The formation consists of early Eocene marine clay and sand.
The Dongen Formation consists predominantly of marine clay, alternated with layers of fine sand or glauconiferous marl.
The clay can also contain glauconite and locally thin layers of tuffite or loam can occur.
All of these units are marine sands and clay cats from the Ypresian (55.8 - 48.6 million years ago) and Lutetian (48.6 - 40.4 million years ago) ages, when the eustatic sea level was relatively high after the Paleocene–Eocene Thermal Maximum.
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