An angular unconformity can form when rocks that were initially horizontal are tilted, eroded and subsequently buried beneath younger layers of sediment. Image credit: Schlumberger.
An unconformity is a surface between two contiguous rock strata representing a break in the normal succession; usually owing to erosion having removed layers of rock before the deposition of the younger stratum.
Unconformities may be parallel (strata parallel), angular (strata not parallel), heterolithic (sediment over intruded igneous rocks, or nondepositional (a genuine break in the depositional pattern). See also sedimentary rocks.