Unconformity

a break in the deposition of sedimentary rocks, allowing erosion of previously formed rock before eventual deposition of further sediments. It is usually represented by an obvious difference in the attitude of the rocks on either side of the unconformity, with the upper lying unconformably on the lower.

Taken from Dictionary of Science

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An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe any break in the sedimentary geologic record. The phenomenon of angular unconformity (see below) was discovered by James Hutton, who found examples at Jedburgh in 1787 and at Siccar Point in 1788.

The rocks above an unconformity are younger than the rocks beneath (unless the sequence has been overturned). An unconformity represents time during which no sediments were preserved in a region. The local record for that time interval is missing and geologists must use other clues to discover that part of the geologic history of that area. The interval of geologic time not represented is called a hiatus.

Taken from Wikipedia