I had been looking forward to seeing the Great Unconformity when I visited the Grand Canyon last week, and it did not disappoint.

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Box Canyon Unconformity

This is an angular unconformity at Box Canyon near Ouray, Colorado. The bottom layer, the Uncompahgre quartzites, were deposited horizontally, then uplifted and tilted vertically during mountain building and eroded to a nearly flat surface. The rocks above are the Elbert sandstones deposited during a marine “transgression,” a sinking of the land and resulting ocean encroachment. The gap between the two events is about 1.3 billion years, a vast and very Deep Time. And, yes, this is the same type of Great Unconformity seen at the Grand Canyon, and it occurs in different rock sections around the world.

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