ACTA CARSOLOGICA 28/2 3 77-90 LJUBLJANA 1999 COBISS: 1.08 THE "UNROOFED CAVE" NEAR THE BUNKER (LASKI RAVNIK) BREZSTROPA JAMA PRI BUNKERJU (LAŠKI RAVNIK) MILAN GERŠL1 & UROŠ STEPIŠNIK2 & SIMONA ŠUŠTERŠIČ3 1 Masaryk University Brno, Faculty od Science, Department of Geology and Paleontology, Kotlarska 2, 611 37 BRNO, CZECH REPUBLIC. E-mail: . 2 Student of; Filozofska fakulteta Univerze v Ljubljani, Oddelek za geografijo, Aškerčeva 2, SI-1000 LJUBLJANA, SLOVENIA. E-mail: . 3 Laze 22, 1370 LOGATEC, SLOVENIA. Prejeto / received: 1. 8. 1999 Izvleček: UDK: 551.442(497.4):551.311.24 Milan Geršl & Uroš Stepišnik & Simona Šušteršič: Brezstropa Jama pri bunkerju (Laški Ravnik) Jama leži v Laškem Ravniku, okrog 4 km vzhodno od Planinskega polja. Je najdaljša v ožji okolici, saj njeni denudirani rovi merijo skupaj več kot 200 m. Njena osnovna smer je sever-jug. Jama sestoji iz treh glavnih delov, od katerih je bil vsak preoblikovan na svoj način. Očiten je razpad nekdanjega jamskega stropa, kar daje na površju vtis "lažnih" vrtač, med katerimi je strop delno ohranjen, ali pa ostanki razkrojenega stropa "sedijo" v še neizpranem jamskem polnilu. V smeri vpada skladov so stene rovov razpadle skoraj popolnoma in posamezni bloki so zdrsnili v osrednje, izprane predele nekdanje jame. Vsi trije deli jame so se razvili vzdolž lezik, toda v različnih smereh, tako da se dva danes "dvigneta" nad površje, tretji pa "potone" in je popolnoma zasut z ilovico. Proti jugu se jama nadaljuje v "normalno" jamo. Njeni rovi so nizki in komaj dostopni, v splet pa se vklapljata še dve brezni, verjetno nekdanja freatična skoka. Tega dela tu ne obravnavamo. Ključne besede: brezstrope jame, denudacija, speleogeneza, jamski sistem, kras Slovenije. Abstract: UDC: 551.442(497.4):551.311.24 Milan Geršl & Uroš Stepišnik & Simona Šušteršič: The "unroofed cave" near the bunker (Laški Ravnik) Situated in the Laški Ravnik, about 4 km east of Planinsko polje (Slovenia), this unroofed cave is the longest of many found in the area. More than 200m of denuded passage has been traced at the surface. The cave's general trend is north - south. Three main "passages" making up the unroofed cave have been recognised, and each has been transformed in its own way. Disintegration of parts of the former cave roof is apparent, forming "false solution dolines", between which the ceiling is partly preserved, either in-situ or as isolated blocks lying on cave sediment. In the down-dip direction, passage sides have disintegrated almost completely, and detached blocks have slid down into washed-out portions of the former cave passages. All three passages developed along bedding planes, but in different directions, such that two of them now "emerge" above the surface whereas the third plunges down and is completely choked by loam. The unroofed cave passes southwards into a "normal" underground route, where a barely accessible, low and wide passage is accompanied by two "vertical" shafts, which are presumed to be former phreatic jumps. This part of the system is not considered in this paper. Key words: unroofed caves, denudation, speleogenesis, cave system, karst of Slovenia. INTRODUCTION During the last few years, developments of karstology have abolished the pre-existing spatial constraints within karst caves studies. Speleology is no longer limited to the realm of voids that are accessible to humans, but can now also be carried out at the surface. Consequently, the mode of genesis of some surface karst landforms is no longer a puzzle. Though the appearance of denuded caves has always been regarded as self-evident, their systematic study did not begin until Andrej Mihevc (1996) and his co-workers investigated an unroofed cave near Povir, on the new motorway from Ljubljana to Trieste. In this paper the authors present details of an "unroofed cave" near the Bunker in the Laški Ravnik. Wider study of the denuded caves in this area has been carried out by F. Šušteršic (1998-b). Laški Ravnik lies some 3km northeast of Planinsko polje (Slovenia). It is an 18km-long