Acta geographica Slovenica, 46-1, 2006, 33–55
RELIEF EVOLUTION IN THE HINTERLAND OF THE PIVKA RIVER
RAZVOJ POVR[JA V POVIRJU PIVKE
Gregor Kova~i~
A view across the Petelinsko jezero in the Upper Pivka region
(photograph: Marjan Garbajs).
Pogled prek Petelinskega jezera v Zgornji Pivki (fotografija: Marjan Garbajs).
Gregor Kova~i~, Relief evolution in the hinterland of the Pivka river
Relief evolution in the hinterland of the Pivka river
UDC: 911.2:551.4(497.4) COBISS: 1.01
ABSTRACT: The article discusses relief evolution in the catchment area of the highest situated perma-nent spring of the Pivka river - the Piv{ce karst spring near the village Zagorje. The discussed landscape, which lies on the southernmost edge of the Upper Pivka valley, shows several geomorphological parti-cularities, which are the result of different geomorphogenetic processes in the past. The basic characteristics of the present relief were shaped in the Pliocene when also the original valley of the Pivka river was for-med. The region was more profoundly transformed by Pleistocene processes, which strongly altered the surface. Apart from karstic relief forms, there are several small dry stream channels on the land surface, which were carved by torrents. The two most distinctive relief features are the karst polje situated between the villages Koritnice, Ba~ and Kne`ak, which is covered with rubble and gravel-like clastic sediments, and the dry valley of the former Pivka river lying southwest from Koritnice.
KEY WORDS: geomorphology, relief, karst, Pleistocene, Pivka, Kne`ak, Sne`nik, Slovenia.
The article was submitted for publication on March 24, 2006.
ADDRESS:
Gregor Kova~i~, B. Sc.
University of Primorska, Faculty of Humanities Glagolja{ka ulica 8, SI - 6000 Koper, Slovenia E-mail: gregor.kovacic@fhs-kp.si
Contents
1	Introduction	35
2	Geological setting	36
3	Relief evolution in the Pliocene	36
4	Relief evolution in the Pleistocene	40
4.1	Effects of Pleistocene geomorphogenesis	
	on the present relief structure	40
5	Conclusion	44
6	References	46
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1 Introduction
The Pivka basin is a large depression surrounded by the high karst plateaus Nanos, Hru{ica, Javorniki, Sne`nik and Slavenski ravnik and the flysch basin of the Reka river. The southern part - the Upper Pivka valley- is a depression, about 16 km long and 4-5 km wide, which stretches upstream from the settlement Prestranek and is mainly composed of Cretaceous limestone (Mulec et al. 2005). The landscape is given its character by the Pivka river with its unstable course. Due to the presence of the fine-grained river allu-via which cover the riverbed of the Pivka and the bottoms of larger karst depressions, and given the fact that the rocks are, in some places, less permeable and underground flowing is relatively poor, the prevai-ling karst relief is, in some places, combined with the elements of fluvial relief. This is reflected in the many intermittent karst lakes, the unstable course of the Pivka and nearby flooding.
The Pivka rises from the Piv{ca karst spring (555 m) near the village Zagorje. The catchment area of this highest situated permanent spring of the Pivka covers the southernmost part of the Upper Pivka val-ley, which, towards the south and east, continues into the deeply karstified plateau Sne`nik, which forms the watershed between the Adriatic (the karst springs Podstenj{ek and Bistrica) and Black (the Pivka river) Seas. To the west, the upper part of the basin of the Pivka is bounded by the Taborski hrbet ridge (Gora), which forms a geological and relief boundary between limestone regions and the flysch regions of Brkini and the Reka valley.
The upper part of the basin of the abovementioned spring abounds with special geomorphological features, as some elements of fluvial relief are also present in the area with prevailing karst relief, espe-cially in those parts where flat land converges into the slopes of the Sne`nik plateau. These elements are the result either of a different bedrock (a flysch tectonic window near Kne`ak) or different geomorpho-genetic processes in the past, especially in the Pleistocene, which have left traces in the surface stream of the former Pivka and other torrents. Today, this is evidenced by dry valleys and many fossil ravines. With the exception of the mainly flat, rubble and gravel-covered karst polje situated between the villages Korit-nice, Kne`ak and Ba~, karst relief with dolines of different shapes and sizes prevails. Most of these dolines are shallow and have inconspicuous edges, which gradually continue into somewhat higher and flatter land. In some places there are conical-shaped hills.
Figure 1: The karst polje between Koritnice, Kne`ak and Ba~. View from the slope above Koritnice to the west (photograph: Gregor Kova~i~, 1.3.2000).
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Gregor Kova~i~, Relief evolution in the hinterland of the Pivka river
There is no recent literature regarding relief evolution in the discussed area; therefore, the article is based on a critical review of existing investigations, which, however, are not supported by the latest geo-morphological methods and knowledge about the karst relief evolution. The author has conducted detailed field mapping of the area, giving most emphasis to the study of the processes of relief transformation in the Pleistocene. He has undertaken to integrate field observations and results into the theories of relief evolution in the discussed area. These theories still need to be confirmed by other geomorphological methods, possibly through examination of sediments.
2 Geological setting
Medium to well permeable Upper Cretaceous limestones prevail in the region (Figure 2). The dry valley above Koritnice and the upper parts of its margins are composed of dolomite-limestone breccia, which is somewhat more resistant to weathering. In the valley and its surroundings also grained dolomite can be found, which weathers very fast. The presence of well tectonically crushed dolomite (the valley is deve-loped along the Selce fault) probably contributed to the faster deepening of the former Pivka river dry valley above Koritnice, especially in the Pleistocene, when strong mechanical weathering was taking place ([iki} et al. 1972; [iki} and Pleni~ar 1975; Krivic et al. 1983).
The Taborski hrbet ridge is built mostly of Upper Cretaceous light crystalline limestone, as well as of less permeable Paleogene dark limestone, and somewhere marly limestone characterized by karstic and karst-fissured porosity can be found. In the tectonic window near Kne`ak, Eocene flysch rocks outcrop to the surface (Pleni~ar 1959). In the tectonic sense, the flysch rocks are a fraction of the recumbent fold of the Komen thrust sheet, which lies underneath the overthrust of the Sne`nik thrust sheet, built most-ly of Upper Cretaceous limestone (Placer 1981). The flysch barrier in the base of this area prevents karst water from flowing away underground from the discussed region towards the Reka river and directs the ground waters into the Pivka river basin. The Podstenj{ek karst spring is an exception. The presence of underlying flysch rocks, which outcrop in the tectonic windows near Kne`ak and Zagorje and whose exi-stence is additionally evidenced by a well near Zagorje situated only 2300 m from the thrust edge, at a depth of 100 m, is probably the reason for the formation of a shallow karst aquifer, which enables the rise of karst groundwater to the surface during high waters.
Flysch rocks near Kne`ak cover an area of 0.2 km2 and are an exception in the discussed area. The fly-sch surface is altered by several small streams, which have short channels. Some of the streams are even captured for local drinking water supply. At the foothill, the streams converge into an uplifted artificial ditch, which is inundated only in rainy season. At the end of the ditch, the stream sinks into the ponor Kne{ke ponikve, where underground water connection with the intermittent Videm{~ica karst spring near Zagorje has been proved (Habi~ 1975).
The karst depression between Koritnice, Ba~ and Kne`ak is covered with unconsolidated rubble and gravel clastic sediments, which in some places exceed 10 m of depth. In the Pleistocene, the material was accumulated by the former Pivka river in an alluvial fan at the exit of its source valley. According to its morphology, the abovementioned karst depression can be classified as a karst polje since it has the main characteristics of karst poljes as defined in Slovenian karst terminology) (Gams 1973), less distinctive being only its shape. In addition, Quaternary sediments cover the bottoms of other larger karst depressions.
Beside the abovementioned well-defined thrusting deformation of the Sne`nik thrust sheet over the Komen thrust sheet, the discussed area is dissected also by several other, less distinctive faults. The source valley of the former Pivka river is formed along the Selce fault. Southwest from the lake [embijsko jezero is the [embije fault. Frequent earthquakes that occur along the faults at a depth from 7-17 km (Vidrih et al. 1996; Ceci} et al. 1999) confirm that the area is tectonically still very active.
3 Relief evolution in the Pliocene
The elongated Pivka basin was predominately shaped by the erosional activity of the Pliocene Pivka river, which formed a 3-4km wide valley extending from the village Koritnice towards the town Postojna (Me-lik 1951; 1955). In the east, it is surrounded by the Javorniki plateau (1268 m asl), whereas its upper part lies on the slopes of the Sne`nik plateau (1796 m asl). The western side of the basin is bounded by the
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Acta geographica Slovenica, 46-1, 2006
Figure 2: Geological setting of the southern part of Upper Pivka, compiled by the author (after Pleni~ar 1959; [iki} et al. 1972; Placer 1981 Krivic et al. 1983; Ravbar and [ebela 2004). Legend: 1 - alluvium (Quaternary); 2 - flysch (Eocene); 3 - brown and dark grey limestones (Paleogene); 4- dark and marly limestones (Paleogene); 5- light grey and white crystalline limestones (Upper Cretaceous); 6- light limestones (Upper Cretaceous); 7 – dolomite-limestone breccia (Upper Cretaceous); 8 – established and covered fault; 9 – stronger and weaker thrust fault; 10 – karst spring, ponor; 11 – permanent, periodical water flow; 12 – karst periodical lake; 13 – hill with indication of elevation (above sea level); 14 – settlement, road.
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Gregor Kova~i~, Relief evolution in the hinterland of the Pivka river
Volovja reber ridge with the peak of Velika Milanja (1099 m asl), which is part of the Sne`nik plateau, and by the Taborski hrbet ridge stretching between the settlement [embije and the hill Primo` (718m asl) above the settlement Pivka. According to Melik, this is a long normal valley of the Pliocene Pivka, which once ran from the upper part of the basin situated in the elevated Sne`nik plateau underneath the peaks of Velika Milanja and Devin (1028m asl) towards Postojnska vrata (the Postojna Gap) and onwards to the basin of the Ljubljanica river. This is evidenced by a terrace stretching eastwards from the settlements Pivka, Zagorje and Kne`ak, and featuring wide, levelled ridges and smaller round-shaped peaks lying at an elevation of between 610 and 630 m. Today, the surface stream of the Pivka is preserved only in the lowest parts of Upper Pivka.
