QUESTIONS OF DRy VALLEy S IN KARST: CASE STUDy OF MALI DOL, KRAS (SLOVENIA) VPRAŠANJE SUHIH DOLIN NA KRASU, PRIMER MALEGA DOLA, KRAS (SLOVENIJA) Jure KOŠUTNIK 1 Izvleček UDK 911.2:551.435.83(497.4 Mali dol) Jure Košutnik: Vprašanje suhih dolin na Krasu, primer Male­ ga dola, Kras (Slovenija) V članku je predstavljena su� a dolina Mali dol, ki je ena izmed su�i � dolin, ki prečkajo 45 km dolgo in do 15 km široko uravnavo Krasa. Mali dol je 10 km dolga in med 50 in 75 metrov globoka su� a dolina, ki meandrira po Komen- skem Krasu v prečnodinarski smeri prečno na vse reliefne in geološke strukture. Zaradi sedimenta in debelejše odeje prsti so se v dno dolinaste oblike poglobile globoke in velike koro- zijske vrtače. Te vrtače so večje in globlje od vrtač na sosed- njem, uravnanem kraškem površju. Lokalno pretrte cone, ko- menski ploščati apnenci z roženci in debela odeja sedimenta dovoljujejo površinsko odtekanje vode s pobočij v dno doline. Na nekateri� mesti � so se zato oblikovali globoki in strmi ero - zijski jarki. Geomorfološke značilnosti, starost dolinske oblike in debelina sedimenta govorijo o poligenetskem nastanku su� e doline. Oblika je nastala med uravnavanjem platoja v višini piezometričnega nivoja. Ključne besede: Geomorfologija, su� a dolina, kraško uravna- vanje, Mali dol. 1 Petričeva 14, SI-1230-Ljubljana-Črnuče, jurekosutnik@gmail.com Received/Prejeto: 17.08.2007 COBISS: 1.01 ACTA CARSOLOGICA 36/3, 425-431, POSTOJNA 2007 Abstract UDC 911.2:551.435.83(497.4 Mali dol) Jure Košutnik: Questions of dry valleys in Karst: Case study of Mali dol, Kras (Slovenia) In t� e article t� e geomorp� ology of t� e Mali dol dry valley is presented. It is one of at least two dry valleys t� at cross about 45 km long and up to 15 km wide Kras plateau. Mali dol is a 10 kilometers long, 50 to 75 meters deep dry valley. It meanders on t� e leveled surface across t� e Kras plateau in direction NE- SW , perpendicularly to all structures. Because of t� e sediment and t�icker soil cover deep and large solution dolines inside t � e basic valley-like s� ape were formed. These dolines are larger and deeper t� an t� e dolines formed on t� e nearby surface of t� e plateau. Local tectonically crus� ed zones, limestones wit� c� erts and t�ick sediment cover allow some overland flow into t� e valley. In suc� places some smaller but deep and steep ero - sion gullies were formed. From t� e morp� ology, t� e position of t� e dry valley and t� e sediments we can infer t� e polygenetic origin of t� e valley. It was formed during t� e planation of Kras in t� e � eig� t of t� e piezometric level. Keywords: Geomorp� ology, dry valley, karst planation, Mali dol. INTRODUCTION Kras, because of its �istoric importance for karstology, also called classical Kras and because it lies in t� e �in - terland of Trieste also called Trieste Kras, is around 500 km2 big limestone plateau in SW Slovenia (Gams, 1974). Geologically Kras plateau is an anticlinorium leveled by a longer period of planation and denudation (Gams, 1974). It is part of t� e Dinaric Mountains. In Mesozoic Kras was part of t� e dinaric carbonate platform. The oldest and most widely spread rocks on t� e surface of Kras are cretaceous limestones. In t� e smaller extent also Paleocene and lower Eocene limestones can be found. Result of t� e deepening of sedimentation envi- ronment (Jurkovšek, 1996) and erosion in Alps are t�ick Eocene flysc� beds w �ic � covered SW Slovenia (Placer, 1981; Habič, 1984; Gams, 2003) and are t� e youngest rocks in t� e area. ACTA CARSOLOGICA 36/3 – 2007 426 Kras inclines evenly from SE towards NW; from 435 meters above see level at Divača to 98 meters above sea level at Doberdo. Three �ig � er elevation ridges, wit� � eig� ts above 450 meters are present (Slovenija: pokra- jine in ljudje, 2001). Trstelj �ills (Trstelj 643 m), t � e nort� ern most part of Kras borders wit� Vipava valley. The second ridge starts near