Geomorfology

Geomorfology

KARST RELIEF

Inapproachable, inscrutable, desolate, unfeeling and resplendent, these eternal intermediaries have stood unmoving for eons of ceaseless creation between the sea in the west and the land in the east, between the depths and the sun, communing with the winds, the mighty thunder, the wild raging elements and the life-giving power of the bright sun.

I. Krajač, PhD, Hrvatski planinar, 1930, 8th edition, pp. 243-244

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).

Rillenkarren (“žlibe”)

Rillenkarren (furrow like karren) are formed in compact rocks, free of crevices or fissures, in small stone blocks (measuring only several dm2) as well as in large rocks. Their length can vary from mere millimetres to dozens of metres.

In the Park's peak area, and especially the Hajdučki kukovi and Rožanski kukovi area, which gets a lot of fog and clouds, precipitation originating from direct adherence of water particles from the clouds and the fog contributes greatly to the diversity of the grikes. For example, the area had a dense fog for every day of December in 1981 and 1990, and it had 160 cloudy days in 1984. 

FOR THOSE WHO WANT TO KNOW MORE

Rillenkarren are usually inclined at more than 30 degrees. The greater the inclination, the deeper and narrower the rillenkarren is, and the smaller the inclination, the shallower, broader and rounder they get, because water drainage is slower. Their edges then become increasingly thin and sharp, with secondary rillenkarren forming at their sides, smaller and shallower than the primary rillenkarren. Secondary rillenkarren often appear at inclinations of less than 12 degrees.


DOLINE (Sinkholes)

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.

FOR THOSE WHO WANT TO KNOW MORE

Solution dolines are usually funnel-shaped. They are formed by water insurgence in crevice systems in the surface layer of the karst, which is usually fragmented and very susceptible to corrosion. Soil often occurs on their floor, and plays a major role in further karstification. Measurements have shown corrosion to be up to three times stronger on doline floor, under the soil, than at doline walls, whether denuded or bearing a thin soil cover, which are only about 15 m away.

Cover-subsidence dolines and cover-collapse dolines are usually well- or kettle-shaped.  They are formed when the ceiling of underground cavities subsides or collapses due to the loss of roof stability.  Examples of these two types of dolines are frequent in the strictly protected reserve  of Hajdučki kukovi and Rožanski kukovi. Since this area is comprised of rocks made up of clasts, which wear more quickly than the binder holding them together, in tectonically fragmented terrains, dissolution, weathering and sheet wash are intensified. The processes cause the formation of ever larger cavities under the ground. The expansion of the cavities makes the cover unstable, and leads to gradual subsidence of the surface. Cover-collapse dolines occur when the roof of the doline suddenly collapses. They are shaped by momentary collapse of the roof deposits.

Dolines can also be formed when water-permeable rocks occur underneath impermeable surface rocks. Rock dissolution and sheet wash in the floor then lead to subsidence and the formation of dolines.



UVALE

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.

FOR THOSE WHO WANT TO KNOW MORE

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.


KRŠKA POLJA

Karst polje 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 polje is tectonic depressions, or areas that have “sunken” in relation to their surroundings. There is a single karst polje 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.

FOR THOSE WHO WANT TO KNOW MORE

Štirovačko polje is a flattened area measuring up to 3 km in width and 8 km in length, stretching from the north to the south, and spanned by steep adjacent hill slopes (Mrkvište, Javornik, Crni Vrh, Jasenovac, Pivac, and others). Morphologically speaking, it is a depression, and structurally speaking, Štirovačko polje is an anticline.


“KUKOVI”

”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.  

 

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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.

FOR THOSE WHO WANT TO KNOW MORE

Faults are fractures along which individual blocks of rock move (rise, fall or move longitudinally), and cracks and fissures are all crevices in a rock. All faults are cracks, but not all cracks are faults.

During the Last Glacial Period, the alternations of colder (glacial) and warmer (interglacial) periods, caused melting of the glaciers, so they consequently released large amount of water and it significantly contributed to formation of the pits. In that time, large amount of water flowed down the slopes and dolines (sinkholes). Water strongly dissolved the carbonate substrate and found its way to mostly steep to vertical fault and crack systems. By concentrated flows, mechanical (erosion and abrasion) and chemical (dissolution) forces, water increased the volume of cavities (pits). The largest verticals are located at the intersections of these systems.


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