What Killed the Dinosaurs

I n 1977, Walter Alvarez, a young geologist, was studying m rocks near Rome, Italy. He was puzzled about a particular sedimentary rock sample. There were three layers. The bottom was a thick layer of white limestone, filled with microscopic seashell fossils. It dated from the end of the Cretaceous Period (65 million years ago). The second was a thin layer of chalky clay. Walter did not know when or where it was from.

The third layer was red limestone. It lay on top of the other two layers. It had no fossils in it. This red rock was common in that part of Italy. It was known to be from the early Tertiary Period—the time immediately after the Cretaceous. It made sense that there were no fossils in this red rock. There were not many creatures living immediately after the Cretaceous mass extinction. But what was this mysterious clay layer? Little did Alvarez know! The rock he was holding was evidence that an asteroid may have caused the extinction of the dinosaurs.

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