To preserve the natural shape of a Rock window, the place was designated as a natural monument in 1970 with protected area of 0.2ha. It is situated in the extremely beautiful Big glen of Kostino village, close to the villages of Krushka and Golyama bara. The window rock is an extraordinary natural rock bridge shaped by the turbulent waters of Kostino gully. Going back in the geological past the columns were three, but the forces of Nature have separated them. Presently only the second and third shape the bridge with the last being a big stone mushroom with the hood being part of the upper layer. Its height is 10m, length of 15m, width of 7m and upper layer thickness of 1.5m. The three columns are equal in height formed in solid green volcanic tuffs. The upper layer is white formed of thin-layered limestone rocks and very resistant to erosion. This is the reason it stays above the columns. The Rock window belongs to the same geological period as the Stone Wedding and the Mushroom Rocks, of similar materials and in a similar way. Note: The monument is suitable for tourism with a guide. There is a marked route, but not easy to follow. From there starts also a route to Utroba cave, but the terrain is quite hard.

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