ACTA CARSOLOGICA	29/2	15	201-212	LJUBLJANA 2000
COBISS: 1.08
VELIKA JERSANOVA DOLINE - A FORMER COLLAPSE
DOLINE
VELIKA JERŠANOVA DOLINA - NEKDANJA UDORNICA STANKA [EBELA 1& JOŽE ČAR2
1	Karst Research Institute ZRC SAZU, Titov trg 2, SI-6230 POSTOJNA, SLOVENIJA
2	Univerza v Ljubljani, Geologija, Aškerčeva 12, SI-1000 LJUBLJANA, SLOVENIA
Izvleček	UDK: 551.44(497.4)
551.435.83(497.4)
Stanka Sebela & Jože Car: Velika Jeršanova dolina - nekdanja udornica
Velika Jeršanova dolina (n.m.v.=535 m) leži na površju nad sistemom Postojnskih jam. Njeno poglabljanje je nedvomno prekinilo nadaljevanje Pisanega rova (n.m.v.=535,5 m) proti S. Cez Veliko Jeršanovo dolino poteka teme Postojnske antiklinale v smeri SZ-JV. Prav tako smer ima tudi Jeršanov prelom. Vpad in smer tankoplastnatih turonijskih in cenomanijskih apnencev sta v temenu antiklinale zelo neurejena. Apnenci vpa-dajo pod kotom 5-25°. Velika Jeršanova dolina danes nima značilne oblike udornice. Kot najpomembnejši vzrok netipične udorne oblike Velike Jeršanove doline zagovarjamo njen nastanek v temenskem delu Postojnske antiklinale, oblikovanje v tankoplastnatih bolj glinastih apnencih, ki so ob Jeršanovem prelomu precej pretrti ter intenzivno korozijsko zniževanje terena. Glede na današnjo oblikovanost pobočij in zunanjih robov je Velika Jeršanova dolina relikt nekdanje izrazite udornice. Ključne besede: Velika Jeršanova dolina, udornica, sistem Postojnskih jam, Slovenija.
Abstract	UDC: 551.44(497.4)
551.435.83(497.4)
Stanka Sebela & Jože Car: Velika Jeršanova doline - a former collapse doline
The Velika Jeršanova doline (a.s.l.=535 m) is situated on the surface above the Postojnska Jama cave system. Its deepening undoubtedly interrupted the continuation of Pisani rov (a.s.l.=535,5 m) towards N. Through the Velika Jeršanova doline the Postojna anticline crest runs in the direction of NW-SE. The same direction has also the Jeršan fault. Strike and dip of thin bedded Turonian and Cenomanian limestones are disordered on the anticline's crest. The limestones dip 5-25°. The Velika Jeršanova doline today does not have the typical shape of a collapse doline. The main cause for the untypical collapse shape of Velika Jeršanova doline is its formation in the Postojna anticline crest, its shaping in thin bedded clay - rich limestones and intensive erosional lowering of the area. Regarding the actual shape of the slopes and outer edges, the Velika Jeršanova doline is a relic of a former well shaped collapse doline.
Key words: Velika Jeršanova doline, collapse doline, Postojnska jama cave system, Slovenia.
INTRODUCTION
Velika Jeršanova doline (a.s.l.=535 m) is one of the big dolines situated on the surface above the Postojnska jama cave system. Its deepening disconnected the continuation of Pisani rov (a.s.l.=535,5 m) towards North (Fig. 1). The Velika Jer{anova doline doesn't have a typical shape of collapse doline. With tectonic and lithological mapping we wanted to prove that it's a collapse doline due to the formation.
ABOUT COLLAPSE DOLINES
Collapse dolines are among the most important surface karstological features. We have many definitions of their origin and development. The most widespread definition of collapse doline is based on H.E. Cramer's (1944) work. So the collapse doline should be a direct result of a cave roof
Fig. 1: Tectonic map of the area of Velika Jeršanova doline. Sl. 1: Tektonska karta področja Velike Jeršanove doline.
collapse, appearing on the karst surface. An additional condition is very usual, viz. »the diameter exceeds the depth« (Gams et al., 1973). Habi~ (1963) stated that collapse dolines enlarge with underground water flow taking the collapse material away. Collapse dolines result simple fallingin of cave roofs. Calculations show that the primary cave chambers reduce, when moving towards the earth surface, owing to the loo-sening of the collapsed mass ([u-{ter{i~, 1968). According to Lowe & Waltham (1995) the collap-se sinkholes represent a variety of closed depressions that form by collapse of the rock above an existing cave passage or cham-ber.
