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.


http://www.lpl.arizona.edu/SIC/impact_cratering/Chicxulub/Discovering_crater.html