The K-Pg boundary as exposed along the side of Interstate 25 near Raton Pass in southern Colorado. The obvious white layer is the K-Pg ejecta layer. It contains elevated levels of iridium and shocked mineral grains. Pollen and spores from Cretaceous plants are found immediately below this layer but not above it, a pattern that is seen from the southern United States all the way north to the Arctic Ocean. This direct link between impact ejecta and plant extinction suggests a very strong cause and effect relationship between impact and extinction.
Credit: Kirk Johnson, Denver Museum of Nature & Science
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