The matrix is reddish-brown, silty, calcareous and optically active with small amounts of calcareous sand and rare shell. The quartz silt is about 20% and also contains some heavy minerals (including mica and hornblende) as well as common opaques. The calcareous sand consists of rounded chalk grains (up to 100µm). Rare are fossilized mollusc shell fragments (up to 200µm).