Great Unconformity

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The Great Unconformity is a geologic feature where two adjacent layers of rock were not formed one after another in the earth's development. The lower layer was always at a high point, undergoing erosion, and in this case for an extended time, even in geological terms. The Great Unconformity is visible in different places in the U.S.. It is one of many unconformities that exist around the world and one of the largest.

At Frenchman Mountain in Nevada, there is a gap of about 1.2 billion years where the rock jumps from 500 million years old to 1.7 billion years old. In the Grand Canyon, the clearly visible discontinuity represents a time gap that varies from about 250 million years to 1.2 billion years. The Great Unconformity's missing 1.2 billion years represents about 25% of the earth's history.