The Kortrijk Formation (Dutch: Formatie van Kortrijk; French: Formation de Courtrai; abbreviation: Ko; named after the West Flemish city of Kortrijk) is a geologic formation in the Belgian subsurface. This formation crops out in northern Hainaut, southern West and East Flanders and in Walloon Brabant. The formation consists of marine clay from the Ypresian age (early Eocene, about 54 million years old).