'Mergel' is the regional name for the soft chalk rock which in most parts of southern Limburg is found in isolated outcrops or under a thin soil cover. Depending on the amount of iron oxide (rust) which it contains, the colour varies from light white to light yellow or light brown. The 'mergel' consists almost entirely of fragments of animals that lived here at the end of the Cretaceous era, when the area was flooded by the sea. In fact, it represents an extremely thick pile of fossils.