[1] It is stratigraphically equivalent to the middle part of the Chalk Group of England, and named after the German city of Aachen.

The formation crops out in southern Belgian and Dutch Limburg and adjacent areas in Germany.

The formation can also be found in the subsurface of West Flanders, where it forms an aquifer from which drinking water is won.

It was deposited during the Santonian and Campanian ages (85.8 to 70.6 million years ago) of the Cretaceous Period.

The formation rests unconformably on top of Carboniferous rocks of the Limburg Group, which are more than three times as old.