Aachen Formation, Geological formation in Liège region, Belgium.

The Aachen Formation is an Upper Cretaceous layer of glauconite-bearing sand that crops out in Belgian and Dutch Limburg and adjacent Germany. It rests unconformably on Carboniferous rocks of the Limburg Group and is overlain by the Vaals Formation. In West Flanders it lies in the subsurface as an aquifer.

The Aachen Formation was deposited during the Santonian and Campanian ages, roughly 85.8 to 70.6 million years ago, when a sea advanced from the north over Carboniferous rocks of the Limburg Group. It is named after the German city of Aachen and consists of glauconite-bearing sand. The overlying Vaals Formation followed after an erosion gap.

The name comes from the German city of Aachen. The glauconite-bearing sand was long known in German literature as Aachener Kreide, a term now dropped because the sequence also contains sands and clays, not only chalk. In West Flanders the formation lies in the subsurface and yields drinking water from an aquifer.

The formation crops out in Belgian and Dutch Limburg and adjacent parts of Germany; the photographed cut shows the glauconite-bearing sands with dark and rusty bands dipping gently north. In West Flanders it lies in the subsurface and serves as an aquifer tapped for drinking water. The overlying Vaals Formation and the Carboniferous Limburg Group below are more than three times older.

The formation is glauconite-bearing sand, not limestone; its green colour comes from finely divided glauconite formed in shallow seawater. Reported fossils include the coral Cunnolites, the plant Nicolia, conifers, cycads and pollen. In West Flanders the formation lies in the subsurface and yields drinking water as an aquifer.