The orographic watershed between the Pivka river basin and the Reka river basin in the south is about the same age. It is well expressed with a relief barrier extending from the hill Osojnica across the Taborski hrbet ridge and onwards across the lower surface around [embijsko jezero towards the Volovja reber ridge. In the carbonate bedrock, karstic runoff has developed, with groundwater running towards the basins of the Adriatic and Black Seas (Habi~ 1968).
On the southern side of Volovja reber is a dry hanging valley, created by a onetime stream, which pro-bably flowed into the Reka river basin during the Pliocene. Since it was less subjected to erosion in the past, and, on the other hand, also strongly modified by recent karstification, the discussed valley is, com-pared with the dry valley above Koritnice, less markedly incised into the surrounding surface. According to the morphology of the area, it can be presumed that the former tributary of the Reka joined its basin (flysch bedrock) somewhere in the area called @lebovi lying to the north of Ilirska Bistrica. @lebovi is an erosional gap in the thrust edge, where the flysch bedrock indents for about 700 m inwards into the lime-stone bedrock of the Sne`nik plateau. Similar limestone erosional gaps can be observed along the entire thrust edge, however the discussed one is the most evident one. It was formed by the faster erosional lowe-ring of flysch rocks, which caused headward erosion and the prolongation of the valley into the limestone on the flysch-limestone joint. Later, the surface runoff was interrupted and groundwater circulation developed.
Similar dry and hanging valleys are also found in other parts of Upper Pivka, showing the existence of Pliocene surface streams flowing into the Pivka river valley. Vla~no and the valley between Juri{~e and Pal~je are the most obvious (Habi~ 1968).
The formation of surface drainage on carbonate rocks was possible due to the impermeable layers of Tertiary rocks, which, in the Pliocene, on all sides bounded limestones, which, as a result, lied in a lower position and were blocked (Radinja 1972). Such conditions hindered the process of vertical corrosion. Therefore, groundwater circulation did not develop.
The Pliocene fluvial phase was followed by the karstification period in the conditions of subtropical and tropical climate (Melik 1951; Habi~ 1968). This can be deduced from the preserved land features and the processes which are nowadays in progress in tropical karst areas in the world (Radinja 1972). The erosional activity of the Pivka, its tributaries and the tributaries of the Reka, which developed a regular surface drainage pattern in the discussed area, was replaced by the corrosion activity of aggressive precipitation
Figure 3: The source valley of the Pliocene Pivka river has been modified by torrents in the Pleistocene. View from the Volovja reber ridge to the east (photograph: Gregor Kova~i~, 1.3.2000).
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Acta geographica Slovenica, 46-1, 2006
water, which exhausted its strength soon after it had started to flow underground. A series of hanging ter-races on different elevations on the western side of the Javorniki indicates the multi-stage lowering of the surface as well as the deepening of the Pivka valley (Habi~ 1968). The older, higher terrace of the Pivka river lying at elevations of 600-650 m was intensively transformed by karst processes, which is evidenced by a somewhat dissected surface where several dolines have developed.
The former valley on the southern side of Volovja reber also transformed into a hanging valley, which is now situated 100 m above the depression of [embijsko jezero. This former valley was transformed by subsequent karstification. Therefore, many dolines are present mainly at the bottom of the valley, while, due to inclination, fewer of them are found on the slopes. The latter are usually also of less regular shapes.
In contrast with the other valleys in Upper Pivka, the source valley of the Pliocene Pivka above Korit-nice is opened towards the karst polje between Koritnice, Ba~ and Kne`ak. It seems that its evolution and deepening have not been interrupted.
As was probably the case with the other fossil valleys, the discussed one was also hanged above the lower surface in the first phase of karstification. However, due to the intensive erosional activity of Plei-stocene torrents, it was re-deepened to the level of the already mentioned karst polje, and the load of the torrents accumulated in the form of an alluvial fan at the end of the valley. The former bottom of the source valley of the Pliocene Pivka could be the pass (750 m asl) on the western side of the conical peak Strma reber (822 m asl). Following this explanation, the ending part of the valley on the eastern part of the abovementioned peak would only be the result of very intensive erosional deepening during the Pleistocene.
One of the most probable explanations is that, through the entire Pliocene period, the surface flow of the Pivka was preserved at the bottom of Upper Pivka. Subsequent karstification lead to the forma-tion of a system of karst springs in the area around Koritnice, which hindered the fossilification of the mountain valley above Koritnice. At that time, karst springs, which created a pocket valley called Ra{a and a less pronounced pocket valley under the church of St. Hieronymus (698 m asl), were active. The size of the Ra{a pocket valley shows that the valley must have been created by a water-abundant spring. The catchment area of the abovementioned springs was the higher positioned karstified part of the Sne`nik plateau with already developed karst drainage. In such conditions, it was possible that there was a sink-ing river at the bottom of a more or less homogeneous karst polje, which probably extended from the springs near Koritnice to the ponors in the Postojnska jama cave, but there is no solid evidence to con-firm this theory.
The bottom of the dry Ra{a valley is several metres wide and on the western side reaches up to 50 m. The spring lies at an elevation of 740 m. At the spring, the valley is deeply cut into the surrounding surface for about 20 m. Somewhat downstream, it is even deeper and wider. The valley is approximately 600 m long and its starts with a 10 m high wall. The erosional power of the former flow must have been con-siderable, since it formed such a remarkable relief feature in the relatively undissected surrounding surface. There is a lot of rubble on the slopes, collected in small screes. The edge of the valley is in some places bounded by 8 m high walls. Slightly above the ending wall of the Ra{a valley there is a 1 km long shallow and dry stream channel cut into the rocky bottom of the surface. Its creation is related to one of the pre-vious karstification phases in the region, when several karst springs on different elevations were active in Upper Pivka. With the lowering of the water table, the karst spring in the Ra{a valley became active and created a deep valley, while the channel of the higher situated karst spring hanged above it and remained preserved until today. Habi~ (1968) also writes about pocket valleys on different elevations that were created along karst tributaries under the Sne`nik and Javorniki plateau. Morphologically, the dry channel above the Ra{a valley is very similar to the shallow trough-shaped stream channels in the hinterland of [embijsko jezero, which are of much younger origin. I do not exclude the possibility that the intensive deepening of the Ra{a valley also dates to the Pleistocene period.
Isolated hills (»humi«), which are situated within the karst polje between Koritnice, Ba~ and Kne`ak and on its margins, give evidence of the Pliocene karst phase. Viewed from above, the conical-shaped hills have the shape of a more or less regular circle. The hills gently rise above the surrounding levelled surface. Such hills are Velika Obroba (644m asl), Mala Obroba (641 m asl) and the neighbouring hill situated at an elevation of 644 m. Their almost identical elevation testifies to their common origin. Well noticeable are also Galari~ica and Obrobca (both 600 m asl). In the vicinity there are also some other isolated hills on higher elevations. The karstified base of the bottom of the karst polje between Koritnice, Ba~ and Kne`ak
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Gregor Kova~i~, Relief evolution in the hinterland of the Pivka river
has been confirmed by investigations conducted in wells (Krivic et al. 1983). Small karst caves filled with clay can be seen on different depths, while closer to the surface some of the conduits are also filled with limestone debris.
At the same time, the Pliocene karst surface of the Pivka basin was altered by erosion-denudation and corrosion morphogenetic processes with intensive corrosion planation. Both processes supplemented each other and led, on the one hand, to the development of dolines, which became increasingly deeper due to the lowering of gradient, and on the other hand, to the development of terraces on different elevations.
4 Relief evolution in the Pleistocene
In his study of Slovenian karst poljes, Melik (1955) established that, during the Pleistocene, the mecha-nical weathering and disintegration of carbonate rocks on the surface intensified due to very frequent fluctuation in air temperature around zero and intensive freezing, which occurred mainly on higher elevations. Therefore, a lot of rock debris accumulated on the surface, which was either denuded to lower positions by solifluction or washed away by running water. Due to the filling up of caves, shafts and ponors, surface drainage was re-established and the dry valleys became active again. Among the factors contri-buting to the interruption of the karstification process was also the frozen ground, whereas, due to the impoverished soil and vegetation coverage, corrosion mainly took place in fractures.
In periglacial regions (the discussed area), chemical weathering was therefore rather weak, which, geo-morphologically speaking, means a weakening of the karstification process. The predomination of mechanical weathering, solifluction and denudation led to the filling up of existing swallow holes. A clear example is the karst polje Grobni~ko polje above the city of Rijeka in Croatia, where Pleistocene torrents incised some deep and narrow gorges and thickly filled the karst polje with debris. Moreover, the structure of accumulated material suggests that the karst polje was subject to periodical inundation ([iki} and Ple-ni~ar 1975).
There is a similar assumption, according to which there was a Pleistocene lake lying between Korit-nice, Ba~ and Kne`ak (Cumin 1929; Melik 1955) and extending in two branches from Koritnice to Kne`ak and Ba~, but no solid evidence is available to confirm this. The presence of fine-grained lake sediments has not been proven by wells (Krivic at al. 1983). Therefore, it is difficult to speak about a long lasting inundation of the karst polje. On the other hand, we cannot exclude the possibility of periodical inun-dations in the Pleistocene, similar to the present-day flooding of karst poljes, during which the debris fills up the narrower parts of swallow holes, disabling them to swallow all the incoming water and other load. During the raining period, the karst polje between Koritnice, Ba~ and Kne`ak is in its lowest parts regu-larly inundated, while during extremely high waters extensive flooding of short duration occurs, causing flood damage (Kova~i~ 2005).
