The water barriers of the Idrijca are among the strangest industrial monuments in Europe: massive stone and timber dams built across a narrow mountain river not to store water for drinking or power, but to float logs. Locals call them klavže, and the biggest have been nicknamed the Slovenian pyramids for their huge, carefully cut blocks. They lie in the deep, wooded gorge below the Vojsko plateau, a short ride from the town of Idrija.
The reason they exist is mercury. Idrija's mine, opened at the end of the fifteenth century, was for centuries one of the largest sources of mercury in the world, and the smelters burned enormous quantities of wood. The forests upstream were cut, the logs piled behind the dams, and then the gates were opened and the flood carried the timber down the gorge to the town. The mine and the town are now a World Heritage site together with Almadén in Spain, and the old miners' houses, the castle and the shaft buildings make a fine short walk.
Riding up here is a good test. The roads from Idrija towards Vojsko and along the Idrijca are narrow, twisty and often in deep shade, with mossy edges, damp corners and occasional loose stones from the steep slopes above. There are hairpins, tight sections with no centre line and very little traffic. The plateau of Vojsko itself is a high, open landscape of scattered farms and meadows with views across the forested ridges towards the Julian Alps. In wet weather or early spring, take it slowly; the shade means patches stay damp long after the rain stops.
The Idrijca is clear and cold, flowing over pale limestone, and the forest is mostly beech and spruce. In summer it smells of resin and wet stone, and in autumn the whole valley turns copper. During the Second World War these forests hid partisan hospitals; Franja, the most famous, is in a gorge not far away near Cerkno.
Fuel, food and coffee are in Idrija, about nine kilometres away. While you are there, order žlikrofi, small dumplings filled with potato and herbs that are a protected local speciality, and look into a lace shop: Idrija bobbin lace is another tradition that grew up around the mine, made by miners' wives to bring in extra money.