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Rivers Ran to It

By Jocelyn SelimJanuary 2, 2004 6:00 AM


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The Grand Canyon has a fancy East Coast pedigree, say geologists Bill Dickinson and George Gehrels of the University of Arizona. Their research explains both the origin of Arizona’s colorful gorge and the fate of the Appalachians, an eastern mountain chain that was once as mighty as the Rockies.

Most geologists believed that the rocks of the Colorado Plateau, surrounding the Grand Canyon, originated in a mountain range that ran from New Mexico to Colorado 300 million years ago. Presumably, sand eroded away from that range, then washed west and solidified into Arizona’s sedimentary rocks from 300 million to 150 million years ago.

Dickinson and Gehrels used uranium-lead dating to study hundreds of grains and match their ages with those of rocks throughout the United States and Canada. Their findings reveal a different history.

About half the sand in the Colorado Plateau originated 1.2 billion to 500 million years ago in the Appalachian mountains, carried away by a system of immense rivers that have long since dried up. The other half is derived from the ancestral Rocky Mountain range, approximately 1.5 billion years old, and from another, even older section of the Canadian shield, stretching north from the Great Lakes.

This analysis reveals an unknown era in North American history, when mighty waters tore down the towering mountains of the East. “Those rivers would have been as big as the Ohio, the Colorado, or the Missouri,” Dickinson says.

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