A Late Neolithic culture of the Paris basin of northeast France c 3400-2800 BC, named after three rivers. It is best known for its megalithic tombs of gallery-grave type (hypogées), semi-subterranean funerary houses, and allées couvertes. The megalithic tombs often include port-hole slabs. In the chalk country of the Marne, rock-cut tombs were similarly made and some have hafted axes or schematized 'goddess' figures carved on their walls. Native artifacts include transverse arrows, antler, daggers, and rough, plain flat-based pots of cylinder and bucket shapes. The pottery type is the coarseware flat-based flower pot. Trade brought copper, Callaïs stone and beads, and Grand Pressigny flint to the region. The culture seems to have a composite origin, and certain elements of the assemblage occur in other - perhaps unrelated - cultures outside the SOM area proper. The SOM type of megalithic tomb is found from Brittany to Belgium, Westphalia, and Sweden, while similar crude pottery occurs in Brittany, west France, Switzerland (Horgen), and Denmark.