The formation consists of marine clay and silt, alternating with more sandy layers.

The Hannut Formation was formed during the early to middle Thanetian age (Late Paleocene, about 57 million years ago).

The formation is named after the town of Hannut in the province of Liège.

In the northern and eastern parts of Flanders the Hannut Formation lies stratigraphically on top of the Heers Formation (middle Paleocene sands and marls).

This article about a specific stratigraphic formation in Europe is a stub.