and 1 to 3km-wide strip of relatively lowered relief, extended along the Dinaric trend. The karst surface is generally flat, though interrupted by many solution and collapse dolines. Jurassic and Cretaceous dolomite beds dip at 25° to 30° towards the west and west-north-west. Beneath Laški Ravnik are the underground streams of the sinking river Ljubljanica, which cross the area on their way from the karst poljes of central Notranjska towards the Vrhnika springs. F. Šušteršic (Pers. Comm.) has studied about 2.5km2 of the surface in great detail, without yet encountering the limits of the denuded flow corridor. Denuded outcrops of allochtonous clayey cave sediment are distributed across at least a 100m vertical span. All the denuded cave passages so far found in the Laški Ravnik are of phreatic origin, generally comprising short oblique channels and phreatic jumps, all oriented along initial guiding structures. According to Šušteršic (Pers. Comm.), the lower parts of a still active flow corridor were filled with mudflow debris, consisting of brown loam with fragments of Carnian oolitic bauxite and black chert. Such material can be still seen in situ within the catchment area of the Cerkniščica river, which now flows on the surface into the Cerkniško polje. No traces of epiphreatic shaping have been found, and Šušteršic concluded that either no such phase had been passed through or, if there had been an epiphreatic phase, all of its products have since been removed. The focus of this study is an unroofed cave "Near the Bunker", on the eastern slope above the Laški Ravnik, which lies about 40m higher than most of the caves studied by Šušteršic (Pers. Comm.). In the Bunker area there are fragments of large denuded cave channels, up to several metres in diameter, with a cumulative length of about 200m. These were part of a passage system that formed initially along bedding planes, partly along the strike and partly oblique to it. The whole of the known cave lies within Jurassic limestone and its guiding inception horizon(s) are not clearly recognisable. Visible channels partly plunge back below the undisturbed land surface or they "vanish in the air", due to the complete denudation of the parent rock that held their original continuations. In some locations they seem to end at phreatic jumps. At such sites the loamy fill is well preserved, whereas elsewhere it is not. There is no doubt that the cave beneath the Bunker is only a part of an originally much larger system. On the basis of slightly different landforms visible at the surface, probably reflecting uneven levels of cavern developments as well as different effects of denudation and rock weathering, the area can be split into three sectors. Fig. 1: The "unroofed cave" near the bunker (Laski Ravnik). Sl. 1: Brezstropa Jama pri bunkerju (Laski Ravnik). • The "Northern Sector" is a cave channel developed approximately along the strike, where approximately 50m of the cave is at outcrop. In one direction the cave ends in a phreatic jump, whereas in the other the "end" is obscured by the road. • The "Southern Sector" contains an open channel, which continues to the "Underground Sector" in one direction. In the other direction it branches into two passages. The westernmost passage "sinks" beneath the undenuded rock surface, and the eastern passage "rises" above the karst surface, leaving indications of a short phantom cave below it. • The "Underground Sector" comprises three speleological objects that are in contact with the "Southern Sector". Close by there are several areas where bedrock arches have partially disintegrated, so that blocks of rock "sit" rather chaotically on the inferred cave filling. Clearly, the features in these areas must be related to neighbouring caves. The latter have previously been explored and documented by cavers, but their data are insufficient for the purposes of this study, and the caves must be revisited. Only the "Northern Sector" is considered in detail here. It fits partly to Sušteršič's (Pers. Comm., Fig. 