Diagram showing dating of selected igneous rocks

The age of the volcanic ash bed and the igneous dike are determined directly by radiometric methods. The layers of sedimentary rocks below the ash bed are obviously older than the ash, and all the layers above the ash are younger. The igneous dike is younger than the Mancos Shale and Mesaverde Formation but older than the Wasatch Formation because the dike does not intrude the Tertiary rocks.

From this kind of evidence, geologists estimate that the end of the Cretaceous Period and the beginning of the Tertiary Period took place between 63 and 66 million years ago. Similarly, a part of the Morrison Formation of Jurassic age was deposited about 160 million years ago as indicated by the age of the ash bed.

Return to Radiometric Time Scale

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Last updated July 28, 1997
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