A slightly different explanation of morphogenesis in the Pleistocene is offered by Radinja (1972), who argues that climate changes caused the selective lowering of different types of rock, which changed hypso-graphic relations between them and led to the creation of gradient and deep karst circulation, which, due to the bandage of impermeable rocks around carbonates, had not been able to develop during the pre-vious period. Therefore, corrosion dynamics transformed from Pliocene surface and planation corrosion into deep (point-like) karstification, as aggressive water, with the climate being colder and the vegetation and soil coverage poorer, lost its power of dissolving not until much further on its path. According to Radinja (1972), the predominant morphogenetic processes are point-like karstification and erosion, the effects of the latter being more pronounced on the surface of the discussed area.
4.1 Effects of Pleistocene geomorphogenesis on the present relief structure
Pleistocene geomorphic processes have left many traces in the discussed area. In the area above Koritnice, well noticeable traces of very intensive erosional activity of the Pivka, which sprang under the peaks Devin and Velika Milanja, are present. At the exit from the mountain valley, the river accumulated a huge alluvial fan, composed of rubble and gravel-like debris. The fan is inclined from Koritnice (625 m asl) towards Ba~ (580 m asl) and Kne`ak (580 m asl). The material was transported from the nearby slopes, where strong mechanical weathering and solifluction took place in periglacial conditions. By its structure, the debris
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Acta geographica Slovenica, 46-1, 2006
is more similar to rubble than it is to gravel, which proves that it was transported only for a short period and on a short distance. In order to make farming easier, local farmers arranged some terraces on the more inclined slopes of the alluvial fan near Koritnice.
The highest situated springs of the former Pivka river are those in the @lebovi area (950 m asl), situa-ted directly at the foot of the Volovja reber ridge. On a very steep slope with an inclination of more than 22°, torrents created several deep and steep gullies.
At @lebovi, gullies are abundant and they are generally orientated towards the northwest, with the exception of the gullies under Velika Milanja. In their upper part, they are orientated towards the north, but as soon as they reach the lower parts of the valley they turn towards the northwest. @lebovi is the left and upper source branch of the Pleistocene Pivka river. From the right, the other branch of the Pivka runs in a 1km long channel. The spring of this branch lies at an elevation of 740 m. Here, the Pivka created a 10 m high pocket valley, which is separated from the surrounding surface. Above the ending wall of this pocket valley, there are several gullies, which are most noticeable under Devin, where, according to our assumptions, there were the highest positioned springs of the right branch of the Pleistocene Pivka. Under Devin, the surface is not as strongly dissected as it is in the @lebovi area. In the Pleistocene, torrents under Devin most probably also sprang at an elevation of 950 m.
At the convergence of both branches, approximately 1 km before the hill Strma reber (822 m asl), the channel of the Pleistocene Pivka becomes somewhat wider. Since the gradient decreases, some accumula-tion of unconsolidated rubble-like material mixed with soil can be observed in the area. In the Holocene, running water formed some very small and narrow, 0.5 m deep channels in these deposits. The latter are probably a result of one of the last phases of Pleistocene accumulation. Where these sediments are not denuded, their depth can exceed several metres.
At the foothill of Strma reber, the channel of the former Pivka river is 1.5 m deep and more than 3 m wide. It is developed in unconsolidated material. An interesting shape of the Pivka riverbed can be observed shortly ahead of Koritnice. In this 350 m long section of its path, the Pivka river incised a narrow, 4 m deep channel (chut), which has given the name to the nearby settlement of Koritnice (»korito« = »chut«).
Figure 4: The dry riverbed of the former Pivka river northwest from Koritnice (photograph: Gregor Kova~i~, 7.10.2005)
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Gregor Kova~i~, Relief evolution in the hinterland of the Pivka river
Further from Koritnice towards the northwest, the Pivka riverbed gradually becomes shallower and, only after a few hundred metres, disappears. The position of fossil riverbeds indicates that the Pivka has changed its course several times. Dry riverbeds can be observed along the road leading from Koritnice to Ba~ and Kne`ak. Especially well developed is a dry riverbed at the foothill of Tu{~ak (786 m asl) on the right side of the Koritnice-Ba~ road (Figure 4), where some interesting meanders can be observed. Its bottom is 2 m deeper than the surface of the alluvial fan above it. This confirms the assumption that the majority of accu-mulation ended in the Pleistocene and that in the Holocene some weak incision activity took place. During the extreme precipitation events of November 2000, the abovementioned dry riverbed was filled up with water in its lower part and a surface stream appeared, which flooded the settlement Ba~ (Kova~i~ 2005).
The Pleistocene Pivka was, measured from the springs at @lebovi to the dry riverbeds under Tu{~ak, more than 4 km long. On this relatively short course, its gradient was relatively steep. A steeper gradient means more erosional power as well as easier and faster transportation of the mechanically weathered and disintegrated material, which formed an alluvial fan at Koritnice. The thickness of the accumulated material, which in spots exceeds 10 m, is therefore not surprising.
The structure of the material can be examined on several spots in the abovementioned dry riverbed below Tu{~ak and in some of the excavations alongside the Koritnice-Ba~ road. In this dry riverbed, the 2 m deep profile is composed of material of different sizes. In the uppermost part of the profile, there is a 20 cm thick layer of dark soil mixed with gravel. The size of individual fragments differs, however, those with a dia-meter of 1-6 cm prevail, and some of them reach the size of 25 cm. The material is not well rounded and resembles more rubble than gravel. There is also some sand in it. With the exception of the upper part of the profile, where some thin layers of sorted material can be observed, the fragments are usually irregularly spread in the mixture of fragments of different sizes, which proves the torrent character of the Pleistocene Pivka river.
Figure 5: Structure of the alluvial cover in the karst polje between Koritnice, Kne`ak and Ba~ in the excavation alongside the Koritnice-Ba~ road (photograph: Gregor Kova~i~, 7.6.2006).
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Figure 6: Geomorphological map of the discussed area, drawn on the basis of a digital elevation model (25 ×25 m). Legend: 1 - conical-shaped hill; 2 - karst spring; 3 - intermittent karst lake depression, bigger karst depression; 4 - karst polje; 5 - narrow, wider dry trough-shaped stream channel; 6– dry stream channel formed in alluvium, pocket valley; 7– directions of the Pleistocene Pivka accumulation; 8– tectonic
window; 9 - hill with indication of elevation (above sea level); 10 - surface flow (permanent or temporary); 11 - figure location.
In the excavation alongside the Koritnice-Ba~ road (Figure 5), situated about 700m away from the first profile, the conditions are similar. Here, the size of individual fragments is slightly smaller; fragments with a diameter of less than 2 cm prevail. The biggest fragments do not exceed 16 cm in diameter. Also here, there is a lot of sand mixed among the fragments. A slightly more rounded are the smaller fragments.
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Gregor Kova~i~, Relief evolution in the hinterland of the Pivka river
The material is irregularly spread in the profile and there is no trace of stratification. At the bottom of this 2.5 m deep profile, the onetime surface of the karst polje is exposed to the surface, which is reflected in subcutaneous rocky relief features and the presence of red clay inside the pockets.
Traces of Pleistocene activity can also be found in a hanging valley on the southern side of the Volov-ja reber ridge. At the end of the valley, on a flat area (Kamen{~ina) above the depression of [embijsko jezero, material of different sizes is deposited. The material was accumulated by a Pleistocene stream, as well as by solifluction processes, which transported the weathered material from the slopes to the lower parts of the valley. Compared with the material in the karst polje between Koritnice, Ba~ and Kne`ak, the frag-ments at Kamen{~ina are much smaller.
The material structure can be studied on the basis of excavations in three dolines. In the examined profile (Figure 7), layers of various materials can be seen in one of the excavated dolines. Beneath a 25 cm thick layer of dark soil there is a 0.5 m thick layer of unconsolidated and irregularly spread rubble mixed with soil. Individual fragments in this layer do not exceed 2 cm in diameter. Beneath this layer, there is a 0.9 m thick layer of brown clay containing some isolated larger pieces of rubble, followed by a 0.8 m thick layer of well-calcified, fine-grained rubble, which is lithified into the breccia. In the breccia, some stratification of fragments of different sizes can be observed. Beneath the breccia layer, there is alayer of unconsolidated fine-grained rubble, which is stratified in thin layers. The thickness of this layer cannot be established, however, it is estimated that this figure is higher than 0.8 m. In the examined 4 m deep profile, the size of individual fragments in the breccia as well as in the layer of rubble beneath it does not exceed 5 cm in diameter, which indicates small transport capacity of surface flow in the valley in the past. The shape of the fragments indicates that the origin of the material is in the nearest hinterland. The rocky bottom of the doline is not noticeable, and there are no fragments of dripstone found in the material. The sediments in the other two excavations have a similar structure, however rocky basement is well noticeable at the bottom and margins of these two dolines filled up with earth and other material.
In the middle part of the valley, there is a small dry channel, 200 m long, which was created as a result of the erosion activity of a small surface stream, which was fed with water from melting snow below the Volovja reber ridge. The stream probably flowed onwards across the edge of the valley into the depression of [embijsko jezero, where a small channel incised into the rock above it can be observed. The channel carried surface waters and fine-grained material to the bottom of the present depression of [embijsko jezero and to a hollow depression called Nari~e, which was also flooded during the extreme precipitation events of November 2000. A similar, but somewhat longer channel runs from the northwesternmost slopes of the Volovja reber ridge and is initially orientated towards the northwest. Afterwards, it turns to the west and ends at Nari~e. The channel is very narrow and shallow and has a rocky bottom.
5 Conclusion
The study of present relief features in the discussed area has revealed that the fluvial relief features have been heavily transformed by the recent lowering of the surface due to the corrosion of precipitation water. Nevertheless, some elements of the erosional activity of surface running waters from previous periods in the hinterland of today's Pivka river karst spring are still well preserved. Moreover, these elements are the most recognizable ones in the discussed area.
The basic features of karst relief in the hinterland of the present Pivka river were developed in the period of warm and humid Pliocene climate.
The previously developed karst features were altered by Pleistocene processes. Due to increasing mec-hanical weathering of limestone, gradual slopes were formed and the surface became well dissected.