Michler (1954) stated that collapse dolines can help us to determine the underground water channels. With detailed tectonic-lithological mapping of the area around Planina polje Car (1982) included also collapse dolines.
According to Car's (1982) geological mapping the smaller collapse dolines are developed inside stronger fault zones. Bigger collapse dolines can be expected in fissured zones and the biggest ones in tectonically less destro-yed rocks where we have weaker fissured zones. The crossing of two fissured zones is especially favorable for development of col-lapse doline. Generally we can expect many smaller collapse dolines in an area with similar fis-sured zones. Less expressed and smaller collapse dolines can be
LEGENDE (to Fig. 2):
1.	light gray micritic limestone with remains of shells and pelagic microfossils
2.	thin bedded dolomized limestone
3.	collapse blocks
4.	crest of Postojna anticline
5.	cross plane of Postojna anticline plane axis
6.	stronger fault
7.	less expressed fault
8.	crushed zone
9.	broken zone
10.	fissured zone
11.	dip direction of bedding planes
12.	dip direction of fault plane
13.	groundplan (outer edge) of collapse doline
14.	groundplan and cross section of doline
15.	cave passage in groundplan and cross section
16.	polygon point in Pisani rov of Postojnska jama cave system
17.	the direction of the profile.
LEDENDA (k Sl. 2):
1.	svetlo siv mikritni apnenec z ostanki skoljk in pelagi~nih mikrofosilov
2.	tanko plastnati dolomitizirani apnenec
3.	podorni bloki
4.	teme Postojnske antiklinale
5.	prese~nica osne ravnine Postojnske antiklinale
6.	mo~nejsi prelom
7.	šibkejši prelom
8.	zdrobljena cona
9.	porušena cona
10.	razpoklinska cona
11.	upad plasti
12.upad	prelomne ploskve
13.	tloris (zunanji rob) udornice
14.	tloris in profil vrtače
15.	jamski rov v tlorisu in profilu
16.	poligonska točka v Pisanem rov Postojnskega jamskega sistema
17.	potek profila.
found in the areas where the rock is more crushed (in areas of fault zones). Bigger but few collapse dolines are situated in areas of less tectonically broken blocks (Car, 1982).
In Slovene karst a lot of collapse dolines are known. In 1996 (Sebela) collapse dolines above Postojnska Jama cave system were studied. Some as Stara apnenica and Kafrna dolina have characteristic funnel shapes, but others as Velika Jersanova doline (the Big Jersan doline) do not have
steep slopes. That was the main reason that we decided to determine if Velika Jeršanova doline is really a collapse doline or not.
VELIKA JERŠANOVA DOLINE AND ITS SURROUNDINGS
The upper edge of Velika Jeršanova doline measures 325x300 m. The highest point of the doline is 575 m a.s.l. which is 40 m higher than its bottom (535 m). Geodetic measurements of the bottom of Velika Jeršanova doline in regarding to the position of Pisani rov were done in 1969 (Gospodarič) and 1993 (Sebela, 1994 a). With a laser theodolite we determined the bottom of the doline to be 535 m a.s.l., while on topographic map 1:5000 it is at 539,9 m (Sebela, 1994 a).
Pleničar (1960, 1970) studied the general geology of the area. Buser et al. (1967) are the aut-hors of the geological map, Sheet Postojna 1:100.000. Gospodarič (1976) studied geological structure of Postojnska jama cave system as well as the surface above the cave. Velika Jeršanova doline was included in detailed geological mapping (1:5.000) done in 1994 (Sebela b). Geological mapping of the area above the Postojnska jama cave system in the scale 1:10.000 was done by Rižnar (1997).
GEOLOGY OF VELIKA JERŠANOVA DOLINE
LITHOSTRATIGRAPHY
Regarding the geological mapping at the scale 1:5.000 (Sebela, 1994 b), the northern part of Velika Jeršanova doline is built of thin bedded Turonian limestone which is in accordance with studies of Gospodarič (1976). According to Rižnar (1997) the southern part of Velika Jeršanova doline is built of light brown micritic limestone with shells and pelagic microfossils (33K12). The northern part of Velika Jeršanova doline is composed of light brown dolomitic limestone and rudist lumakelle (23K12). Rižnar (1997) established that Velika Jeršanova doline is formed in rocks of Upper Cenomanian.
