Dinosaur Killer - Four Revolutions in the Earth Sciences - page 178

FIGURE 24 . Luis and Walter Alvarez at Gubbio. Walter has his fingers on the K-T Boundary,
when the dinosaurs disappeared. Source : Lawrence Berkeley National Laboratory.
Right at the boundary, in between the K and the T, Walter found a one-
centimeter-thick layer of reddish clay with no fossils. On his return, he showed a
hand specimen that included the boundary clay to his father and explained that it
marked the time when the dinosaurs had gone extinct. Walter estimated that the
boundary clay might have taken five thousand years to form, which if true sugges-
ted that dinosaur extinction had happened in a geologic eyeblink. Luis was fascin-
ated and proposed that they try to measure the time span. Since no known method