Erosion is very effective over the course of millions of years. The crater corresponding to the Cretaceous impact wasn't discovered until this decade. Geologists working for the Mexican national oil company discovered rocks that had been shocked and melted, hidden away beneath the sediments of the Yucatan peninsula. Further investigation showed that the disturbed rocks formed a circle 180 kilometers in diameter. The circular structure was actually a buried impact crater.
Iridium is a dense metal, like platinum and gold, which is very scarce in terrestrial rocks.
When the Earth was molten, dense metals like iridium had a tendency to sink to the Earth's core. However, asteroids and comet dust are relatively rich in iridium. Thus, if an asteroid or comet struck the Earth, we would expect iridium-rich eject to be thrown high in the atmosphere and settle down gradually all over the Earth. The rocks at the bottom right date from the Cretaceous period. The dark band between them is a layer of clay in which the iridium density is hundreds of times the normal level.