It consists of shallow marine sandstone and calcareous sands, deposited in the sea that covered Belgium 45 million years ago, in the Eocene.
The Brussels Formation crops out in Walloon Brabant and the north of the provinces of Namur and Hainaut.
The formation consists either of cross-bedded non-calcareous sands alternating with thin beds of marl and clay or calcareous fine sands with traces of bioturbation.
The formation was formed during the early Lutetian age (about 46 million years ago) and is part of the Zenne Group.
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