K-T boundary layer rock samples. The K-T boundary is a thin layer of clay containing iridium, separating older Cretaceous (K) and younger Tertiary (T) rocks. The boundary layer is only a few centimetres thick, but is found worldwide. It is thought to have been formed from material ejected by the impact of a large asteroid around 66 million years ago. This is thought to have been responsible for the extinction of the dinosaurs and 75 percent of life on Earth. This sample is from Agost, Spain. the largest piece measure approximately 2 centimetres.