Today, evidence of this collision can be found in the
rock record. Geologists have discovered a thin layer of clay
containing a high concentration of the element iridium
between 2 particular rock layers. This boundary marks the
end of the Mesozoic and the beginning of the Cenozoic era.
This iridium-rich layer has been identified at the Mesozoic-
Cenozoic boundary at many different locations around the
world. Iridium, while rare on Earth, is a common substance
in meteorites and asteroids.