The Brussel Formation is a geologic formation in the subsurface of central Belgium. The formation is named after the Belgian capital, Brussels. It consists of shallow marine sandstone and calcareous sands, deposited in the sea that covered Belgium 45 million years ago, in the Eocene. The Brussel Formation crops out in Walloon Brabant and the north of the provinces of Namur and Hainaut. It is normally about 30 meters in thickness. In Flemish Brabant and Antwerpen, the formation can be found in the subsurface.