Sesljan (Sistiana), it reac� es t� e biggest � eig� ts above Trieste and continues into Slo- venia. On t� e top of t� e middle ridge Volnik �ills (Vol - nik 546 m) t� e state border between Italy and Slovenia is running. Volnik �ills divide two main plateaus, t � e nar- rower Nabrežina-Bazovica (Aurisina-Basovizza) and t� e wider Sežana-Komen plateau (Gams, 2003). Micro relief in Kras is represented in numerous dolines and scarce low, rounded �ills. Depression forms are represented also wit� large rounded dolines of collapse origin and valley- like features, wit� additional, sometimes prolonged, so - lution dolines in t� e bottom. The primary goal of t� e researc� was to determinate t� e origin of t� e valley-like features. Alt� oug� fluvial erosion seemed to be t� e most probable process t� at created t� e forms, also ot� er possibilities were considered. Karst � as its beginning and its end, its �istory in w �ic � it goes t�roug � a variety of evolu - tion p� ases. The definition of karst we know from t� e literature but if we take a �istorical time - line of a certain karst region and we slice it somew� ere in its past evolution and analyze t� e state of t� e karst and make a definition out of our findings, we will many times find t� at our definition is just opposite of t� e one from t� e text books. If we are aware of t�is and if we know t � at karst is in a way conservative, t� at it to some extend preserves t� e sur- face forms, t� e combination of different �istorical land forms in karst surface is somet�ing we must seriously consider during our studies. I can say t� at dry valleys are c� aracteristic and important karst land form. Dry valleys ap- pear also in t� e fluvial geomor- p�ic system but t � eir identifica- tion in karst is easier because of t� eir longer life spend. Their im- portance lies in t� eir key-role in interpretation of development of certain karst surface. Researc� ers named dry valleys in Kras doli (singu- lar: dol). In� abitants of Kras use t� e expression dol for bigger dolines, w�ic � are common also in t � e bottom of dry valleys (Radinja, 1965). The word dol is of preslavic origin and meant cave, cavern, valley and also into t� e valley and into t� e cave (Snoj, 2003). Mali dol got its name after t� e village of t� e same name in its bottom, so we can distinguis� it from Brestovica and Doberdo dry valleys. For t� e purpose of understanding t� e processes in t� e dry valley-like form and its origin detailed geomor- p� ologic mapping was done, w�ic � resulted in a geomor - p� ologic map in t� e scale 1:5000 wit� legend. Mali dol meanders on t� e leveled surface across Kras plateau in t� e direction NE-SW , perpendicularly to lit� o- logical and structural units of prevailing dinaric direc- tion. It starts above Branica river valley wit� a pass in t � e Trstelj �ills on 293 meters above sea level. It � as a distinc- Fig. 1: Kras plateau with mali dol dry valley (Slovenija: pokrajine in ljudje, 1999). JURE KOŠUTNIK ACTA CARSOLOGICA 36/3 – 2007 427 tive prolonged depressional s� ape, c� aracteristic for river valleys. In its bottom large solution dolines were deepened. Transition between t� e surrounding planated karst surface and Mali dol is quick but not s� arp. The difference in elevation between Komen plateau and t� e bottom of Mali dol is in its nort� ern part 40, in its central part between 50 and 60 and in t� e sout� ern part up to 75 meters. The bottom of t� e valley-like form is at its largest extent more t� an 250 and in its smallest extant only 40 meters wide. The total lengt� of Mali dol is 10 kilometers. In Mali dol, t�roug � out its lengt� t � e rocks are of upper cre- taceous age. Between Tomačevica and Mali dol village c� aracteristic t�in-layered limestones wit � c � erts can be found. At t� e edge of Kras anticlinorium in Trstelj �ills also layers of Paleocene and Eocene age appear. On t� e opposite end of Mali dol in t� e Divača