I had to look it up. Found this from universetoday.com,
K is actually the traditional abbreviation for the Cretaceous period, and T is the abbreviation for the Tertiary period. So the K-T boundary is the point in between the Cretaceous and Tertiary periods. Geologists have dated this period to about 65.5 million years ago.
A layer of dark sediment found worldwide, unusually rich in iridium, which has been attributed to material from a catastrophic meteor impact, 65 million years ago. It is believed the event is concurrent to the cause of the Chicxulub crater, off the Mexican Yucatán.
A huge mass extinction has been traced to this time period, to likely include the last days of dinosaurs. No fossils of identical species have been found on both sides of the KT sediment line. As such, it has also been called “the Tombstone layer”.
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