A geological formation is a pack of bedrock which geologists consider to be part of the same type for posessing similar characteristics. Since all formations are defined in the field these are mainly characteristics that are visisble to the naked eye, like rock lithologies.

Usualy bedrock from one formation formed within the same period, but this isnt always the case. Often rocks of a formation at one location are older or younger than rocks of the same formation in a different location. This means that the characteristics are characteristic for a certain depositional environment. This depositional environment could have been formed in different times on different places. The formation is then labelled diachronic.

With thanks to Johan Vellekoop.