fault lower creta- ceous limestones and dolomites ap- pear (Osnovna geološka karta SFRJ, 1973). Age of rocks generally de- creases in t� e NE direction. RELIEF FORMS IN MALI DOL Evolution of relief forms in Mali dol presumably � appened gradually, all forms are not of t� e same age and all forms were not made only by cor- rosion. The most important relief forms are dolines and erosion gul- lies. Up to 20 meters deep solution dolines are deepened into valley’s bottom. Dolines are in average wid- er t� an dolines on t� e surrounding karst plateau and usually prolonged in t� e general valley direction. Ex- tremely wide irregular dolines can also be found in t� e bottom of Mali 500 400 300 200 150 100 50 0 500 400 300 200 150 100 50 0 100 300 500 700 900 1100 (m) Branica river valley Komen Veliki dol Trstelj hills Trstelj hills Veliki dol Komen Branica river valley Longitudinal profile of Mali dol Longitudinal profile of Komen plateau Fig. 2: Longitudinal profile of mali dol dry valley. Geomorphologic mapping of mali dol Fig. 3: DEm of m ali dol and it’s surrounding. QUESTIONS OF DRy VALLEy S IN KARST: CASE STUDy OF MALI DOL, KRAS (SLOVENIA) ACTA CARSOLOGICA 36/3 – 2007 428 dol. According to t� eir size we can call t� em uvalas but t� ey can be also of collapse origin. The number of dolines on a square kilometer of planated surface near Komen and on t� e square kilometer of Mali dol is at a ratio of 5 to 1. In t� e slopes of Mali dol, in t� e slopes of dolines in t� e bottom of Mali dol and between dolines erosion gullies were developed. Big- ger gullies are less steep and wider t� an smaller ones, commonly related to tectonically fractured zones. Usually t� ey end in dolines in t� e bottom of Mali dol, w� ere alluvial fens were formed. Gullies in t� e slopes of Mali dol reac� t � e planated Komen plateau and can still bee traced as a series of s� allow dolines. Gullies were probably formed, because lo- cally t�icker, less permeable soil and sediment cover allows surface runoff. Evidence of t� at was commonly found. Surface water flow � as an ability to car- ry sediment and organic parti- cles from t� e plateau and slopes to t� e bottom of Mali dol. Fig. 5: Legend of the geomorphologic map. Fig. 4: Geological map of mali dol (Osnovna geološka karta, list Gorica). Fig. 6: part of the geomorphologic map of mali dol (TTN 1:5000). JURE KOŠUTNIK ACTA CARSOLOGICA 36/3 – 2007 429 ple removed t� e sediments and used t� em elsew� ere, even after several meters bedrock was not reac� ed. The origin of t� e sedi- ment is polygenetic. Part is t� e so called insoluble limestone residue, w�ic � is more important only w� ere ot� erwise very pure limestone contains c� erts. Origin of t� e ot� er part of t� e sediment can be related wit� impermeable flysc� layers, from w �ic � particles were broug� t on today already denudated karst surface by sinking streams. Two important processes are enabled by t� e sediment presence. Firstly it allows surface run-off wit� SEDIMENTS AND FLOWSTONE IN MALI DOL Bottom of Mali dol is covered by a t�ick layer of sedi - ments and soil. At some locations in dolines, w� ere peo- Fig. 7: prolonged doline in the bottom of mali dol. Fig. 8: Doline with anthropogenicaly changed bottom. Fig. 9 and Fig. 10: Erosion gullies in the slopes of mali dol. Fig. 11: Thick sediment in the bottom of mali dol. Fig. 12: Rock particles and pebbles found in the sediment. QUESTIONS OF DRy VALLEy S IN KARST: CASE STUDy OF MALI DOL, KRAS (SLOVENIA) ACTA CARSOLOGICA 36/3 – 2007 430 transportation to lower parts of t� e valley and formation of gullies. Secondly its capability of water retention in- creases t� e accelerated corrosion on t� e contact between sediment and limestone. Aloc� tonous and autoc� t� onous rock particles were found in t� e sediment of Mali dol. Autoc� t� onous are t� e semi-rounded parts of c� ert rubble and some sub- cutaneously rounded limestone particles, bot� deriving