Idrija and Cerkno: the hill roads between the Alps and the Karst

Between the Julian Alps and the Karst there is a band of steep, wooded hill country that most visiting cyclists ride straight through on the way to Bovec. Idrija and Cerkno sit in the middle of it. The roads are narrow, the valleys are deep, and there is nothing here over 1,700 m, which is exactly why it works: the passes open earlier than Vršič, the traffic is light, and a single day can hold three separate climbs of 400–600 m without ever leaving tarmac.

This is a guide to what the riding consists of, what the weather does up on the plateaus, and where the tarmac runs out. If you want the wider picture of the country first, start with our overview of cycling in Slovenia.

Where you are

Idrija is a mining town. Mercury was found in the stream here in 1490, the mine ran for five centuries, and in 2012 it was inscribed on the UNESCO World Heritage list together with Almadén in Spain as the two largest mercury mines in the world. The town now sits inside the UNESCO Global Geopark Idrija, which covers almost 300 km² of the surrounding hills. Cerkno is a short way up the valley to the north-west, a small town in a basin under Porezen, at 1,630 m the highest summit of the Cerkno Hills, with Škofja Loka 39 km away over the pass.

What that means on a bike is that both towns sit low in river valleys, and everything interesting is up. There are three broad directions.

• North to Cerkno and over Kladje. The valley road from Idrija follows the Idrijca river to Želin and turns up to Cerkno. From Cerkno the road east to the Poljane valley crosses the Kladje pass, which is the climb that decides Maraton Franja.
• West on to the Vojsko plateau. A dead-end climb from the Idrijca valley to the highest village in the Primorska region, and the coldest place in this post.
• South to Črni Vrh and the edge of the Karst. Roads climb out of the Idrija side on to a limestone plateau of sinkholes and dry fields, then drop into the Vipava valley on the far side.

Kladje, the Franja climb

Kladje is the one road in this region that people outside Slovenia have heard of, because it is the decisive climb of Maraton Franja, the country's biggest road sportive, which comes through Idrija and Cerkno on its 156 km loop from Ljubljana. The organiser's route description gives it as 7 km at an average of 6.3% with a maximum of 10%, ridden from the Cerkno side, and warns that the 10 km descent that follows towards Trebija has sections of poor surface and is treated as dangerous on race day.

Ridden on an ordinary day, it is a good climb rather than a great one: steady, wooded, with the steep pitches in the upper third. The Cerkno tourist board's own circular route over Kladje calls it the most difficult part of the Franja course, which is fair, and pairs it with a return through Bevkov vrh (1,051 m) and the farm villages above Otalež, though that loop mixes tarmac with forest track and is not one for 25 mm tyres. If you are riding Franja, this is the climb to know: we cover how to pace a gran fondo with power separately, and the short version is that Kladje comes at 87 km, after the Vrhnika hill and 60 km of rolling valley, so the number you can hold on it is lower than the number you could hold fresh.

Vojsko, the highest village in Primorska

The road on to the Vojsko plateau climbs out of the Idrijca valley west of Idrija to a scattered village that the Geopark describes as the highest in the Primorska region. It is a cul-de-sac for road bikes: the Geopark's 26.5 km Vojsko trail continues across the plateau on gravel roads and woodland tracks past the wartime partisan printing works, and it is marked as easy, but on a road bike you climb up, look at the view, and come back down.

It is worth doing for the climate alone, which is the most instructive thing in this post. ARSO, the national met office, runs a station at Vojsko at 1,067 m, and its 1981–2010 normals are the reference for what the plateaus here do:

Two things to take from it. First, 2,275 mm a year is nearly as wet as Bovec, and the autumn months are the worst of it, so a September or October trip here needs the same jacket discipline as the Soča valley. Second, the plateau carries snow for well over a hundred mornings a year, and the record depth in the normals runs to over two metres, so the high roads are not reliably rideable until spring is properly under way. The valley roads between Idrija and Cerkno are lower and clear earlier.

Črni Vrh and the plateau roads

South of Idrija the road climbs to the Črni Vrh plateau, a stretch of karst fields, sinkholes and dry valleys at the top of the hills that separates the Idrija side from the Vipava valley. The Idrija tourist board's gEoBIKE circuit through Dole, Gore and the Črni Vrh plateau is 46 km with 1,046 m of climbing and is filed as a demanding trail, and it finishes back down in the Idrijca valley by the river bathing spots. Their shorter High Karst trail across the same plateau, from Predgriže through Idrijski Log and Zadlog, is 7.5 km on a mix of asphalt and macadam, which tells you the important thing about this plateau: the through roads are tarmac, the field roads between the villages are not.

Once you are up, the plateau is the easiest riding in the region, gently rolling on quiet lanes between the villages, and the descent on the far side into the Vipava valley drops you into vineyards and a completely different climate. That makes a long loop possible: Idrija up to Črni Vrh, down to the Vipava valley, back over towards Godovič and the Franja route into Idrija. It is a big day with a lot of climbing, and the Vipava side is where the burja blows in winter, so check the wind forecast before committing to the far side out of season.

The honest caveats

• The main valley road is the main road. Between Idrija and Cerkno and on to the Soča, the Idrijca valley road carries the region's through traffic, including on Franja weekend. It is not busy by most countries' standards, but it is the one road here that is not quiet, and the tunnels and rock cuttings near Želin are narrow.
• Half the marked routes are not road routes. The Geopark publishes a numbered set of trails, and the ones that read as cycling routes are mostly gravel and forest road. Check the surface line before assuming a listed route suits a road bike; our guide to reading surfaces in a planner covers how to do that.
• There is no big pass. Nothing here reaches the altitude of Vršič or the Mangart road. The reward is quantity, not height: three or four separate 400–600 m climbs in a day rather than one long one.
• Services are sparse on the plateaus. Idrija is where the shops are; up on Vojsko and Črni Vrh, carry what you need.

Planning it

Base in Idrija for the plateau roads and Kladje, or in Cerkno if the point of the trip is the Franja climb and Porezen. Late May to early October is the season for the plateaus; the valley roads work from April. Build the loops in a planner that shows you the climbing before you commit, because the descents here are steep and twisting and a 60 km loop can contain 1,500 m without looking like it on a map. Moveee's route builder shows the profile of a line before you ride it, and a route saved there can be ridden indoors in Moveee Indoor in the months when the plateau is under snow.

The short version: come for Kladje if you are racing Franja, come for Vojsko and Črni Vrh if you want the quietest tarmac climbs within an hour of Ljubljana, and bring a jacket in every month with an R in it.