3) stages B to D, but with sufficient differences for it to remain of interest to a wider audience. THE NORTHERN SECTOR As shown on the plan (Fig. 1), the Northern Sector is a kind of trench, about 50m long, which runs parallel to the surface and descends gradually towards the north-west (i.e. diagonal to the dip or strike). The trench is about 2 to 4m wide and, at the most, 2m deep. Rock fragments of various origins are distributed chaotically, and larger blocks, which have crept down-dip from the eastern side, are more obvious. Openings inaccessible to humans lead downwards for about 1m between the boulders. The cave channel itself has not yet been revealed. Denudation simply made the roof thin, to such an extent that it could no longer support itself, and the loosened blocks settled gently onto the loamy cave fill. Where the loam has been partly washed away, the bottom of the trench is locally relatively lower. It appears as if the boulders have been "swallowed" or "engulfed". This is especially clear at the northern limit, where the trench becomes wider and deeper. This is deduced to be the position of a "downward" phreatic jump, but such an impression is subjective, as there is no indication of the water flow direction when the channels were active. This sector is of interest due to the processes that transform the trench slopes. The clearest way to describe these is by reference to individual cross-sections (Fig. 1), which are chosen to illustrate the various slope processes, though some details of the trench floor can also be recognised. A. The trench is somewhat less than 2m wide and a metre and a half deep, and its floor is covered by dead leaves and organic litter, with some small pieces of rock protruding. The counter-dip wall of the trench is vertical in its upper part, with a small overhang in the lower part. A staircase-like form on the opposite slope has steps formed in partly disintegrated strata whose loosened blocks tumbled down into the centre of the trench. This relatively gentle slope is all within solid rock. B. Here the trench is somewhat wider and shallower. The slope on the counter-dip side of the trench is low, gentle and staircase-like, resembling the up-dip slope of profile A. The opposite slope is virtually non-existent. There is just one large block, detached from the solid rock, along the whole depth of the trench, and this has moved only slightly towards the centre. It is stressed that this is an isolated block with its own identity. C. The trench here is about 2m wide, and slightly more than 1m deep. Both slopes are near vertical and there is no great difference between them. In the centre of the trench a block protrudes from the organic litter, and is possibly a roof remnant, left after the rest of the roof sank through the cave fill. D. At this point the trench is highly asymmetrical. It is about 4m wide and at its deepest position -where there is an opening leading downwards between boulders - is shifted in the down-dip direction nearly to the foot of the scar. The slope there is nearly vertical, whereas the opposite one, which is exactly parallel to the strata, cannot be distinguish from the trench floor. The lower part it is covered by dead leaves and the upper part is effectively a bedding plane. Some blocks lie on its surface, but most of them have crept into the central part of the trench. "Swallowing" of blocks by the cave fill must be quite extensive here, as not only are the chaotic blocks missing, but there are also openings leading downwards. E. The trench is essentially wider (to 4m) and deeper (to 2.5m). Nearly all the slopes are of solid rock and more or less vertical. Loosened blocks are visible at several locations, set free along extension cracks, and the largest of them, which is completely detached, lies just in the line of the profile. In the uneven floor, which is mostly covered by chaotic blocks and partly by organic litter, there are some openings leading downwards. The margin to the northwest of the bottom is of special interest. As at