and believed to be entirely of Cretaceous age, implying errors as great as 40 m.y. We can now understand why this dating seemed satisfactory at the time, because it did not conflict with Charles Lyell’s view that there should be a huge hiatus corresponding to a major faunal overturn like the Cretaceous-Paleogene (K-Pg) boundary, and because thrust faulting that contradicted it had not yet been discovered. The K-Pg boundary was correctly placed within the Scaglia in 1936 when Otto Renz identified the foraminifera in thin section. Renz wrote with pleasure of a field trip with Bonarelli, who later presented Renz’s new dating to the Società Geologica Italiana on a 1940 field trip to Gubbio. These two are the predecessors of all the geologists who have worked in the Bottaccione Gorge since the Second World War.