While the hanging valley on the southern side of the Volovja reber ridge was largely transformed by more recent karstification processes, the elements of fluvial relief are still predominant in the dry valley of the former Pivka river. This is evidence of the great erosional power of Pleistocene running waters, cau-sing more intense deepening of this tectonically crushed area compared with the surrounding areas. Thus, most of the effects of Pleistocene morphogenetic activity are still noticeable and are prevailing on the surface. Therefore, several small dry gullies and trough-shaped stream channels are still preserved.
Rubble and gravel-like clastic debris, which had been transported by Pleistocene running waters from the foothills of Devin and Velika Milanja, thickly covered the depression of the karst polje between Korit-
44
Acta geographica Slovenica, 46-1, 2006
nice, Ba~ and Kne`ak. It can be concluded from the structure of deposits and the presence of dry river-beds that the majority of accumulation ended in the Pleistocene, while in the Holocene some weak incision activity took place. Except in the area of the abovementioned karst polje, deposits from the Pleistocene period and some of later origin can also be found in other depressions.
Today, dispersed point-orientated corrosion is a prevailing geomorphic process in the discussed region. In the upper parts of the Volovja reber ridge and in the @lebovi area, the mechanical weathering of rocks is also strong, especially in the colder part of the year. Relief evolution in the region is, to a great extent, conditioned by the groundwater level. The water table in the karst polje between Koritnice, Ba~ and Kne`ak is usually 30-40 m below the surface and is inclined from Koritnice, where it reaches 560 m asl, towards Ba~, where it reaches 540 m asl (Krivic et al. 1983). Hydrologically, the area under review is a bifurcation zone between the Adriatic and Black Seas. Groundwater flows away from the karst polje mainly towards the Pivka spring, whereas the depression of [embijsko jezero and its catchment area belong to the Reka river basin, as the water table is inclined towards the Podstenj{ek karst spring.
If we integrate the geomorphogenetic evolution of the discussed area into the broader context of relief evolution in the entire Pivka basin or at least in its upper part (Upper Pivka), the question of further gemorp-hogenetic development of this very interesting region arises. In their study of intermittent lakes in the Pivka basin as well as in their comparison between hydrological conditions in Upper Pivka and those in the Cerknica karst polje, Ravbar and [ebela (2004) establish that the Pivka basin is currently in the pre-liminary development phase of a »typical« karst polje, where karst depressions deepen to the level of karst groundwater, as is the case with the intermittent lakes of Upper Pivka.
However, another explanation is possible. According to the results of hydrogeological investigations, the groundwater gradient in the Javorniki area is orientated towards the Malni karst springs (Habi~ 1968). Strong, deep groundwater circulation towards the northeast is present, which has been proved by tracer
Figure 7: Profile through sediments in one of the excavated dolines situated at Kamen{~ina (photograph: Gregor Kova~i~, 7.6.2006). Legend:
A - soil; B - fine-grained rubble; C - clay; D - breccia; E - fine-grained rubble in layers.
45
Gregor Kova~i~, Relief evolution in the hinterland of the Pivka river
experiments (Habi~ 1989). Considering these facts, we can assume that the deepening of karst depres-sions has not been interrupted and is still in progress. Today, the relief of the Pivka basin, with its many periodically inundated karst depressions and the unstable course of the Pivka river, can be characterized as a dissected karst polje. As an interesting detail, I would like to mention the structure of the karst depres-sion of the lake Malo Drskov{ko jezero, which is reminiscent of a classical overflow karst polje, with the spring on one side and the ponor on the other.
The Pivka basin (Pivka, the Pivka karst polje) (Kranjc 1987) is one of the most spectacular and most interesting regions of the Slovenian classical karst. This article, based on critical review of literature and fieldwork, sets out to explain relief evolution in the southeasternmost part of the basin. The region has gone through different geomorphogenetic phases, with many different processes interchanging with one another. Today, the relief structure is predominantly characterized by the effects of Pleistocene processes.
6 References
Ceci}, I., @iv~i}, M., Deterding, M., Gosar, A. 1999: Potresi v Sloveniji. Ujma 13. Ljubljana.
Cumin, G. 1929: Le piane alluvionali ed i fenomeni idrologici del' alto carso della Piuca. Bulletino di Societa Geographia Italiana. Roma.
Gams, I. 1965: H kvartarni geomorfogenezi ozemlja med Postojnskim, Planinskim in Cerkni{kim poljem. Geografski vestnik 37. Ljubljana.
Gams, I. 1973: Slovenska kra{ka terminologija. Filozofska fakulteta. Ljubljana.
Habi~, P. 1968: Kra{ko pore~je Zgornje Pivke in javorni{ki podzemeljski tok. Vodni viri za oskrbo Postojne. Elaborat, In{titut za raziskovanje krasa ZRC SAZU. Postojna.
Habi~, P. 1975: Pivka in njena kra{ka jezera. Ljudje in kraji ob Pivki. Kulturna skupnost Postojna. Postojna.
Habi~, P. 1989: Kra{ka bifurkacija na jadransko ~rnomorskem razvodju. Acta carsologica 18. Ljubljana.
Kova~i~, G. 2005: Poplava na obmo~ju Ba~a, Kne`aka in Koritnic novembra 2000. Geografski zbornik 45/1. Ljubljana.
Kranjc, A. 1987: O pokrajinskem imenu Pivka. Geografski vestnik 59. Ljubljana.
Krivic, P, Juren, A., Bizjak, M., Ravnikar, B. 1983: Hidrogeolo{ke raziskave zaledja Zgornje Pivke, I. faza. Elaborat, Geolo{ki zavod Ljubljana. Ljubljana.
Melik, A. 1951: Pliocenska Pivka. Geografski vestnik 23. Ljubljana.
Melik, A. 1955: Kra{ka polja Slovenije v pleistocenu. Dela in{tituta za geografijo SAZU 3. Ljubljana.
Mulec, J., Mihevc, A., Pipan, T. 2005: Presihajo~a jezera na Piv{kem. Acta carsologica 34/3. Ljubljana.
Placer, L. 1981: Geolo{ka zgradba jugozahodne Slovenije. Geologija 24/1. Ljubljana.
Pleni~ar, M. 1959: Tektonski okni pri Kne`aku. Geologija 5. Ljubljana.
Radinja, D. 1972: Zakrasevanje v Sloveniji v lu~i celotnega morfogenetskega razvoja. Geografski zbornik 13. Ljubljana.
Ravbar, N., [ebela, S. 2004: The karst periodical lakes of Upper Pivka, Slovenia. Acta carsologica 33/1. Ljubljana.
[iki}, D., Pleni~ar, M., [parica, M. 1972: Ilirska Bistrica. Osnovna geolo{ka karta SFRJ 1:100.000. Zvezni geolo{ki zavod. Beograd.
[iki}, D., Pleni~ar, M. 1975: Ilirska Bistrica - tolma~. SFRJ 1:100.000. Zvezni geolo{ki zavod. Beograd.
Vidrih, R, Ceci}, I., Godec, M. 1996: Potresa na obmo~ju Ilirske Bistrice 22. maja 1995 - geolo{ke in seizmolo{ke zna~ilnosti. Ujma 10. Ljubljana.
46
Gregor Kova~i~, Razvoj povr{ja v povirju Pivke
Razvoj povr{ja v povirju Pivke
UDK: 911.2:551.4(497.4) COBISS: 1.01
IZVLE^EK: Prispevek obravnava razvoj povr{ja v povirnem zaledju najvi{je le`e~ega stalnega izvira reke Pivke, kra{kega izvira Piv{ce pri Zagorju. Obravnavana pokrajina na skrajnem ju`nem robu Zgornje Pivke izkazuje {tevilne geomorfolo{ke posebnosti, ki so rezultat razli~nih geomorfogenetskih procesov v preteklosti. Osnovne poteze dana{njega povr{ja so bile za~rtane v ~asu pliocena, ko se je tudi izoblikovala prvotna dolina Pivke. Pokrajino so izraziteje preoblikovali pleistocenski procesi, ki so povr{ju dali poseben pe~at. Poleg kra{kih reliefnih oblik, so v reliefu prisotne {tevilne manj{e suhe struge nekdanjih hudourni{kih tokov. V povr{ju izstopata kra{ko polje med Koritnicami, Kne`akom in Ba~em, ki je nasuto z gru{~nato-prod-natim drobirjem ter suha dolina nekdanje Pivke jugovzhodno od Koritnic.
KLJU^NE BESEDE: geomorfologija, relief, kras, pleistocen, Pivka, Kne`ak, Sne`nik, Slovenija.
Uredni{tvo je prejelo prispevek 24. marca 2006.
NASLOV:
Gregor Kova~i~, univ. dipl. geog.
Univerza na Primorskem, Fakulteta za humanisti~ne {tudije Glagolja{ka ulica 8, SI - 6000 Koper, Slovenija E-po{ta: gregor.kovacic@fhs-kp.si
Vsebina	
1          Uvod	49
2          Geolo{ka sestava	49
3          Razvoj povr{ja v pliocenu	50
4          Razvoj povr{ja v pleistocenu	52
4.1       U~inki geomorfnega dogajanja v pleistocenu	
na dana{njo izoblikovanost povr{ja	52
5          Sklep	54
6         Viri in literatura	55
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Acta geographica Slovenica, 46-1, 2006
1 Uvod
Piv{ka kotlina je velika depresija med visokimi kra{kimi planotami Nanosom, Hru{ico, Javorniki, Sne`-nikom, Slavenskim ravnikom ter fli{nim povodjem reke Reke. Ju`ni del, Zgornja Pivka, je okrog 16km dolga in 4-5 km {iroka depresija, ki se razteza gorvodno od Prestranka in je izoblikovana prete`no v apnencih kredne starosti (Mulec in ostali 2005). Pokrajini daje pe~at reka Pivka s svojim nestanovitnim tokom. Prevladujo~e kra{ko povr{je se zaradi prisotnosti drobnozrnatih re~nih naplavin, ki prekrivajo dno struge Pivke in dna ve~jih kra{kih depresij, ter mestoma slab{e prepustnih kamnin in omejenega podzemeljskega pretakanja, ponekod prepleta z elementi fluvialnega povr{ja. To se odra`a v {tevilnih presihajo~ih kra{-kih jezerih, nestalnem toku Pivke ter poplavah ob njej.
