Of the many unconformities (gaps) observed in geological strata, the term Great Unconformity is frequently applied to either the unconformity observed by James Hutton in 1787 at Siccar Point in Scotland,[1][failed verification] or that observed by John Wesley Powell in the Grand Canyon in 1869.[2] Both instances are exceptional examples of where the contacts between sedimentary strata and either sedimentary or crystalline strata of greatly different ages, origins, and structure represent periods of geologic time sufficiently long to raise great mountains and then erode them away.
- ^ Rance, H (1999) Historical Geology: The Present is the Key to the Past. Archived 3 December 2008 at the Wayback Machine QCC Press, New York
- ^ Merten, G (2005) Geology in the American Southwest: New Processes, New Theories In MF Anderson, ed., A Gathering of Grand Canyon Historians. Proceedings of the Inaugural Grand Canyon History Symposium, January 2002. Grand Canyon Association, Grand Canyon, Arizona.