K/T Boundary

Boundary between two time periods (the Cretaceous and the Tertiary) marked by a  sedimentary layer that is approximately 65 million years old. It contains abnormally high levels of non-terrestrial iridium, which leads many scientists to believe that an extra-terrestrial object may have impacted the Earth at this time, distributing iridium all over the planet. The iridium was preserved in the thin layer of clay, shown in the photograph.