Pivka izvira v izviru Piv{ce (555 m) pri Zagorju. Zaledje tega najvi{je le`e~ega stalnega izvira Pivke obsega najju`nej{i del Zgornje Pivke, ki proti jugu in vzhodu prehaja v globoko zakraselo planoto Sne`-nik, kjer poteka razvodnica med jadranskim (kra{ka izvira Podstenj{ek in Bistrica) in ~rnomorskim (reka Pivka) povodjem. Proti zahodu povirje Pivke omejuje Taborski hrbet (Gora), ki predstavlja geolo{ko in reliefno mejo s fli{no pokrajino Brkinov in doline Reke.
Povirje omenjenega izvira se pona{a s {tevilnimi geomorfolo{kimi posebnostmi, saj se na obmo~ju s prevladujo~im kra{kim zna~ajem povr{ja, zlasti na prehodih uravnanega sveta v vzpeti svet Sne`ni{ke planote, pojavljajo tudi elementi fluvialnega povr{ja. Ti so bodisi rezultat druga~ne kamninske podlage (fli{no tektonsko okno pri Kne`aku) bodisi delovanja razli~nih morfogenetskih procesov v preteklosti, zlasti v pleistocenu, ki so v pokrajini pustili sledove o povr{inskem toku nekdanje Pivke in drugih hudourni{kih vod. To dokazujejo dana{nje suhe doline ter {tevilne fosilne grape. Z izjemo prete`no ravnega nasutega dna kra{kega polja med Koritnicami, Ba~em in Kne`akom, prevladuje zakraselo povr{je z vrta~ami raznih oblik in velikosti. Najve~ je plitvih vrta~ z neizrazitimi robovi, ki v blagih pregibih prehajajo v nekoliko vi{ji in bolj uravnan svet med njimi, ponekod so kopaste vzpetine.
Novej{e literature o razvoju reliefa na omenjenem obmo~ju nimamo, zato ~lanek temelji na kriti~nem pregledu obstoje~ih raziskav, ki pa niso podprte z novej{imi geomorfolo{kimi metodami in znanjem o razvoju kra{kega povr{ja. Avtor je opravil podrobno terensko kartiranje obmo~ja, pri ~emer je najve~ji poudarek namenil preu~itvi procesov preoblikovanja povr{ja v pleistocenu. Terenska spoznanja in izsledke je sku{al uokviriti v hipotezo o razvoju povr{ja na omenjenem obmo~ju. Hipotezo bi bilo potrebno potrditi {e z drugimi geomorfolo{kimi metodami, mo`nosti se nakazujejo zlasti pri preu~evanju sedimentov.
Slika 1: Kra{ko polje med Koritnicami, Kne`akom in Ba~em. Pogled s pobo~ja nad Koritnicami proti zahodu (fotografija: Gregor Kova~i~,
1.3.2000).
Glej angle{ki del prispevka.
2 Geolo{ka sestava
Na obmo~ju prevladujejo srednje do dobro vodoprepustni apnenci zgornjekredne starosti (slika 2). Na obmo~ju suhe doline nad Koritnicami in na njenih vr{nih robovih je apnen~asto-dolomitna bre~a, ki je nekoliko bolj odporna na preperevanje. V suhi dolini in v njeni okolici naletimo tudi na vlo`ke zrnatega dolomita, ki hitro prepereva. Prisotnost tektonsko mo~no pretrtega dolomita, dolina je namre~ izoblikovana ob Sel{kem prelomu, je najverjetneje prispevala k hitrej{emu poglabljanju suhe doline nekdanje Pivke nad Koritnicami, {e posebej v pleistocenu, ko je potekalo mo~no mehani~no preperevanje
Slika 2: Geolo{ka sestava ju`nega dela Zgornje Pivke, sestavil avtor (po Pleni~ar 1959; [iki} in ostali 1972; Placer 1981; Krivic s sodelavci 1983; Ravbar in [ebela2004. Legenda: 1 - aluvij (kvartar), 2 - fli{ (eocen), 3 - rjavi in temnosivi apnenci (paleogen), 4 - temni in lapornati apnenci (paleogen), 5 - svetlosivi in beli prekristalizirani apnenci (zg. kreda), 6- svetli apnenci (zg. kreda), 7- dolomitno-apnena bre~a (zg. kreda), 8- ugotovljen in prekrit prelom, 9- mo~nej{i in {ibkej{i nariv, 10- kra{ki izvir, ponor, 11 - stalni, ob~asni tok vode, 12- presihajo~e jezero, 13- vzpetina z nadmorsko vi{ino, 14- naselje, cesta. Glej angle{ki del prispevka.
Taborski hrbet gradijo poleg svetlih prekristaliziranih apnencev zgornjekredne starosti {e za vodo manj prepustni temni apnenci in ponekod lapornati apnenci paleogenske starosti s kra{ko in kra{ko-razpoklin-sko poroznostjo. V tektonskem oknu pri Kne`aku izdanja na povr{je fli{ eocenske starosti (Pleni~ar 1959).
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Gregor Kova~i~, Razvoj povr{ja v povirju Pivke
Fli{ne kamnine so v tektonskem pogledu del prevrnjene gube Komenske narivne grude, ki le`i pod narivom Sne`ni{ke narivne grude, zgrajene prete`no iz zgornjekrednih apnencev (Placer 1981). Fli{na zapora v podlagi narivnega obmo~ja prepre~uje podzemni odtok kra{ke vode z obravnavanega obmo~ja proti Reki in usmerja podzemne vode v pore~je Pivke. Izjema je kra{ki izvir Podstenj{ek. Spodaj le`e~e fli{ne kamnine, ki so razen v tektonskih oknih pri Kne`aku in Zagorju dokazane tudi z vrtino pri Zagorju 2300 m od narivnega roba v globini 100 m (Krivic in ostali 1983), so najverjetneje vzrok za obstoj plitvega kra{kega vodonosnika, kar omogo~a ob visokih vodah dvig kra{ke podtalnice na povr{je.
Fli{ne kamnine pri Kne`aku, ki izdanjajo na pribli`no 0,2 km2 povr{ine, so posebnost obravnavanega obmo~ja, saj jih s svojimi kratkimi strugami preoblikujejo {tevilni manj{i potoki. Nekateri med njimi so celo zajeti za lokalno vodooskrbo. Studenci se ob vzno`ju pobo~ja zdru`ijo v izgonsko strugo manj{ega potoka, ki te~e izklju~no v de`evnem obdobju. Potok ponika v Kne{kih ponikvah, od koder je bila dokazana podzemeljska zveza z ob~asnim kra{kim izvirom Videm{~ica pri Zagorju (Habi~ 1975).
Kra{ko kotanjo med Koritnicami, Ba~em in Kne`akom prekrivajo nesprijeti gru{~nato prodnati kla-sti~ni sedimenti, ki so ponekod globoki ve~ kot 10 m. Gradivo je v obliki vr{aja ob izhodu iz povirne doline v ~asu pleistocena nasula tedanja Pivka. Po morfologiji lahko kotanjo ozna~imo kot kra{ko polje, saj izkazuje bistvene zna~ilnosti kra{kih polj po Slovenski kra{ki terminologiji (Gams 1973), nekoliko nezna~ilna je zgolj njena oblika. Kvartarni sedimenti pokrivajo tudi dna ostalih ve~jih kra{kih kotanj.
Razen `e omenjenega izrazitega narivnega preloma Sne`ni{ke narivne grude na Komensko narivno grudo sekajo obravnavano obmo~je {e {tevilni drugi, manj izraziti prelomi. Povirna dolina nekdanje Pivke je izoblikovana ob Sel{kem prelomu, jugozahodno od [embijskega jezera poteka [embijski prelom. Obmo~je je tektonsko aktivno {e danes, kar dokazujejo pogosti potresi, ki nastajajo ob prelomih na globini 7-17km (Vidrih in ostali 1996; Ceci} in ostali 1999).
3 Razvoj povr{ja v pliocenu
Melik (1951; 1955) navaja, da je Piv{ko podolje rezultat erozijskega delovanja pliocenske Pivke, ki je od Koritnic proti Postojni izoblikovala 3-4 kilometre {iroko dolino. Na vzhodu jo obdajajo Javorniki (1268 m), v zgornjem delu pa je izoblikovana na obronkih Sne`ni{ke planote (1796 m). Na zahodni strani dolino obdaja gorski hrbet Volovje rebri z Veliko Milanjo (1099 m), ki je del Sne`ni{ke planote, in Taborski hrbet med [embijami in Primo`em (718 m) nad naseljem Pivka. To je po njegovem mnenju dolga normalna dolina pliocenske Pivke, ki je tekla od povirja v visokem svetu Sne`ni{ke planote pod Veliko Milanjo in Devinom (1028m) proti Postojnskim vratom v pore~je Ljubljanice. To dokazuje terasni nivo, ki se razprostira vzhodno od Pivke, Zagorja in Kne`aka. Povsod je izra`en s {irokimi, uravnanimi hrbti in manj{imi zaobljenimi vrhovi v nadmorskih vi{inah 610-630 metrov. Povr{inski tok Pivke je danes ohranjen le v najni`jih delih Zgornje Pivke.
Enako stara kot dolina Pivke naj bi bila tudi orografska razvodnica med Pivko in pore~jem Reke na jugu, ki je danes opazna po markantni pregradi od Osojnice ~ez Taborski hrbet in dalje prek ni`jega sveta v zaledju [embijskega jezera na Volovjo reber. Na karbonatni podlagi je razvit kra{ki odtok z raztekanjem voda v jadransko in ~rnomorsko povodje (Habi~ 1968).
Slika 3: Izvirna dolina pliocenske Pivke je preoblikovana zaradi delovanja hudournikov v ~asu pleistocena. Pogled z grebena Volovje rebri proti vzhodu (fotografija: Gregor Kova~i~, 1.3.2000). Glej angle{ki del prispevka.
