What is the earth's tombstone layer?

The term 'tombstone layer' is a colloquialism associated with an extinction event ~ 66 million years ago leading to the demise of the dinosaurs and about 70% of other life on earth. The layer is a thin (few inches) clay deposit containing iridium (a very rare element) at a level about 100 times its normal distribution in the earths crust. The layer has been positively linked to the Chicxulub impact crater discovered in the Yucatan Penisula, Mexico and credited with being the extinction event that ended in the mass extinction, thus the term 'tombstone layer'.