Conversation so far:
Did Luis Walter Alvarez make many discoveries about dinosaur extinction?
In 1980 Alvarez and his son, alongside nuclear chemists Asaro and Michel, uncovered a calamity that literally shook the Earth and is one of the great discoveries about Earth's history“
Did Luis Walter Alvarez go on any digs?
During the 1970s, Walter Alvarez was doing geologic research in central Italy
What did Luis Walter Alvarez find on his Italy dig?
There he had located an outcrop on the walls of a gorge whose limestone layers included strata both above and below the Cretaceous–Paleogene boundary.

Question: What was the significance of that find?
Self-contained rewrite: What was the significance of that find of limestone layers?