old marl quarry mine on mount saint peter or sint-pietersberg with bare trees

old marl quarry mine mount saint peter or sintpietersberg bare trees maastricht autumn cave caving cement day earth environment erosion europe explore formation geologic geology ground hill holland landscape limburg limestone mining mountain mysterious nature nederland nobody opening plateau reserve rock sand scenery scenic slope speleology stone surface texture trail tree tunnel underground underworld velvet view wall winter part caestert sunny south netherlands