Large boulders and small pebbles: these have all been brought here by the River Maas during the last two million years. In the course of time the river’s drainage basin has changed considerably. This explains why Maas gravel is to be found virtually throughout southern Limburg. It also explains why very different types of river-borne rock are found in different places: the erosional products of the Eifel, Ardennes and Vosges and Lorraine.