section D, parts of the covering bed have crept along the bedding plane into the trench, but the actual bedrock foundation is no longer compact. Because there was a mass deficiency in the centre of the trench the blocks have moved slightly apart, forming a kind of lapiaz, with grikes that are slightly transformed extension cracks. This effect can be seen elsewhere along the margin of the trench, but not to such an extent. Many similar features are also found in the Southern Sector. The question of whether the whole trench is a denuded cave at all, rather than simply being a kartstified fault (bogaz), is obvious. As said above, the trench follows neither the strike nor the dip, but is directed diagonally across both, with slight zigzagging. Therefore, the impression is that the fundamental direction is weaving through a mosaic of blocks, formed along strike- and dip-aligned cracks. In the region of stations 48, 45, 63 and 64 a joint crosses the trench. Its direction is characteristic of (primarily) tension and (secondarily) strike-slip faults, which are the result of movements along the Idria Fault, and which possibly brought about the formation of the Rakovska kukava (F. Sušteršič, 1998-a). A small vertical shaft has formed along the same joint (between stations 63 and 64), but evidently the joint had no influence on the initial development of the cave. Possible conclusions are that the joint is either younger than the cave, or that such joints have no major importance in early cave formation. FORMS OF THE SLOPES AND PROCESSES ACTING UPON THE DENUDED PASSAGE The cross-sections of the denuded/deformed cave channel presented above offer an opportunity to distinguish a number of processes acting upon the slopes and floor of the trench. The shape of conséquent slopes Consequent slopes are those that lower in an approximatively down-dip direction. Description Process Profile Vertical wall After the cave ceiling has settled the rock remains undisturbed C Staircase-like slope along the bedding planes Several beds disintegrate at the same time; blocks move towards the centre A Vertical wall, some blocks detached from it Single block detach from the bed, dipping towards the mass deficiency. The bulk of the parent rock remains untouched B, E, possibly C Smooth slope along the bedding plane The upper bed has crept predominantly or completely into the centre D, E Apparent lapiaz along the bedding plane Smooth slope along the bedding piane, the basement has moved apart towards the mass deficiency E The shape of obsequent slopes Obsequent slopes are those that lower in an approximatively counter-dip direction. Description Process Profile Vertical wall in the upper part, overhang below After settling of the cave roof the bedrock remains undisturbed A Uniform vertical or very steep wall After settling of the cave roof the bedrock remains undisturbed C, D Staircase-like slope along the bedding planes No explanation bound directly to the mass deficiency B Vertical wall, some blocks detached from it The influence of the mass deficiency outweights the influence of the dip E The shape of the trench Description Process Profile Well arranged, settled down blocks Disintegrated cave roof, sitting on top of the cave fili Not in the profiles Chaotically arranged smaller tumbled blocks During weathering the blocks that are set free tumble down into the centre Not in the profiles Single isolated larger blocks Single blocks break away from the walls B, E Partly sunken larger blocks Blocks sink into the basement; possibly due to downwashing and undermining, or due to intensified corrosion C Dead leaves and humus Organic litter covers (predominantly) chaotic blocks A,B,C,D,E Holes, leading downwards between the boulders Due to settling down, smaller blocks and organic litter cannot remain on the surface D DETAILS FROM OTHER SECTORS • Loam can be found between the chaotically arranged blocks. This might mean that a filled cave passage lies below the blocks and the blocks are "floating" in the fill. • Denuded phreatic jumps