(
Geology)
A boundary between
rock strata where there is a
discontinuity in the
time sequence where the rocks were created. "Unconformity" also refers to the time gap between the two strata.
Unconformities can be created by
faulting, but they most often appear where a
landscape was exposed to the surface, eroded for awhile, then much later, buried with sediment or lava.
A major unconformity appears over much of
North America, at the boundary between the
Precambrian and later periods, sometimes with a gap of as much as 300 million years. This unconformity is dramatically revealed at the
Grand Canyon, where
John Wesley Powell spotted it and named it
The Great Unconformity during an
1876 expedition.