Clock in the rock: Scientists measure Earth's history

The mass extinction of dinosaurs occurred about 66 million years ago, at the end of the Cretaceous Period (K) and the beginning of the Tertiary Period, known as the K-T boundary. A massive asteroid slammed into what is now the Gulf of Mexico about that time.

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<p><a href="http://www.physorg.com/news74869350.html">Clock in the rock: Scientists measure Earth's history</a></p> <p>The mass extinction of dinosaurs occurred about 66 million years ago, at the end of the Cretaceous Period (K) and the beginning of the Tertiary Period, known as the K-T boundary. A massive asteroid slammed into what is now the Gulf of Mexico about that time.</p>