appear on the surface at the end/beginning of an intermediate branch of the denuded cave channel, or in complete isolation. All of them are filled with loam (plus bauxite and chert). • Some denuded passages pass beneath the surface, "ending" below an arch. Some arches are well-expressed, appearing as small caves; others are merely indicated. CONCLUSIONS • The cave near the Bunker is more "horizontal" than any other yet encountered in the Ravnik area. • The Northern Sector is trench-like, and formed by the settling down of the disintegrated cave roof onto the cave filling, without any significant collapse. • Though the studied section is quite short, the effects of various slope-transforming processes are recognised. These were conditioned as much by the pre-existing geology as by the external weathering processes. • There is an urgent need to develop a standard set of specific graphical symbols to represent all of the features mentioned (and others, too), to make map drawing possible. However, before this is done, we should ensure that most, if not all, of the processes and the features they produce are known. ACKNOWLEDGEMENTS Our special thanks to: Dr Andrej Mihevc, who "invented the wheel", Professor France Sušteršič, who "taught us to drive", and Dr David Lowe, who "smoothed" the English text. REFERENCES Mihevc, A., 1996: The cave Brezstropa jama near Povir (in Slovene, English abstract). Naše jame, 38, 65 - 75, Ljubljana. Sušteršič, F., 1998-a: Rakovska kukava - collapse or tumour doline? Acta carsologica 27, 1, 321259, Ljubljana. Sušteršič, F., 1998-b: Interaction between a cave system and the lowering karst surface. Case study: Laški ravnik. Acta carsologica 27, 2, 115 - 138, Ljubljana. BREZSTROPA JAMA PRI BUNKERJU (LASKI RAVNIK) Povzetek UVOD Razvoj krasoslovja je v zadnjih letih odpravil prostorske omejitve proučevanja kraških jam. Speleologija ni več omejena na območje človeku dostopnih kraških votlin in proučevanje jam se je preneslo tudi na površje. Tako so nekatere površinske kraške oblike postale genetsko določljive. Kljub samoumevnosti jamskih izdankov v preteklosti se je sistematičnega proučevanja lotil šele dr. Andrej Mihevc (1996) s sodelavci, ki so prvi temeljito proučili brezstropo jamo na trasi primorske avtoceste. Podpisani želimo predstaviti del brezstrope "Jame pri bunkerju" v Laškem Ravniku. Denudirane jame v Laškem Ravniku je podrobneje proučeval F. Sušteršič (1998-b). Laški Ravnik se nahaja 3 km severovzhodno od Planinskega polja. V celoti je dolg 18 km, širok pa od 1 do 3 km in se vleče v dinarski smeri. Površje v tem delu je uravnano ter prekrito s številnimi vrtačami in udornicami. Skladi jurskih in krednih dolomitov ter apnencev upadajo pod kotom 25 o-30o proti severozahodu. Doslej je F. Sušteršič (o.c.) v Laškem Ravniku podrobno kartiral približno 2.5 km2 površja in odkril okrog 150 izdankov denudiranih rovov, vendar bližine robov svežnja ni zaznal nikjer. Denudirana nahajališča alohtonega ilovnatega jamskega polnila se raztezajo v višinskem razponu najmanj 100 metrov. Izdanjajoči deli jamskega sveženja so freatičnega nastanka; kratki poševne rovi ali vmesni freatični skoki, vsi pa se ravnajo po smeri inicialnih struktur. Po Sušteršiču (o.c.) je spodnje dele freatičnega svežnja najprej zaplavil blatni tok, ki so ga sestavljali rjava ilovica s kosi oolitnega karnijskega boksita in črnega roženca. Takšne boksitne prodnike je danes moč najti v povirju Cerkniščice, ki površinsko teče do Cerkniškega polja. Material blatnega toka je torej alohtonega izvora. Glede na to, da nikjer ni opaznega epifreatičnega preoblikovanja jamskih kanalov, sklepa, da ta faza sploh ni nastopila, ali pa da so vsi njeni produkti že denudirani. Predmet našega proučevanja je brezstropa "Jama pri Bunkerju" na vzhodnem pobočju nad Laškim Ravnikom, to je okrog 30 m višje od večine jam, ki jih je proučeval F. Sušteršič . V našem primeru gre za dele več metrov velikih denudiranih jamskih kanalov v skupni dolžini okog 200m. Potekajo ob leziki; delno vzdolž slemenitve, delno nekoliko poševno