What is Unconformity?
[[File:Siccar Point red capstone closeup.jpg|thumb|220px|Siccar Point, [[Berwickshire]]. The rugged point shows gently sloping beds of 345 million year old [[Devonian]] Old Red [[Sandstone]] above vertical beds of 425 million year old [[Silurian]] [[greywacke]].]] [[File:Taum Sauk precambrian-cambrian unconformity.jpg|thumb|220px|right|The upper angled layer of grey [[dolomite]] is approximately 500 million years old. The reddish [[rhyolite]] on which the dolomite rests is approximately 1.5 [[billion]] years old. There is a billion years of geologic history missing in this picture. [[Missouri]] [[Ozarks]].]] An '''unconformity''' is a [[wikt:gap|gap]] in the sequence of [[Rock (geology)|rocks]] in a geologic column. It is a buried [[erosion]] surface separating two rock [[strata]] of different ages. It shows that [[sediment]] deposition was broken for a time, usually because the land was above sea level at that time. Usually, the older layer was exposed to erosion for some time before deposition of the younger. However, the term is used to describe any break in the [[Sedimentary rock|sedimentary]] record. Many millions of years may separate the rocks above and below the discontinuity. [[James Hutton]] found examples of unconformity in [[Scotland]], at Jedburgh in 1787 and at Siccar Point in 1788.<ref>{{Cite web |url=http://www.snh.org.uk/publications/on-line/geology/elothian_borders/hutton.asp |title=Hutton's Unconformity |access-date=2011-01-07 |archive-date=2015-09-24 |archive-url=https://web.archive.org/web/20150924104617/http://www.snh.org.uk/publications/on-line/geology/elothian_borders/hutton.asp |url-status=dead }}</ref><ref name=monty>{{cite web |url=http://nagt.org/files/nagt/jge/abstracts/Montgomery_v51n5.pdf |title=Siccar Point and teaching the history of Geology |accessdate=2008-03-26 |author=Keith Montgomery |date=2003 |format=pdf |work= |publisher=University of Wisconsin }}</ref> The rocks above an unconformity are younger than the rocks beneath (unless the sequence has been overturned). An