In the valley of the stream Djedovica, along the Voćin-Kamenska road, on the south entrance to Voćin, there is a unique geological monument of nature - Rupnica. The monument is placed in the area of the former quarry. This is the unique locality in Croatia where are expressed columnar jointing on volcanic rocks. The pillars are mostly vertical apart from those in the middle, which are bent. When one takes a first look of these pillars, stays wondering and surprised with the fact that this is the act of nature, that is of a volcano, which is known only for destructions.

The origin of such symmetrically shaped pillars, usually four-sided, is related to the appearance of the extraction of volcanic effusive rocks, and it is formed when the lava of homogeny composition gets cold. By cooling, the volcanic pile tightens so the area that gets first cold breaks in the shape of polygon (usually a square, pentagon, or hexagon). As the surface cracks as they get colder penetrate deeper into volcanic pile, this is the final indicator of the mentioned process of extraction. The rocks are mostly of green-grey colour, of a porphyry structure with holo-crystal fine granulated basis. Locality of Rupnica is a part of petrologically heterogeneous volcano pile in which there are different varieties of basalts, andesites, rhyolites tuffs and volcanic agglomerates.

Although such morphological characteristics are not know in Croatia, in the world this phenomenon can be found in the Czech Republic highlands, in New Zeeland, and the most impressive pillars up to 75 m high, can be find in the Yellowstone National Park. The locality is still not explored in details, so that according to some scientists the rocs are from the Cretaceous period (approx. 70 millions years old), while other claim that they are from Miocene period (approx. 16 million years old). Locality of Rupnica was proclaimed first protected geological monument in Croatia, on October 14, 1948.