K-T Boundary
The K-T Boundary (also known as the K-Pg Boundary) is a stratum – a distinctive layer of rock – that runs around the world and marks the transition between the Cretaceous and Tertiary geological periods. It was laid down about 66 million years ago, coinciding with the end of the dinosaur age. The layer is rich in iridium, an element that is rare in the Earth’s crust, but common in meteorites, providing some of the strongest evidence for what caused the mass extinction. SB