Unconformity

An unconformity is a widespread surface separating rocks above and below, which represents a gap in the rock record. Unconformities occur when either erosion wears away rocks, or rock deposits never form. Therefore, a time gap exists between when the rocks below the unconformity formed and when those above it formed.

Unconformities are classified as three types. The most easily recognized are angular unconformities, which show horizontal layers of sedimentary rock lying on tilted layers of sedimentary rock. The upper layers may not be perfectly horizontal, but they do not lie parallel to the lower layers. The second type of unconformities are disconformities, which lie between parallel layers of sedimentary rock. The third type are nonconformities, which divide sedimentary layers from metamorphic and intrusive (cooled inside the earth) igneous rocks. Common to all three, erosion causes them to form, and younger rocks sit on older rocks.