The Voort Formation consists of shallow marine sands with a late Oligocene age (Chattian stage, about 25 million years old).
In Belgian lithostratigraphy the Voort Formation is part of the Rupel Group, though other formations in this group are slightly older (of the early Oligocene Rupelian stage).
In the Netherlands the Voort Member is part of the Veldhoven Formation.
The Voort Formation consists of a 60 meters thick layer of greyish to greenish micaceous and glauconiferous fine sand.
crags or pyrite rich clay layers can occur locally.