Na ju`ni strani Volovje rebri je suha in obvisela dolina nekdanjega vodotoka, ki je verjetno v pliocenski fluvialni fazi pritekal v pore~je Reke. V primerjavi s suho dolino nad Koritnicami je omenjena dolina manj izrazito vrezana v okoli{ki svet, saj je v erozijskem procesu nekoliko zaostajala, mo~no pa jo je preoblikovalo tudi recentno zakrasevanje. Glede na izoblikovanost povr{ja lahko sklepamo, da se je nekdanji vodotok vklju~eval v fli{no pore~je Reke nekje na obmo~ju @lebov severno od Ilirske Bistrice. @lebovi so erozijska vrzel v narivnem robu, kjer se fli{na podlaga pribli`no 700 m zajeda v notranjost apnen~astega sveta Sne`ni{ke planote. Podobne apnen~aste erozijske vrzeli lahko opazujemo vzdol` celotnega narivnega roba v okolici Ilirske Bistrice, omenjena pa je najizrazitej{a. Njen nastanek lahko razlagamo z izrazitej{im erozijskim zni`evanjem fli{nih kamnin, ki je povzro~ilo zadenjsko erozijo in podalj{evanje doline na stiku fli{a v apnen~asti svet, dokler ni pri{lo do prekinitve povr{inskega toka in prestavitve odtoka v podzemlje.
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Acta geographica Slovenica, 46-1, 2006
Podobne suhe in obvisele doline, ki ka`ejo na prisotnost pliocenskih povr{inskih tokov usmerjenih v dolino Pivke, so tudi drugje na Zgornji Pivki, v povr{ju pa najbolj izstopata Vla~no in dolina med Juri{-~ami in Pal~jem (Habi~ 1968).
Razvoj povr{inske re~ne mre`e na karbonatnih tleh je po mnenju Radinje (1972) omogo~ila odeja neprepustnih terciarnih kamnin, ki je v pliocenu na {iroko obdala apnence, da so bili ti v nizki legi in zaprti. To je onemogo~alo globinsko korozijo, zaradi ~esar se ni razvila globinska cirkulacija.
Pliocenski fluvialni fazi je sledilo obdobje zakrasevanja, ki je potekalo v subtropski oziroma tropski klimi (Melik 1951; Habi~ 1968). To sklepamo po ohranjenih oblikah in procesih, ki potekajo danes v kra{-kih tropskih pokrajinah (Radinja 1972). Korozijsko raztapljanje agresivne padavinske vode, ki je svojo mo~ iz~rpala `e na za~etku svoje poti, je zamenjalo predhodno izrazitej{e erozijsko delovanje reke Pivke in njenih pritokov ter pritokov Reke, ki so na obravnavanem obmo~ju za~rtali linearno strukturo povr{-ja. Razli~ni nivoji obviselih teras na obmo~ju zahodnih Javornikov nakazujejo ve~stopenjsko zni`evanje povr{ja oziroma poglabljanja doline Pivke (Habi~ 1968). Kra{ki procesi so zelo izrazito preoblikovali nekdanjo starej{o teraso Pivke v vi{inah 600-650 m, saj je raz~lenjena s {tevilnimi vrta~ami.
Obvisela je tudi nekdanja dolina na ju`ni strani Volovje rebri, ki v 100 m stopnji visi nad kotanjo [em-bijskega jezera. Kasnej{e zakrasevanje je preoblikovalo nekdanjo dolino, {tevilne vrta~e so zlasti na dnu, zaradi ve~jih nagibov pa so na pobo~jih manj pogoste in nepravilnih oblik.
V nasprotju z ostalimi dolinami na Zgornji Pivki, je povirna dolina pliocenske Pivke nad Koritnica-mi odprta proti kra{kemu polju med Koritnicami, Ba~em in Kne`akom in ne ka`e, da bi bila njen razvoj in poglabljanje kakorkoli prekinjena.
Morda je tako kot ostale fosilne doline tudi omenjena gorska dolina v prvi fazi zakrasevanja sicer obvisela, vendar je zaradi intenzivnega hudourni{kega erozijskega delovanja v pleistocenu pri{lo do ponovne poglobitve doline do ravni `e omenjenega kra{kega polja, gradivo pa se je na izhodu iz doline v obliki vr{aja odlo`ilo v dnu polja. Dno obvisele povirne doline pliocenske Pivke bi lahko bilo preval na zahodni strani kopastega vrha Strma reber (822 m) v vi{ini 750 m, ter da je spodnji del doline izoblikovane na vzhodni strani omenjene vzpetine rezultat zgolj zelo intenzivnega erozijskega poglabljanja v ~asu pleistocena.
Zelo verjetna je tudi razlaga, da se je v dnu Zgornje Pivke ves ~as pliocena ohranil povr{inski tok Pivke. Kasneje je z zakrasevanjem nastal sistem kra{kih izvirov na obmo~ju Koritnic, kar je zavrlo fosilizacijo gorske doline nad Koritnicami. Takrat so bili aktivni kra{ki izviri, ki so izoblikovali izrazito zatrepno dolino Ra{e in nekoliko manj izrazito in kraj{o suho dolino, ki se vije izpod cerkve Sv. Jeronima (698 m) skoraj do Koritnic. Velikost zatrepne doline Ra{e nakazuje, da je {lo za precej vodnat izvir. Napajalno zaledje omenjenih izvirov je bil vi{je le`e~i zakraseli svet Sne`ni{ke planote z `e izoblikovano kra{ko drena`o. V tak{nih razmerah bi imeli opraviti z reko ponikalnico v dnu dokaj homogenega kra{kega polja, ki se je morda raztezalo od Koritnic do ponorov v Postojnski jami, vendar pa o smereh odtekanja vode nimamo pravih dokazov.
Dno suhe doline Ra{e je {iroko nekaj metrov, na zahodu pa dose`e 50 m. Izvirni del doline je v nadmorski vi{ini 740 m. Dolina je `e pri izviru globoko vrezana v okoli{ki svet za pribli`no 20 m. Ni`je je {e bolj vrezana, vendar je tu dno doline `e {ir{e. Ra{a je dolga pribli`no 600 m. Zatrepni del je 10 m visoka stena. Erozijska mo~ nekdanjega vodotoka je morala biti zelo velika, da je lahko izoblikoval tako markantno zajedeno dolino v ne preve~ raz~lenjeno povr{je. Na pobo~jih se je nabralo veliko gru{~a, ki tvori manj{a meli{~a. Rob doline ponekod obdajajo tudi do 8 m visoke stene. Neposredno nad zatrepom Ra{e se slab kilometer v notranjost Sne`ni{ke planote vije plitva struga, vrezana v `ivoskalno dno. Njen nastanek povezujemo z eno od starej{ih faz zakrasevanja, ko so bili na Zgornji Pivki aktivni kra{ki izviri v vi{jih nadmorskih vi{inah. Z zni`anjem piezometra je postal aktiven izvir v Ra{i, ki se je izrazito poglobil v okoli{ki svet, suha struga vi{jega izvira pa je obvisela nad njim in se ohranila do danes. O zatrepih, ki si v stopnjah slede z vi{jih v ni`je uravnave in so nastali ob kra{kih pritokih izpod Sne`ni{ke planote in Javornikov, pi{e tudi Habi~ (1968). Seveda so mo`ne {e druge razlage, ki jih lahko povezujemo s pleistocenskimi procesi, saj je zlasti suha struga nad zatrepom Ra{e po morfologiji zelo podobna plitvim koritastim strugam v zaledju [embijskega jezera, ki jim pripisujem kasnej{i nastanek. Ne izklju~ujem mo`nosti, da je tudi do izrazite poglobitve suhe doline Ra{e pri{lo v ~asu pleistocena.
O pliocenski kra{ki fazi pri~ajo kopaste vzpetinice, ki se dvigajo v obrobju in znotraj kra{kega polja med Koritnicami, Ba~em in Kne`akom. Kopaste vzpetinice imajo tloris v obliki skoraj pravilnega kroga. Humi se z blagim nagibom dvigajo iz sosednjega ve~inoma ravnega sveta. Lepo so opazne zlasti vzpetinice Velika Obroba (644m), Mala Obroba (641 m) ter sosednji hum vi{ine 644m. Njihova skoraj enaka
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nadmorska vi{ina pri~a o skupnem nastanku. Dobro opazni sta {e vzpetinici Galari~ica in Obrobca, ki sta v nadmorski vi{ini 600 m, v okolici so {e druge kopaste vzpetine v vi{jih nadmorskih vi{inah. Zakra-selost podlage v dnu kra{kega polja med Koritnicami, Ba~em in Kne`akom dokazujejo vrtine (Krivic in ostali 1983). Na razli~nih globinah naletimo na manj{e kra{ke jame zapolnjene z glino, bli`je povr{ju pa so nekateri rovi delno zapolnjeni tudi z apnenim gradivom.
Pliocensko kra{ko povr{je Pivke sta isto~asno preoblikovala tako erozijsko-denudacijski kot korozijski morfogenetski proces z mo~no ploskovno korozijo. Procesa sta se medsebojno dopolnjevala. Tako so nastajale doline, ki so se zaradi zni`evanja gradienta vedno bolj poglabljale, in terase na razli~nih vi{inah.
4 Razvoj povr{ja v pleistocenu
Melik (1955) je s preu~evanjem slovenskih kra{kih polj ugotovil, da se je v pleistocenu pove~alo mehansko preperevanje ter razpadanje karbonatnih kamnin na povr{ini kot u~inek izredno pogostega kolebanja temperatur okoli ni~le in intenzivnega zmrzovanja, kar se je dogajalo zlasti v vi{jih predelih. Tako se je na povr{ju nabiralo veliko kamninskega drobirja, ki ga je mo~na denudacija, bodisi v obliki soliflukcije, bodisi s fluvialnim odna{anjem transportirala v ni`je predele. Zaradi zatrpavanja jam, brezen in ponikev je pri{lo do obnovitve povr{inskih vodotokov in do o`ivljanja suhih dolin. K prekinitvi kra{kega procesa je pripomogla tudi zamrznjenost tal, osiroma{ena pedolo{ka in vegetacijska odeja pa je korozijo vezala predvsem na razpokanost tal.
