Polhov Gradec Hills


The Polhov Gradec Hills are a pre-Alpine hilly region of northwestern Slovenia. To the north they border the Škofja Loka Hills, and to the south they border the Ljubljana Basin and the Ljubljana Marsh. The largest watercourse is the Gradaščica River. In 1971, a proposal was put forward to protect the region as a nature park, but it was never formally adopted.

Name

The hills are named after Polhov Gradec, the central settlement in the range. In German, they were known as the Billichgrazer Gebirge or Billichgrazer Bergen. Slovene geographers have rejected the Slovene name Polhograjski Dolomiti as inappropriate.

Geography and setting

The hills sit on the southeastern margin of the Ljubljana Basin, where steeper relief reflects vigorous tectonic uplift and a dense network of north-east⁠–⁠south-west faults. Slopes often exceed 40° and descend into narrow ravines such as Jevc Ravine above the Little Božna valley. Numerous weak but permanent springs issue where permeable Middle-Triassic dolomite overlies impervious Permian flint-sandstone; these contacts lie just above valley floors and supply the headwaters of the Gradaščica river system.

Geology and relief

Much of the massif is built of heavily fractured Middle Triassic dolomite—a magnesium-rich carbonate rock that weathers more quickly than adjacent limestones. Overthrusting has left the dolomite riding on softer sandstone, producing a pronounced structural slope that funnels run-off into deep, debris-choked gullies. Field measurements in one 130 m-long gully recorded annual denudation of about 170 tonnes per hectare, an order of magnitude greater than chemical solution rates on nearby plateaux.
Physical weathering dominates the steep, vegetation-poor upper slopes, where freeze–thaw processes and rain-wash loosen blocks that are transported downslope during cloudbursts. Within the colluvium, water normally seeps through coarse debris, but exceptional storms briefly raise the water table, triggering debris flows that can lower the gully floor by half a metre in a single event. Springs emerging along bedding planes and faults typically align with the 50° westerly dip of the dolomite strata, underlining the structural control on local hydrology.
Although corrosion removes only a fraction of the mass lost through slope failure, it sculpts the gentler, clay-mantled hollows and foot-slopes that support meadows and smallholdings. The resulting patchwork of forested scree, grassy clearings and cultivated pockets is characteristic of dolomite terrains throughout western Slovenia, but is particularly well expressed in the Polhov Gradec Hills.

Peaks