What is the K-t boundary layer?

K-T stands for Cretaceous-tertiary boundary. The K is used instead of a C to help that there is no confusion between the Cretaceous and Carboniferous Periods; as the Carboniferous Period was among one of the first periods. The K-T boundary is marked by the element named Iridium, the K-T boundary contains approximately 31% iridium, this element is rare on Earth, common in space debris, such as asteroids and meteors/meteorites, such as the one believed to have hit the Earth causing the extinction of the dinosaurs. The Cretaceous was the last of the dinosaur bearing periods. The Tertiary period is the period we as humans live in currently.