Kemi~no preperevanje je bilo v periglacialnih predelih (obravnavano obmo~je) torej zelo {ibko, kar v geomorfolo{kem smislu pomeni oslabitev procesa zakrasevanja. Prevlada mehani~nega razpadanja, soliflukcije ter denudacije je povzro~ila zasipavanje `e obstoje~ih kra{kih po`iralnikov. Zelo nazoren primer je Grobni~ko polje nad Reko na Hrva{kem, kjer so pleistocenski potoki v pobo~ja vrezali prave soteske, polje pa na debelo nasuli z drobirjem. Sestava sedimentov ka`e celo na ob~asno ojezeritev ([iki} in Ple-ni~ar 1975).
Podobne so tudi domneve o pleistocenskem jezeru med Koritnicami, Ba~em in Kne`akom (Cumin 1929; Melik 1955). Jezero naj bi se raztezalo v obliki dveh krakov od Koritnic proti Kne`aku in Ba~u, vendar pa pravih dokazov o njegovem obstoju nimamo. Vrtine v polju (Krivic in ostali 1983) niso dokazale prisotnost drobnej{ih jezerskih sedimentov, zato te`ko govorimo o dalj{i ojezeritvi. Ne moremo pa izklju~iti mo`nosti, da je v pleistocenu kljub vsemu prihajalo do ob~asnih ojezeritev, ko je nasuti drobir zadelal pretesna grla po`iralnikov in ponikve niso mogle ve~ sproti sprejemati vse vode, plavja ter nanosa, podobno kot so dana{nje poplave na kra{kih poljih. Kra{ko polje med Koritnicami, Ba~em in Kne`akom je v najni`jih predelih zalito ob vsakem dalj{em de`evnem obdobju, ob izjemno visokih vodah pa prihaja tudi do obse`nej{ih kratkotrajnih poplav, ki povzro~ajo {kodo (Kova~i~ 2005).
Nekoliko druga~no razlago morfogenetskega dogajanja v ~asu pleistocena ponuja Radinja (1972), ki navaja, da je s podnebnimi spremembami usmerjeno diferencirano zni`evanje razli~nih kamnin spremenilo hipsografska razmerja med njimi, kar je omogo~ilo nastanek gradienta in globinske kra{ke cirkulacije, ki se zaradi zajezenosti karbonatnih kamnin z vododr`nimi kamninami v prej{njem obdobju ni mogla razviti. Pri{lo je do spremembe korozijske dinamike, ki je pre{la iz pliocenske povr{inske, usmerjene in ploskovne korozije v razpr{eno in globinsko (to~kasto) korozijo, saj so agresivne vode ob hladnej{i klimi in skromnej{i vegetacijski in pedolo{ki odeji iz~rpale svojo mo~ {ele na dalj{i poti. Kot prevladujo~a mor-fogenetska procesa navaja globinsko korozijo in erozijo. V povr{ju obravnavanega obmo~ja izstopajo predvsem u~inki slednjega.
4.1 U~inki geomorfnega dogajanja v pleistocenu na dana{njo izoblikovanost povr{ja
Geomorfni procesi so v pleistocenu obravnavano obmo~je mo~no zaznamovali. V terenu nad Koritnicami so dobro opazni sledovi zelo intenzivnega erozijskega delovanja reke Pivke, ki je izvirala izpod Devina in V. Milanje. Ob izhodu iz gorske doline (grape) je reka nasula ogromen gru{~nato-prodnati vr{aj, ki je nagnjen od Koritnic (625 m) proti Ba~u (580 m) in Kne`aku (580 m). Drobir so dobavljala tudi sosednja pobo~ja, na katerih sta v periglacialnih razmerah vladala mo~no mehansko preperevanje kamnine ter solifluk-
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cija. Drobir je bolj podoben gru{~u kot produ, kar dokazuje, kako malo ~asa so ga valile vode. Da bi si olaj{ali kmetovanje, so doma~ini v bolj nagnjenih predelih vr{aja pri Koritnicah uredili manj{e terase.
Najvi{ji izviri nekdanje Pivke segajo v @lebovih do nadmorske vi{ine 950 m in so neposredno pod slemenom Volovje rebri. V strmem pobo~ju z nakloni prek 22° so hudourniki izdolbli globoke in strme grape.
Hudourni{ke grape v @lebovih so zelo {tevilne, na splo{no pa so usmerjene proti severozahodu. Izjema so hudourni{ke grape pod Veliko Milanjo, ki so v vrhnjem delu usmerjene bolj proti severu, vendar se potem ko pridejo v ni`ji svet, obrnejo proti severozahodu. @lebovi so levi, vi{ji povirni krak pleisto-censke Pivke. Z desne pa Pivka priteka po slab kilometer dolgi strugi. Izvir tega povirnega kraka je pribli`no na vi{ini 740 m. Pivka je tu izoblikovala 10 m globok zatrep, ki dolino lo~i od okolice. Nad zatrepom so hudourni{ke grape, ki so najbolj opazne pod Devinom, kjer je bil predvidoma najvi{ji izvir desnega kraka Pivke. Pod Devinom povr{je ni tako mo~no razjedeno kot v @lebovih. Podobno kot v @lebovih so tudi pod Devinom hudourni{ke vode v pleistocenu najverjetneje izvirale na vi{ini okoli 950 m.
Struga pleistocenske Pivke se po zdru`itvi obeh krakov pribli`no 1 km pred Strmo rebrijo (822 m) nekoliko raz{iri. Strmec se zmanj{a, zato na povr{ini zasledimo sipko gradivo, sestavljen iz drobnega gru{~a, pome{anega s prstjo. V to naplavino je voda v holocenu izoblikovala zelo majhne in ozke struge, 0,5 m globoke. Ti predeli so najverjetneje rezultat ene od zadnjih faz re~nega zasipavanja v Pleistocenu. Kjer ni denudirano, debelina tega gradiva ponekod zna{a tudi ve~ metrov.
Ob vzno`ju Strme rebri je poldrugi meter globoka in nekaj ve~ kot 3 m {iroka struga v kratkem odseku nastala v drobirju, ki ga je prinesla Pivka iz zaledja. Zelo zanimivo obliko dobi tik pred Koritnicami. Tu je Pivka s svojo razdiralno mo~jo na razdalji 350 m v `ivoskalno podlago izdolbla do 4 m globoko in ozko strugo (korito), kar je pripomoglo k imenu Koritnice (Melik 1955). Struga Pivke postaja od Korit-nic proti severozahodu vedno plitvej{a, dokler je po nekaj 100 m ni ve~ opaziti. Polo`aj nekdanjih strug nakazuje, da je Pivka ve~krat preusmerjala svoj tok. Suhe struge lahko opazujemo ob cesti, ki vodi od Korit-nic proti Ba~u in proti Kne`aku. Zlasti je lepo izdelana suha struga pod Tu{~akom (786 m), ki se v meandrih vije desno od ceste Koritnice-Ba~ (slika 4). Njeno dno je do 2 m globlje od hrbtov vr{aja nad njimi, kar dokazuje, da se je ve~ji del nasipavanja zaklju~il v pleistocenu, v holocenu pa se je vr{ilo {ibko vrezova-nje. Ob izjemnih padavinah novembra 2000 se je struga v spodnjem delu zapolnila in pojavil se je povr{inski vodotok, ki je poplavil naselje Ba~ (Kova~i~ 2005).
Slika 4: Suha struga nekdanje Pivke severozahodno od Koritnic (fotografija: Gregor Kova~i~, 7.10.2005). Glej angle{ki del prispevka.
Pleistocenska Pivka je od izvirov v @lebovih do obmo~ja suhih strug pod Tu{~akom v dol`ino merila nekaj ve~ kot 4 km. Pivka je imela na tej razmeroma kratki poti relativno velik strmec. Ve~ji strmec pa pomeni ve~jo erozijsko mo~ ter la`ji in hitrej{i transport mehani~no preperelega in razpadlega gradiva, ki se je nasul v vr{aj pod Koritnicami. Debelina nasutega gradiva, ki ponekod presega 10 m, zato ni presenetljiva.
Sestavo drobirja je mo~ preu~evati na ve~ mestih v `e omenjeni suhi strugi ter v posameznih izkopanih jamah vzdol` ceste Koritnice-Ba~. 2 m globok prerez v suhi strugi sestavlja drobir razli~ne velikosti. V zgornjem delu prereza je do 20 cm temne prsti, ki je pome{ana z drobirjem. Velikost posameznih delcev je razli~na. Prevladujejo delci, ki v premeru merijo 1-6 cm, najve~ji med njimi pa do 25 cm. Drobir je slabo zaobljen in bolj podoben gru{~u kakor produ. Med drobirjem je veliko peska. Z izjemo gornjega dela prereza, kjer lahko v zelo omejenem obsegu opazujemo sortirano gradivo v tankih slojih, so delci razporejeni kaoti~no, tako da je med seboj pome{ano gradivo razli~ne velikosti. To dokazuje hudourni{-ki zna~aj pleistocenske Pivke.
V izkopu vzdol` ceste Koritnice-Ba~ (slika 5), ki je 700 m oddaljen od prvega prereza, so razmere podobne. Velikost posameznih delcev je nekoliko manj{a, saj prevladujejo delci, ki v premeru merijo do 2 cm, najve~ji drobir pa v premeru ne presega 16 cm. Tudi tu je med slabo zaobljenim drobirjem veliko peska. Nekoliko bolj zaobljeni so zlasti manj{i delci. Drobir je razporejen brez reda, slojevitosti ni opaziti. V dnu 2,5 m globokega prereza je razkrita nekdanja povr{ina kra{kega polja, ki se odra`a v oblikah podtalnega skalnega reliefa in prisotnosti rde~e ilovice v `epih.
Slika 5: Sestava naplavine v kra{kem polju med Koritnicami, Kne`akom in Ba~em v izkopu poleg ceste Koritnice-Ba~ (fotografija: Gregor
Kova~i~, 7.6.2006).
