Great Unconformity

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.