nanjo. Vsa jama se nahaja v jurskem apnencu in zato so začetni horizonti slabo razvidni. Rovi delno potonejo pod še nedenudirano površje ali pa se zaradi popolne denudacije izgubijo "v zrak". Ponekod se končajo s freatičnii skoki. Tu je ilovnato polnilo dobro ohranjeno, drugod pa je večinoma sprano. Nobenega dvoma ni, da je Jama pri bunkerju del večjega sistema. Zaradi nekoliko različnih oblik na površju, ki so enako posledica nekoliko neenotne oblikovanosti podzemskih rovov, kot različne stopnje denudacije in površinskega preperevanja, smo jamo razdelili na tri odseke: • "Severni odsek" obsega jamski kanal ki je nastal približno vdolž slemenitve. Izdanjajoči del rova je dolg približno 50 metrov in se na eni strani zaključi s freatičnim skokom, na drugi pa je zaključek uničila cesta. • "Južni odsek" sestavlja rov, ki se nadaljuje v podzemni del, na drugi strani pa veji v dva. Zahodnejši med njima potone pod še nedenudirano površje, vzhodnejši pa se "dvigne" nad zemeljsko površje. • "Podzemni" odsek obsega tri speleološke objekte, ki mejijo na Južni odsek. V bližini je še več posedov, ki so nedvomno povezani s podzemskim delom. Jamarji so vse tri objekte sicer že raziskali in izmerili, vendar njihovi podatki za bolj podrobno obdelavo ne zadostujejo in jih bo potrebno pregledati še enkrat. Prikazali bomo samo "Severni odsek", ki nekoliko ustreza Sušteršičevim (o.c., Sl. 3) razpadnim stopnjam B do D, se pa toliko razlikuje, da je zato zanimiv. SEVERNI ODSEK Kot je razvidno iz načrta (Sl. 1), predstavlja Severni odsek nekakšen jarek, ki se v dolžini okrog 50 metrov spušča v smeri proti severozahodu, torej diagonalno na vpad ali slemenitev skladov. Jarek je širok 2 do 4 metre, globok pa največ 2 metra. Po dnu je organski drobir, ki leži na - kot kaže - posedlih blokih jamskega stropa. Nekoliko manj je kaotično razmetanega skalovja različnega izvora, dobro pa so opazni večji bloki, ki so pripolzeli s skladnega pobočja. Med njimi so večkrat odprtine, ki vodijio do 1 m globoko pod površje. Denudacija samega jamskega kanala še ni odprla, pač pa je strop stanjšala do takšne mere, da je izgubil samonosilne sposobnosti in se pričel posedati na ilovico v jami. Kjer je že sprana, je tudi dno jarka globlje. Zdi se, kot da tam večje bloke "požira". To je še posebej opazno na severnem koncu rova, kjer se "jarek" razširi in poglobi ter ob enem konča. Domnevamo, da je to mesto freatičnega skoka "navzdol" (če gledamo jamo z današnjimi očmi). Kako je tekla voda po rovu ob njegovem nastanku, ne moremo soditi. Ta odsek je zanimiv zaradi procesov, ki oblikujejo pobočja. Najlažje jih proučujemo na posameznih profilih. Izbrali smo jih tako, da smo predvsem pokrili procese v pobocjih - nekatere podrobnosti v dnu pa le omenjamo. Lahko si zastavimo vprašanje, ali ni ves jarek le posed ob zakraselem prelomu, ki nima z brezstropimi jamami nic opraviti. Kot receno, jama (jarek) ne sledi niti vpadu, niti slemenitvi, temvec poteka naju diagonalno in rahlo cikcaka. Zato se zdi, kot da osnovna smer preseva skozi mozaik blokov, nastalih zaradi krojitve ob slemenitvenih in vpadnih razpokah. Če bi bil to bogaz, bi bil sevda raven. V smeri tock 48, 45, 63 in 64 precka jamo lepo vidna razpoka, ki ima smer, znacilno za (primarno) natezne in (sekundarno - leve) strižne prelome, ki so posledica premikanj ob Idrijski prelomnici in kateri so botrovali npr. Rakovski kukavi (F. Šušteršic, 1998-a). Ob tej razpoki je nastalo celo manjše korozijsko brezence (med tockama 63 in 64), na nastanek jame pa ocitno sploh ni vplivala. Zato lahko zakljucimo, da je bodisi mlajša kot jama, ali pa so take razpoke za nastanek jam nepomebne. OBLIKE POBOČIJ IN PROCESI, KI DELUJEJO NA DENUDIRANI ROV Iz predstavljenih profilov, položenih prek denudiranega/deformiranega rova, ki ga je moc slediti na površju, smo po natancnem pregledu lahko prepoznali vec poglavitnih procesov oz. njihovih produktov, ki oblikujejo pobocja denudiranih jam tega tipa. Torej delujejo na stene jarka, ki nastaja s pogrezanjem razrahljanega jamskega stropa, ali pa samo na jamsko dno. O