K-T Boundary

The K-T Boundary, now called the Cretaceous-Paleogene (K-Pg) boundary, marks a major extinction event about 66 million years ago. It’s the layer of sediment found between the Cretaceous and Paleogene rock formations. This boundary is notable because it shows a sharp change in the types of fossils and a layer of iridium—a rare metal often associated with asteroids—indicating a massive asteroid impact. This event caused widespread destruction, leading to the extinction of the dinosaurs and many other species, and is a key marker in Earth's biological history.