from underling bedrock. Aloc� tonous rock particles are pebbles of quartz and c� erts w�ic � were transported and rounded by sinking streams from flysc�. Quartz pebbles are w�ite, non-transparent and partly limonited. C � ert pebbles, reddis� to pale grey, almost w �ite in color were made from flysc� sandstone and � ave visible layers and grainy texture (Herlec, 2006). More or less rounded par- ticles of limonite iron ore can be found. All speleot� em occurrences were noted because of t� e interpretation of t� e origin of t� e valley. Compact speleot� em blocks were found twice, t� e first time in t� e Bobovec doline nort� of village Mali dol w � ere de- nudation in t� e slope revealed a cave, possibly an abyss filled wit� cave sediments and flowstone. The second time flowstone was found NW of t� e village Škofi. A cave opens in t� e SE slope of doline called Jevščine. Distinc- tive yellowis� cave sediment and t �ick speleot � em come to t� e surface. THE QUESTION OF DEVELOPMENT, ORIGIN AND AGE OF MALI DOL According to older Slovene geomorp� ologists Mali dol was created by fluvial erosion in t� e so called »pre-karst p� ase« of surface development. The valley is a remnant of t� e last p� ase of river down-cutting before it gradually sinked underground in correlation wit� sudden start of karstification process (Radinja, 1965). Gams wrote t� at Mali dol is t� e result of t� e last p� ase of tectonic lowering of t� e western part of Kras as a result of counter-clockwise rotation of t� e Adriatic micro plate and general tectoni- cal lowering of Po river basin (Gams, 1998). Tectonical origin of t� e valley is also accepted by Habič. He wrote t� at Mali dol was created on t� e contact between differ- ent tectonical structures in association wit� t � e c� ange in general ridge direction of Kras from dinaric (NW-SE) to alpine (W-E; Habič, 1984). In addition to possible fluvial and tectonical origin of Mali dol also t� e development from a denudated cave system s� ould be considered. By inspection of t� e geomorp� ologic map and by t� e met� od of exclusion we can state, t� at Mali dol, as we see it today, was made by fluvial erosion. Meandering s� ape excludes its tectonical origin, t� e lack of t�ick-bed - ded speleot� ems and unproportionaly smaller dimen- sions of t� e largest Slovene unroofed cave (unroofed cave in Loza) exclude its unroofed development. Only expla- nation by t� e fluvial erosion development remains. The valley-like feature Mali dol in Kras was at some time in t� e past an active river valley w�ic � is now dry. CONCLUSION Already in t� e early stages of researc� preparation, it be - came clear t� at t� e task is going to be � arder t� an ex- pected. In a seemingly simple equation new, puzzling variables emerged. Buy geomorp�ic mapping polygenetic origin of Mali dol was confirmed. Different processes, acting one by anot� er and simultaneously, � ave affected t� e creation and later development of Mali dol during longer period of time. All affective geomorp�ic processes are closely connected wit� sediments. Less permeable sediment, w� et� er it is insoluble limestone residue, remnant of Eo- cene flysc� cover or filling of denudated caves, � as a key role in t� e geomorp� ological development of Mali dol. The sediments impermeability, t�ickness and retention capability affected t� e accelerated corrosion process and contributed to preservation of t� e land form. Numerous questions remain open. The time-line, age of planation, beginning of t� e last p� ase of karstifica- tion and t� e valley formation s� ould be confirmed wit� absolute datations. As far as we know t� e most promising datation met� od is paleomagnetism w�ic � already gave some results on t� e age of cave sediments in Slovenia. New data about tectonic activity on t� e Miocene/Plio- cene border is being gat� ered, along wit� possible gra - dient c� anges on Kras during t� e messinian crisis and mineralogical c� aracteristic of karst sediments. Specific accelerated corrosion impact and detailed sediment c� aracteristic of Mali dol are jet to be studied. JURE KOŠUTNIK ACTA CARSOLOGICA 36/3 – 2007 431 This article is part of my diploma t� esis presented at Geograp� y department of Ljubljana University. I would like to t� ank Dr. Andrej Mi� evc for guidance and sup- port. ACKNOWLEDGEMENT REFERENCES Državna topografska karta RS 1:25 000. 