Small ‘holes’ in the landscape can be seen here and at other points along the Geotrail. These are mines where iron ore was once mined. The iron deposits are associated with the 360 million year old rocks from the Devonian Period which formed at depths of around 3,000 m. You are actually standing on an old deep sea floor! This hostile sea was home to virtually only spherical radiolarians. Their shells consisted of quartz and were barely one millimetre big. Nevertheless, their skeletons are the source of enormous masses of rock because there were countless radiolarians thriving in the ocean like plankton. Upon their death they accumulated in great masses on the seafloor. The very hard rocks, which consist almost entirely of quartz, can be found in two forms: as compact radiolarite (lydite) and as densely foliated shale.