While Gene Shoemaker was trying to get people galvanized about the potential dangers of the inner solar system, another development—wholly unrelated on the face of it, was quietly unfolding in Italy with the work of a young geologist from the Lamont Doherty Laboratory at Columbia University.
In the early 1970s, Walter Alvarez was doing fieldwork in a comely defile known as the Bottaccione Gorge, near the Umbrian hill town of Gubbio, when he grew curious about a thin band of reddish clay that divided two ancient layers of limestone—one from the Cretaceous period, the other from the Tertiary. This is a point known to geology as the KT boundary,
It is KT rather than CT because C had already been appropriated for Cambrian. Depending on which source you credit, the K comes either from the Greek Kreta or German Kreide. Both conveniently mean "chalk," which is also what Cretaceous means.
and it marks the time, sixty-five million years ago,when the dinosaurs and roughly half the world's other species of animals abruptly vanish from the fossil record. Alvarez wondered what it was about a thin lamina of clay, barely a quarter of an inch thick,that could account for such a dramatic moment in Earth's history.
At the time the conventional wisdom about the dinosaur extinction was the same as it had been in Charles Lyell's day a century earlier—namely that the dinosaurs had died out over millions of years.