Geomorfology

Karst relief

Abundance, diversity and beauty of karst forms have contributed to recognizability of the National Park in proffesional public, both in domestic and in foreign. The carbonate composition and fragmentation of limestone and dolomite rocks, combined with the effects of water, ice, sun and wind, have led to intensive karstification and the formation of distinctive karst relief. 

Karst is a special type of relief, formed by the process of karstification, or chemical dissolution or carbonate rocks, limestones and dolomites, under the influence of stormwaters. Given its carbon dioxide content, the water has the effect of a weak acid, gradually dissolving the calcite in the carbonate rock, and shaping different karstic features on the surface and under the ground.

Surface karst forms Karrens

Karrens are the smallest and most prevalent karstic features. Their multiplicity and distinctiveness in Northern Velebit, and especially the strictly protected reserves of Hajdučki kukovi and Rožanski kukovi, make them one of the most stunning traces left by water in the stone. They are characterized by an exceptional variety of shapes.

In terms of the extent of rock cracking, karrens are categorized into those that are formed in compact rocks, without crevices or fissures (rillenkarren, kamenitzas and biocorrosion roots), and those whose formation is predisposed by crevice and fissure systems (fissure karrens).

Sinkholes – Geomorfology

Dolines (sinkholes) are small, closed stormwater drainage basins, varying in diameter from several metres to several hundred metres. They are usually funnel-, kettle- or well-shaped. Swallets, often found at the bottom of dolines, drain water under the ground, expanding and deepening the dolines.

There are 902 dolines in „Northern Velebit” National Park. The largest are located in the Hajdučki and Rožanski kukovi area. Some measure from 100 m to more than 400 m in diameter, and the largest measure almost 800 m in diameter. Dolines can be 50-100 m deep. Edges of adjacent dolines occasionally come together.

Dolines are categorized into solution dolines, which occur due to the corrosive effect of water, cover-subsidence dolines, and cover-collapse dolines.

Uvale – Geomorfology

Uvala are closed, usually elongated karstic depressions whose length can range from several hundred metres to several kilometres.

Uvala formation is usually associated with larger tectonic crack zones, which are conducive to strong corrosion wear of the fragmented rock complex. As a result, the layout of karst uvala matches the direction in which the zones extend.

Several large uvala were formed in „Northern Velebit” National Park, notably Veliki and Mali Lom, Lubenovac, Tuderevo, Bilensko and Dundović Mirovo, Bilenski padež, Šegotski padež and Dundović padež.

Karst fields

Karst fields is spacious flattened valley, surrounded by hill slopes. The largest morphological karstic features, they are formed by chemical effects of water in carbonate rocks, with plate tectonics playing a major part. Karst field is tectonic depressions, or areas that have “sunken” in relation to their surroundings. There is a single karst field in the National Park area, located in the northern section of the Štirovača – Klepina duliba field. Covered in younger Quaternary sediment, the field features several springs in its northern section, one with spring-water intake. There is a stream cutting across the polje, and several swallets on its south end. The field is flooded when the snow melts and during heavier rainfall in the spring, and it dries up in periods of drought.

Kukovi – Geomorfology

”Kukovi” (in local language) are high, prominent geomorphological features rising above the surrounding terrain. They are formed under the influence of internal (endogenic) and external (exogenic) forces. Internal tectonic forces caused folding, cracking, breaking and crushing of older limestone rocks. External forces (water, ice, sun and wind) caused mechanical and chemical weathering. Thus were formed large blocks and boulders, as well as small fragments of rock material, whose hardening formed new rock type, the Velebit breccia. Further actions of these forces weathered the rocks and the cracks widened, flushing the material to lower and lower positions (selective denundation). The most resilient parts of the rocks – Velebit breccias, are left standing as magnificent witnesses of the turbulent geological past. They are popularly known as “kukovi”.

The most famous ”kukovi” of the Park are: Strogir, Hajdučki i Rožanski kukovi, Begovački kuk, Jurekovački kuk, Vratarski kuk, Krajačev kuk, Mali kuk, Novotnijev kuk, Veliki Kozjak and others.

Subterranean karst forms Pits and Caves

Pits and caves are naturally formed subterranean cavities whose formation is usually associated with the processes of mechanical and chemical effects of water in fissure system zones in carbonate rocks, as well as along bedding planes and other discontinuities. Large verticals and deep channels are main morphological features of subterranean structures in National Park. Horizontal or oblique channels are rare, but the most common are usually narrow and winding. By 2020, 617 subterranean structures were discovered in the National Park, and pits accounted for 95% of this number.  

The chemical action of water in the pits is also evident in the formation of underground rillenkarrens, while vortex pots, on the ceiling and walls of the pits, indicate to rapid vortex movement of water and pebbles. The movement of water through cracks and fissures often creates labyrinths, while along the faults, large verticals and halls are formed, where in some places cave decorations (speleothemes) and underground lakes are formed.