The K-PG Boundary and Dinosaur Extinction (aka "K-T Boundary")

Geologists have found that the boundary between the Cretaceous and Paleogene layers called the “K–Pg” boundary (previously known as the "K-T boundary"), in many regions is marked by high levels of the rare metal iridium. While it is true that nearly all dinosaur fossils are absent above this K-Pg boundary, the areas with high iridium levels are not restricted to the thin zone that defines this boundary. In fact, in the same regions where the K-PG boundary was discovered, recent studies have identified a 4-meter thick layer rich in iridium—not the thin line typically highlighted in most textbooks. These studies reveal that there was not a single iridium “spike,” but rather a horizon of peak values within a sequence of iridium-enriched clays that were most likely deposited by volcanic activity as well as an asteroid or series of asteroids.