Here the basal complex consists of Lower Devonian sandstone of the Klerf layers from the Unterems tier (405 million years). They’re interpreted as mudflat deposits. At the NW edge of the maar remains of river deposits from the Lower Trias (approx. 235 million years) will crop out.
In 2004 the Institute of Physical Geography at Frankfurt University drilled about 25 m deep into the centre of the at the time still dry maar. Evidence of a sequence of glacial detritus and younger, interglacial period lake beds was found in this drilling core.
These approx. 16 m thick lake beds are certain evidence a maar lake existed for an extended period. In the Tranchot map of 1811 of this region the Eichholz maar was still labelled as the water filled Gussweiher. The maar cone has demonstrably been used by people since Roman times. It was dried several time and then again filled for fish farming. 2007 / 2008 the maar lake was renatured again, i.e. it was again filled with water. For this purpose the Gussbach which previously flowed through the dry maar was dammed.
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