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).

[1][2] The Kortrijk Formation consists predominantly of clay, sometimes sandy or silty.

In the westernmost part of Belgium it can be 125 metres (410 ft) thick, but it gradually wedges out to the east.

The Kortrijk Formation forms the lowest part of the Ieper Group and is stratigraphically overlain by the younger Tielt Formation (late Ypresian marine sand), part of the same group.