The formation consists of marine clay and sand, deposited in the shallow sea that covered northern and central Belgium in the Eocene epoch.

The base of the formation consists of glauconiferous clayey sand alternating with organic rich (humus and peat) layers (the Aalterbrugge Member).

On top of this is a sequence of clay, sand and sandstone layers, rich in fossils (the Beernem Member).

The top of the formation consists of fossil rich, glauconiferous fine sand (the Oedelem Member).

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