Conglomerate karst in Udin boršt

There are two different reliefs from the geological point of view: west and south karst-like brink which developed on limestone conglomerate where we can find different springs and fluvial denudation formed by Želinj stream with its numerous tributaries on the impermeable rocks (Oligocene dark-greenish-grey clay) in the east and north part. Its central part was dammed and thus formed into two small lakes called Račnjak or Pri racah (by the ducks). The local name Mlaka (pond) indicates that the ground is marshy in this part.
Despite occasional impermeable rocks, the surface of Udin boršt consists of limestone conglomerates seen only in some areas due to the alluvial deposits covering the terrace. This geological ground presents the most important basis which defines the relief, soil, vegetation, the use of space and biotic diversity of this area. The influence and people's connection with the forest is shown in the cultural heritage.

Udin boršt is 200,000-300,000 years old (late or middle Pleistocene) and thus belongs to the oldest and most karst-like conglomerate terraces in Gorenjska region. It was formed under the influences of Tržiška Bistrica and Sava Rivers both depositing round rocks, gravel stones, which combined into conglomerate stone with the calcite binder. This rock is very water permeable. Underneath is impermeable grey marly soil known as Oligocene dark-greenish-grey clay. Water creates karst caves at the level where they connect. Being 530 to 410 m high, it is not more than 50 m above the younger terrace from würm period and it slopes steeply from north to south. Such geological structure enabled karst-like Udin boršt in an otherwise non-karst region. Apart from all the above mentioned characteristics, the karst in this region has 4 typical definitions. It is first mentioned as a conglomerate type of karst which distinguishes from karst on limestone. Due to rather shallow conglomerate rock (from 10 to 50m) it is also named the shallow karst and because it is separated from other karst regions, solitary karst. Being only 200,000-300,000 years old, it is a very young or eogenetic karst.
Numerous superficial karst forms (sinkholes, rifts, gorges, caves, karst springs) and karst caves can be seen in Udin boršt. Sinkholes are most common in the west and south part of Udin boršt where the terraces are older and have formed in time as a consequence of exogenetic processes. According to cadastre of Slovenian caves there are 14 caves in the area of Udin boršt of which two are longer than 500 meters. All the caves except two are on the western brink of Udin boršt. The two most known are Arneševa luknja in Spodnje Duplje (815m long and 13m deep, the longest cave in Udin boršt), which was mentioned by Valvasor in The Glory of the Duchy of Carniola, and Dupulnek in Zadraga (306m long and 6m deep), which Vrhovnik (1885) described as the most beautiful and biggest cave with impressive interior with more than a meter long whitish stalactites.
In the karst-like area the precipitation seeps underground and resurfaces in the form of springs and in the eastern part the precipitation is accumulated in streams. More important streams in the area of Udin boršt are: the permanent spring in Arneševa luknja, Dupulnek spring, the spring under the entrance of Velika Lebnica cave, Strahinj spring, the spring under the entrance of Kadunčev studenec cave and two bigger and several little ones at Želinj spring. These springs were an important source of water for local people and preserved rural basins still remind us of this (for example the basin in Zadraga, Zgornje Duplje and Spodnje Duplje).
The karst of Udin boršt is, because of its well-developed superficial and underground forms, often described by experts in karst geomorphology (karstologists) as the most beautiful, practically a textbook example of conglomerate karst in Slovenia.