jamskih stenah v našem primeru ne moremo govoriti. Oblikovanost skladnih pobočij Skladna pobocja so tista, ki vpadajo približno v isti smeri kot plasti maticne kamnine. Opis Proces Profil Navpična stena. Po pogreznitvi jamskega stropa ostane matična kamnina nepoškodovana. C Stopničasto pobočje ob leziki. Razpada več plasti naenkrat, bloki odpadajo v sredino. A Navpična stena, od nje so odluščeni posamezni bloki. Od plasti, ki "visi" proti masnemu deficitu se luščijo posamezni bloki, celota ostaja nepoškodovana. B, E, verjetno C Gladko pobočje ob leziki Zgornja plast je v večini/celoti zdrsela v sredino. D, E Navidezne škraplje ob leziki. Gladko pobočje ob leziki, podlaga enakomerno razmaknjena proti masnemu deficitu. E Oblikovanost neskladnih pobočij Neskladna pobočja so tista, ki vpadajo v nasprotni smeri kot plasti matične kamnine. Opis Proces Profil Zgoraj navpična stena, spodaj previs. Po pogreznitvi jamskega stropa ostane matična kamnina nepoškodovana. A Enotna navpična ali zelo strma stena. Po pogreznitvi jamskega stropa ostane matična kamnina nepoškodovana. C, D Stopničasto pobočje ob leziki. Ni neposredne razlage, vezane na masni deficit. B Navpična stena, od nje odluščeni posamezni bloki. Vpliv masnega deficita pretehta vpliv vpada skladov. E Oblikovanost dna jarka Kot rečeno, se jamski rov v pravem smislu besede sploh se ni pojavil na površju, ampak je se vedno v podzemlju. Denudacija je stanjšala jamski strop in le-ta je postopoma izgubil samonosilne sposobnosti. Ker je bila votlina zapolnjena z ilovico, se je v obliki posameznih blokov počasi posedel v rov. Ilovica se v tem procesu izpodriva in izpira. Depresija, ki se pojavlja na površju, je le preslikava spodaj ležeče jame. Posedanje/izpiranje ni povsod enakomerno, ampak je v nekaterih delih intenzivnejše. Zato se tam v dnu pojavljajo majhne depresije. Očitno je, da podorno kamenje, ki se kruši od stene, izgineva nekam v notranojst in akumulirajo se le manjše količine podornega materiala. Zdi se, kot da potone v ilovico, ali pa se celo v njej raztopi. Ce bi šlo za premeščanje in korozijo, enako splošni stopnji denudacije, bi jarek pač ne bil tako globok, ali vsaj manj izrazit. V južnem delu jame, ki je večji, in je ostal tokrat neobdelan, lahko opazujemo še precej več procesov. Med njimi naj omenimo le najpomembnejše. • Denudirani freatični skoki se na površju pojavljajo kot zaključek ali pa povsem samostojno. V Ravniku so vsi zapolnjeni z ilovico in izgledajo kot mahjne okrogle vrtače z ravnim, ilovnatim dnom, v katerem najdemo prodnike boksita in koščke roženca. Sediment predstavlja ugodno podlago za vegetacijski pokrov, ki povzroča pospešeno raztapljanje apnenca v najbližji okolici. Ponekod denudacija površja znižuje okolico freatičnega skoka, zato sediment, ki se nahaja v njem, polzi tja. Drugje se površju ilovice zbira deževnica, ki ilovico na robovih spira v podzemlje. • Nekateri denudirani jamski kanali potonejo pod površje in se zaključujejo pod obokom. Oboki so lahko zelo izraziti in izgledajo kot spodmoli ali pa so le nakazani. Delovanje eksogenih sil povzroča krušenje kamninskih blokov z oboka v dno brezstropega jamskega kanala. Večinoma pod obokom ne prihaja do kopičenja podornega materiala, ker se v gornjem delu ilovice razkraja. Možno pa je tudi, da bloki kamnine tonejo v ilovico in jo izpodrivajo na površje. ZAKLJUČKI Proučili smo le kratek odsek večjega, a v primerjavi s celoto še vedno zelo majhnega dela denudiranega freatičnega svežnja. Kjub temu pa lahko povemo nekaj zaključkov: • Jama pri Bunkerju je bolj "vodoravna" kot večina doslej znanih brezstropih jam v Ravniku. • Severni odsek jame ima obliko nekakšnega jarka, ki je nastal kot posledica posedanja razkrojenega stropa na ilovnato podlago, brez pomembnejšega podiranja. • Kljub temu, da je proučevani odsek kratek, smo lahko ugotovili učinke različnih preoblikovalnih procesov, enako pogojenih z geološko predispozicijo kot popolnoma endodinamičnimi procesi. • Nujno je določiti zanke za posamezne procese in njihove produkte, sicer ne bo mogoče risati načrtov.