A distinctive horizon which is used for regional correlation of lithology. A good marker horizon is distinctive, widespread, and represents a relatively short period of geologic time. For example, ash from a volcanic eruption, debris from a meteorite impact, etc. It is GeoMan's opinion that humans will represent one of the earth's finest marker horizons in several hundred million years. Our effect on the surface is certainly distinctive and widespread, and, at the rate we are going, it is likely that our species will have a relatively short lifespan (speaking in terms of geologic time, of course).

This definition is in context to Earth Science. See more contextual defintions for Marker Horizon (Or Bed).