The formation of Borgloon is a geologic formation from the early Oligocene (Rupelien, between 33.9 and 28.4 million years old) that day sunsets in the south of the Belgian provinces of Walloon Brabant and Limburg.
The formation of Borgloon is 20 to 10 meters thick, to the west it becomes thicker. The formation belongs to the Tongeren Group and is stratigraphically seen above the formation of Saint Huibrechts-Hern and under the formation of Bilzen. To the east, the formation is divided into the member of Henis and the member of Alden Biesen; In the west it is subdivided into the Low of Heide, the Member of Kerkom, the Member of Boutersem and the Hoogbutsel Low.
The formation was deposited in a lagoon or in fluviatile flushes. The foundation of the formation is a thickening layer of green and gray clay, followed by a change of clay and sandy layers. The top is a low yellow sand in which many shell fragments occur. The formation is a well-known find of fossils of Oligocene mollusks.
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