Basal complex: Lower Devonian clay slate, sandstone and greywacke (Siegen tier / Herdorf layers), partly covered by maar tuffs. Particularities Starting in the 1980s, the University of Trier, and later on German Research Centre for Geosciences in Potsdam until today, numerous drilling cores were taken from the Holzmaar and analysed. New research methods have been tested and optimised here. The sensors of the green buoy floating in the lake continuously measure the water parameters and algae growth at various water depths. These are queried by the Kernforschungszentrum Jülich by satellite for monthly analysis. During the 16th century the lake was dammed toward the south to supply the Gillenfeld saw mill in the forest about 2 km to the south.