Glej angle{ki del prispevka.
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Gregor Kova~i~, Razvoj povr{ja v povirju Pivke
V obviseli dolini na ju`ni strani Volovje rebri tudi naletimo na sledove delovanja pleistocena. Ob zaklju~-ku doline, na uravnanem svetu Kamen{~ine nad kotanjo [embijskega jezera, je odlo`eno gradivo razli~ne velikosti. Akumulacija gradiva je posledica delovanja pleistocenskega potoka v dolini ter soliflukcijskih procesov, ki so omogo~ali transport preperelega gradiva po pobo~jih v ni`je predele. Velikost drobirja je v primerjavi z gradivom na kra{kem polju med Ko ritnicami, Ba~em in Kne`akom veliko manj{a.
Vpogled v sestavo gradiva nam omogo~ajo izkopi v treh vrta~ah. V preu~evanem prerezu (slika 7) v eni izmed izkopanih vrta~ lahko opazujemo sloje raznovrstnega gradiva. Pod 25 cm slojem temne prsti je sloj 50 cm nesprijetega in kaoti~no razporejenega drobnozrnatega gru{~a, ki je pome{an s prstjo. Velikost kamninskih delcev v tem sloju ne presega 2 cm. Pod slojem gru{~a je 90 cm debel sloj rjave ilovice, v kateri so tudi posamezni ve~ji kosi gru{~a. Sledi 80 cm debel sloj dobro kalcificiranega drobnozrnatega gru{~a, ki je sprejet v bre~o. V bre~i so opazni posamezni sloji razli~no velikega gradiva. Pod bre~o je nesprijeti drobnozrnati gru{~, ki je tudi sortiran v tanke sloje. Debelina tega sloja ni znana, v prerezu pa meri pribli`no 80 cm. V preu~evanem 4 m globokem prerezu velikost posameznih delcev v bre~i in sloju gru{~a pod njo ne presega 5 cm, kar pri~a o majhni transportni mo~i nekdanjega vodotoka v dolini, oblika delcev pa nakazuje, da je izvor drobirja zelo bli`nje zaledje. @ivoskalno dno v vrta~i ni opazno, med gradivom ni sige. Podobna sestava sedimentov je tudi v sosednjih dveh izkopih, vendar pa je `ivoskalna osnova dna in robov zasutih vrta~ tam dobro vidna.
V srednjem delu doline je 200 m dolga manj{a suha struga, ki je rezultat erozijskega delovanja vodotoka, ki se je v ~asu pleistocena napajal s sne`i{~ izpod Volovje rebri. Vodotok je najverjetneje odtekal tudi prek roba doline v kotanjo [embijskega jezera, kjer je v pobo~ju nad njo v skalo vrezana manj{a hudourni{ka grapa. Hudourni{ka grapa je odvajala povr{inske vode in drobnozrnato gradivo v dno dana{nje kotanje [embijskega jezera in vkotanjo Nari~e, ki je bila ob izjemnih padavinah novembra 2000 tudi zalita z vodo. Podobna, vendar dalj{a hudourni{ka grapa, se vle~e s skrajnih severozahodnih pobo~ij Volovje rebri in je sprva usmerjena proti severozahodu, nato pa zavije na zahod, kjer se na Nari~ah kon~a. Omenjena hudourni{ka grapa je ozka in plitva z `ivoskalnim dnom.
Slika 6: Geomorfolo{ka karta obravavanega obmo~ja izrisana na podlagi digitalnega modela reliefa (25×25 m). Legenda: 1 - kopasta vzpetina 2 - kra{ki izvir, povr{inski vodotok (stalen ali ob~asen), 3 - kotanja presihajo~ega kra{kega jezera, ve~ja kra{ka kotanja, 4 - kra{ko polje, 5 - ozka, {iroka suha koritasta struga, 6 - suha struga oblikovana v naplavini, zatrepna dolina, 7 - smeri nasipanja pleistocenske Pivke, 8 - tektonsko okno, 9 - vzpetina z nadmorsko vi{ino, 10 - povr{inski vodotok (stalen ali ob~asen), 11 - lokacija slike. Glej angle{ki del prispevka.
5 Sklep
[tudij dana{njih povr{inskih oblik je pokazal, da je fluvialni relief na tem obmo~ju `e povsem zabrisalo recentno zni`evanje povr{ja zaradi korozije padavinske vode. Toda nekateri elementi erozijskega delovanja povr{inskih voda iz prej{njih obdobji v zaledju dana{njega izvira reke Pivke so zelo dobro ohranjeni. [e ve~, obravnavanemu obmo~ju dajejo prepoznaven pe~at.
Osnovne oblike kra{kega povr{ja v zaledju dana{nje Pivke so nastale med toplim in vla`nim pliocenskim podnebjem. Pleistocenski procesi so spremenili prej{nje kra{ke oblike. Zaradi pove~anega mehani~nega razpadanja apnencev so nastala polo`na pobo~ja, kasneje pa se je povr{je drobno raz~lenilo.
Slika 7: Prerez skozi sedimente v eni od izkopanih vrta~ na Kamen{~ini (fotografija: Gregor Kova~i~, 7.6.2006). Legenda: A- prst, B - drobnozrnati gru{~, C - ilovica, D - bre~a, E - drobnozrnati gru{~ v slojih. Glej angle{ki del prispevka.
Medtem ko je obviselo dolino na ju`ni strani grebena Volovje rebri precej preoblikovalo mlaj{e korozijsko delovanje, v suhi dolini nekdanje Pivke {e danes mo~no prevladujejo poteze fluvialnega povr{ja. To je dokaz o izjemni erozijski mo~i pleistocenskih voda, ki so tektonsko dobro pretrto obmo~je hitreje poglabljale od sosedstva. U~inki pleistocenskega morfogenetskega dogajanja zaradi kasnej{e korozije tako ve~inoma niso bili zabrisani, zato so v povr{ju dominantni. Tako so v povr{ju ohranjene tudi {tevilne manj-{e hudourni{ke grape in koritaste struge.
Gru{~nato-prodnati drobir, ki so ga pleistocenske vode prena{ale izpod pobo~ij Devina in Velike Mila-nje, je na debelo prekril kotanjo kra{kega polja med Koritnicam, Ba~em in Kne`akom. Tako lahko po
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Acta geographica Slovenica, 46-1, 2006
naplavinah in suhih strugah sklepamo, da se je ve~ina akumulacije zaklju~ila v pleistocenu, v holocenu pa je potekalo rahlo vrezovanje. Razen na obmo~ju omenjenega polja zasledimo naplavine pleistocen-skega in deloma kasnej{ega izvora {e v drugih kotanjah.
Danes je prevladujo~i geomorfni proces razpr{ena to~kovna korozija, v vr{nih predelih Volovje rebri in v @lebovih pa je zlasti v hladni polovici leta intenzivno tudi mehansko razpadanje kamnin. Razvoj povr{-ja je v veliki meri pogojen z gladino podtalnice na obmo~ju, ki se v polju med Koritnicami, Ba~em in Kne`akom obi~ajno nahaja nekje na globini 30-40 m pod povr{jem in je nagnjena od Koritnic, kjer dosega 560 m, proti Ba~u (540 m) (Krivic in ostali 1983). V hidrolo{kem smislu je obravnavano obmo~je bifur-kacijska cona med jadranskim in ~rnomorskim povodjem. Podzemne vode s kra{kega polja odtekajo ve~inoma proti Pivki, medtem ko kotanja [embijskega jezera in njegovo hidrografsko zaledje pripada pore~-ju Reke, saj gladina podtalnice visi v smeri kra{kega izvira Podstenj{ek.
^e posku{amo geomorfogenetsko dogajanje obravnavanega obmo~ja povezati v {ir{i kontekst razvoja povr{ja v Piv{kem podolju ali vsaj njegovega zgornjega dela (Zgornje Pivke) naletimo na vpra{anje, v kak{ni smeri gre nadaljnji geomorfolo{ki razvoj te nadvse zanimive pokrajine. Ravbar in [ebela (2004) v okviru preu~evanja Piv{kih presihajo~ih jezer ter medsebojne primerjave hidrolo{kih razmer na Zgornji Pivki in Cerkni{kem polju ugotavljata, da je Piv{ko podolje trenutno v predhodni fazi razvoja »pravega« kra{-kega polja, ko se kra{ke kotanje poglabljajo do nivoja kra{ke podtalnice, kot je to primer v presihajo~ih jezerih na Zgornji Pivki.
Mo`na pa je tudi druga~na razlaga. Iz hidrogeolo{kih raziskav je razvidno, da je gradient podzemne vode pod Javorniki usmerjen v izvire Malnov (Habi~ 1968), prisotna je mo~na globinska cirkulacija proti severovzhodu, kar je dokazano tudi s sledenji (Habi~ 1989). Iz tega lahko sklepamo, da proces poglabljanja kra{kih kotanj ni zavrt, temve~ se nadaljuje. V tem pogledu bi lahko dana{njo reliefno podobo Piv{kega podolja s {tevilnimi ob~asno poplavljenimi kra{kimi kotanjami in ob~asno povr{insko teko~o Pivko razlo`ili kot razpadajo~e kra{ko polje. Kot zanimivost omenjam zgradbo kotanje Malega Drskov{kega jezera, ki je manj{e izvorno-ponorni{ko polje s kra{kim izvirom na eni in ponorom na drugi strani.
Piv{ko podolje (Pivka, Piv{ko polje) (Kranjc 1987) se uvr{~a med najzanimivej{e in najsliko vitej{e pokrajine slovenskega klasi~nega krasa. Prispevek, ki temelji na kriti~nem pregledu literature in terenskih raziskavah, sku{a razlo`iti razvoj povr{ja v njenem skrajnem jugovzhodnem predelu. Obmo~je je pre{lo skozi razli~ne geomorfogenetske faze razvoja, izmenjevali so se {tevilni procesi preoblikovanja pokrajine. V dana{nji podobi povr{ja nedvomno izstopajo u~inki delovanja pleistocenskih procesov.
6 Viri in literatura
Glej angle{ki del prispevka.
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