<P> The contrast in lithology between formations required to justify their establishment varies with the complexity of the geology of a region and the detail needed for geologic mapping and to work out its geologic history . </P> <P> Formations must be able to be delineated at the scale of geologic mapping practiced in the region . The thickness of formations may range from less than a meter to several thousand meters . </P> <P> Geologic formations are typically named for the geographic area in which they were first described . </P> <P> Strictly, formations cannot be defined on any other criteria except primary lithology . However, it is often useful to define biostratigraphic units based on paleontological criteria, chronostratigraphic units based on the age of the rocks, and chemostratigraphic units based on geochemical criteria . </P>

Where can formations of layered rock be found