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 fault
ing, 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