FRENCHMAN MOUNTAIN, LAS VEGAS, NEVADA

The Great Unconformity
A Time Gap of up to 1.2 Billion Years — from 1.7 to 0.5 billion years ago

The Great Unconformity is a gap in time in the geologic record of up to 1.2 billion years. It was first described at the Grand Canyon where it represents a gap in time between the Vishnu Schist/Zoroaster Granite of the deep, inner canyon and the Tapeats Sandstone of the wide, upper canyon that begins the familiar sequence of horizontal rocks.

At the Grand Canyon, miles of hiking and a descent of thousands of feet are required to reach the Great Unconformity. But there is an easier way to stand next to the Great Unconformity. More than a hundred miles to the west, on the eastern edge of Las Vegas, along State Highway 147 (East Lake Mead Boulevard), is Frenchman Mountain.

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View of Frenchman Mountain from the interpretative turnout along State Highway 147

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The Vishnu Schist (dark) and the Zoroaster Granite (light red)

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The Great Unconformity is the contact between the whitish Tapeats Sandstone (above) and the dark Vishnu Schist (below).

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Tapeats Sandstone

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Vishnu Schist

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Zoroaster Granite

Other locations of the Great Unconformity:

COLORADO—Box Canyon near Ouray, Red Rocks Amphitheatre west of Denver, and Baker's Bridge along the Animas River in the San Juan Mountains

WYOMING—Shoshone Canyon near Cody

UTAH—Ruby Canyon near I-70

OKLAHOMA—Arbuckle Mountains

MISSOURI—St. Francois Mountains near Mark Twain National Forest

WISCONSIN—Chippewa Falls

NEW YORK—south of Ticondergoa on Lake Champlain

ONTARIO—Manitoulin Island