A gap in geologic history, where there is a missing layer of rock or sediment, is called an unconformity.

There are several types of unconformities, each reflecting a different type of geologic event:

* Angular unconformity: Older, tilted or folded rock layers are overlain by younger, horizontal layers. This indicates a period of uplift, erosion, and then subsidence.

* Disconformity: Parallel layers of sedimentary rock are separated by an erosional surface. This indicates a period of erosion and then renewed deposition.

* Nonconformity: Sedimentary layers are deposited directly on top of igneous or metamorphic rock. This indicates a period of uplift, erosion of igneous or metamorphic rock, and then subsidence for deposition.

Unconformities are important to geologists because they provide evidence of past geologic events and help us understand the sequence of events that shaped the Earth's history.