It consists of marine clay, silt and sand, deposited in the shallow sea that covered northern Belgium during the Ypresian age (around 50 million years ago, part of the early Eocene).

The base of the formation is formed by silty clay and clayey silt (Merelbeke Member).

On top of this are laminae of silt (Pittem Member) and beds of very fine sand, disturbed by bioturbation (Vlierzele Member).

On top of the Gentbrugge Formation are deposits of the Lutetian Zenne Group.

In the north of Belgium this is often the Aalter Formation (marine clays and sands).