The Vratna River is a right-side tributary of the Danube (feeds into the Danube downstream of the Iron Gates), on the western rim of the Dacian Basin. It is only 26 km long and its drainage area is partially situated on the eastern slopes of the Carpatho-Balkanides mountain range (Mt. Veliki Greben).
During the Pliocene, the Dacian Basin was filled with water, but the morphological outcomes of that lacustrine period are not preserved in the present time. The upper part of the Vratna drainage area is developed in crystalline schists, while the lowest part is composed of marls and clayey and sandy sediments. In between those two parts, taking the central position, there is a karst area developed in Tithonian limestones, covering about 10 km2. The limestones are clearly stratified and have an eastward inclination. A portion of 3,5 km of the Vratna valley is entrenched in these limestones, as an about 150 m deep gorge.
The outstanding characteristic of the gorge are three natural bridges – Mala Prerast (meaning “small natural bridge”), Velika Prerast (“big natural bridge”) and Suva Prerast (“dry natural bridge”). When summed up, they shelter the river course at the length of 94 m. Velika Prerast and Mala Prerast are situated closely to each other, at the final downstream part of the gorge, while Suva Prerast is 3 km upstream. Mala Prerast shelters the river at the length of 15 m. Its opening has about 30 m of width and height (respectively). Only 100 m upstream from Mala Prerast, there is another natural bridge – Velika Prerast. It is 45 m long, with the opening of approximately 25 x 30 m. The rocky arch has a considerable thickness of 30 m. The upper part of the arch has a form of a peak – the very top of it is at 65 m of relative height above the river bed.
Suva Prerast, the third natural bridge, is located in the most upstream part of the gorge. It is 34 m long, with the opening of about 20 x 15 m. The name “dry natural bridge” originates from the fact that in the late summer all the water from the Vratna disappears in a stream-sink 50 m upstream from Suva Prerast.
Three natural bridges in one relatively short gorge are an outstanding and unique value. The natural value is accompanied with one having cultural significance – the Vratna Monastery, situated right at the downstream exit from the gorge.