145 Opatje selo, 1996. 1. izd. 1:25 000. Ljubljana, Republika Slovenija – Ministrstvo za obrambo in Ministrstvo za okolje in prostor, Geodetska uprava RS. Državna topografska karta RS 1:25 000. 127 Miren. 1997. 1. izd. 1:25 000. Ljubljana, Republika Slovenija - Ministrstvo za obrambo in Ministrstvo za okolje in prostor, Geodetska uprava RS. Gams. I., 1974: Kras.-Slovenska matica, p. 359, Ljublja- na. Gams, I., 1986: Kontaktni fluviokras.-Acta carsologica, 14/15, 71–87, Ljubljana. Gams, I., 1998: Geomorp� ogenetics of t� e Classical Karst - Kras.-Acta carsologica, 17, 2, 181–198, Ljubljana. Gams, I., 2003: Kras v Sloveniji.-Založba ZRC, ZRC SAZU, p. 516, Ljubljana. Gregorič, A., 2005: Analiza razvoja paleonapetosti v dinarskem narivnem sistemu za� odne Slovenije (okolica Nove Gorice).-Univerza v Ljubljani, NTF, Oddelek za geologijo, p. 84, Ljubljana. Habič, P ., 1984: Reliefne enote in strukturnice matičnega Krasa.-Acta carsologica, 12, 5–26, Ljubljana. Habič, P ., 1986: Površinska razčlenjenost Dinarskega Krasa.-Acta carsologica, 14/15, 39–58, Ljubljana. Herlec, U., 2006: Litološka sestava proda.-Personal source, Ljubljana. Jurkovšek B., et al, 1996: Formacijska geološka karta južnega dela Tržaško-komenske planote.-Inštitut za geologijo, geote�niko in geofiziko, p. str, Ljubljana Kras, pokrajina-življenje-ljudje. 1999. Kranjc, A., (ur.). Ljubljana, Založba ZRC, ZRC SAZU, 321 str. Mi� evc, A., 1999: Pomen brezstropi� jam za geološko, geomorfološko in geografsko proučevanje krasa.- Geološki zbornik, 14, 27–29, Ljubljana. Osnovna geološka karta SFRJ. Tolmač lista Gorica. 1973. Beograd, Zvezni geološki zavod, 50 str. Osnovna državna karta, SR Slovenija 1:5 000. B22 Ko- men 19. 1968. 1:5 000. Ljubljana, Zvezna geodetska uprava SFRJ. Placer, L., 1981: Geološka struktura jugoza� odne Slo- venije.-Geologija, 24, 1, 27–60, Ljubljana. Radinja, D., 1965: Nova morfogenetska dognanja na Kra- su.-Univerza v Ljubljani, Oddelek za geografijo, p. 133, Ljubljana. Radinja D., 1966: Morfogenetska problematika matičnega Krasa.-Geografski obzornik, 13, 3–4, 108–114, Lju- bljana. Radinja D., 1974: Matični Kras v luči širšega reliefnega razvoja. Acta carsologica, 6, 22–33, Ljubljana. Slovenija: pokrajine in ljudje. 2001. Perko, D., Orožen Adamič, M. (ur.). Ljubljana, Založba Mladinska knjiga, 735 str. Snoj, M., 2003: Slovenski etimološki slovar. 2. pregledana in dopolnjena izd.-Modrijan, p. 1022, Ljubljana. Šebela, S., 1999: Kras v geološki preteklosti. V: Kras, pokrajina-življenje-ljudje, Kranjc, A., (ur.). Ljublja- na, Založba ZRC, ZRC SAZU, str. 19–25. Šušteršič, F., 1998: Interaction between a cave system and t� e lowering karst surface case study: Laški ravnik.- Acta carsologica, 27, 2, 115–138, Ljubljana. Temeljni topografski načrt, SR Slovenija 1:5 000. B22 Ko- men 28. 1981. 1:5 000. Ljubljana, Geodetska uprava SRS. Temeljni topografski načrt, SR Slovenija 1:5 000. B22 Ko- men 29. 1981. 1:5 000. Ljubljana, Geodetska uprava SRS. Temeljni topografski načrt, SR Slovenija 1:5 000. B22 Ko- men 37. 1981. 1:5 000. Ljubljana, Geodetska uprava SRS. Temeljni topografski načrt, SR Slovenija 1:5 000. B22 Ko- men 38. 1981. 1:5 000. Ljubljana, Geodetska uprava SRS. Temeljni topografski načrt, SR Slovenija 1:5 000. B22 Ko- men 47. 1981. 1:5 000. Ljubljana, geodetska uprava SRS. Temeljni topografski načrt, SR Slovenija 1:5 000. B22 Ko- men 48. 1981. 1:5 000. Ljubljana, geodetska uprava SRS. QUESTIONS OF DRy VALLEy S IN KARST: CASE STUDy OF MALI DOL, KRAS (SLOVENIA)