Great Unconformity

Great Unconformity

Great Unconformity

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Of the many unconformities (gaps) observed in geological strata, the term Great Unconformity is most notably applied to either the unconformity observed by James Hutton in 1787 at Siccar Point in Scotland, or that observed by John Wesley Powell in the Grand Canyon in 1869. Both instances are notable for demonstrating that the contacts between sedimentary formations of greatly different age could represent epochs sufficiently long to raise great mountains and then erode them away.

Hutton's Unconformity at Siccar Point (and other localities) reflects a gap of about 80 million years between certain red sandstones deposited sometime in the Devonion Period (approximately 350 to 416 million years ago) over marine sediments laid down in the Silurian Period (approximately 416 to 444 million years ago).

Powell's unconformity in the Grand Canyon is between the Tapeats Sandstone of Middle or Lower Cambrian Age, about 500 million years ago, and igneous and metamorphic rock of the Statherian Period of the Paleoproterozoic Eon, around 1700 million years old, give or take a few hundred million years, with a gap of over a billion years. (See also geological timescale.)

Unconformities in general tend to reflect long-term patterns of sediments or igneous flows collecting in low-lying areas (often ocean basins, such as the Gulf of Mexico or the North Sea, but also Bangladesh and much of Brazil), then being uplifted and eroded (such as the...
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