With our lithological mapping in the year 2000 we determined that Velika Jeršanova doline (Fig. 1) is built of 3 principal lithological units (Fig. 2). The upper part of the lithological column represents thin bedded laminated limestone (d = 1-8 cm) rich in clays. The middle part of the column is gray bedded limestone (d = 1-3 dm) with foraminifera and intraclasts (it is organogenic, with fossils, particles of foraminiferas, with intraclasta and pellets, biointra micrite or sparite). The lower part of the column represents limestone with cherts which was already mentioned by Gospo-darič (1976) in the cave. On the surface it was determined by remains of cherts in the soil.
STRUCTURAL POSITION OF VELIKA JERŠANOVA DOLINE
Velika Jeršanova doline is situated inside Postojna anticline (Fig. 1). Strike and dip of thin bedded limestones in the area of the anticline axis are disordered. The dip angle of bedding planes in the northern crest of anticline is lower than the one in the southern crest. Inside the principal anticline axis there are probably more smaller anticlines. The formation of the Postojna anticline is connected with overthrusting of Hrušica tectonic unit over Javorniki-Snežnik thrust unit probably in the upper Eocene (Placer & Car, 1997) in the direction from NE towards SW. In Miocene and Pliocene thrusting was accompanied by folding (Placer, 1981). The plane axis of Postojna anticline
is linked for 7-14° towards SW (Fig. 2).
In the beginning the fold axis planes were more or less horizontal. Actual sinking is the result of lifting of SE part of Slovenia which is the result of the raising of the Dinarids and erosion at the same time (Placer, 1981).
Rižnar (1997), Gospodaric (1976) and Sebela (1998) determined that the axis of the Postojna anticline runs on the northern edge of Velika Jeršanova doline. The fact is that in the area of Velika Jeršanova doline the anticline crest is not so well expressed as in the area of Magdalena gora; the anticline crest is more gentle there, because the anticline axis is not very deep.
For the formation of collapse dolines beside the anticline and lithology the fissured zones are also important. Velika Jeršanova doline is situated on the intersections of Dinaric oriented (NW-SE) faults with the tectonic zones in the N-S and NE-SW directions (Fig. 1).
The principal tectonic zone which crosses Velika Jeršanova doline has Dinaric orientation (NW-SE) and is called the Jeršan Fault. It is a 50-75 m wide fault zone which can be observed for a distance of more than 1 km. In the area of Velika Jeršanova doline the Jeršan Fault cuts all other tectonic deformations such as faults and fissures in the general N-S direction. The Jeršan Fault and Postojna anticline axis are parallel in the middle area of Velika Jeršanova doline, but in a regional sense the fault cuts the anticline axis (Fig. 1).
The research area is part of Outhern Dinarids. Faulting deformations originated from the Neogene and Quaternary. Principal tectonic structures are Dinaric oriented (NW-SE). Faults of first generation (NE-SW) were active in all tectonic events, even till the Holocene. The reactivation of faults of the first generation can be shown in the position of collapse dolines as Vodni dol, the blind valley of Risovec and water passages of Crna jama and Pivka jama (Car & Gospodaric, 1984).
MORPHOLOGY OF VELIKA JERŠANOVA DOLINE
In Italian cave maps from years 1933/34 (Cave Register ZRC SAZU) the altitude of 529,5 m which represents the tourist entrance to the Postojnska jama cave system is taken for the base level of all cross and longitudinal sections. That was the reason why for our longitudinal sections (Figs. 2 and 3) the altitude 529,5 m was taken. One of the cross sections is 1.275 m long and shows relation between the surface, Velika Jeršanova doline, Pisani rov and Jama na poti (Fig. 2). The bottom of Jama na poti is at 537,5 m and the bottom of Pisani rov is at 535 m (geodetic point 25). Between the end of Pisani rov passage and Jama na poti the distance is 475 m.
In the E and SE part the slopes of the collapse doline are covered with slope blocks (Figs. 1, 2 and 3). The bottom is covered with red soils. In the E and SE part of the doline some steeper slopes are still preserved representing limestone walls not higher than 3 m whose origin is linked to a parallel fissured zone. The northern and western slopes have gentle morphological shapes with no limestone walls.
The origin and development of Velika Jeršanova doline is directly linked connection with the formation of Velika gora collapse chamber and collapse chambers in Carobni rov of Postojnska Jama cave system.
A non-active stage is shown by a thick red soil which covers the bottom of the doline. With manual drilling of the bottom of Velika Jeršanova doline on 18th May 2000 we (Leon Drame, Jure Hajna and Stanka Sebela) found that after 2,5 m we reached cave sediments. The first change in
Nemčji vrh
Velika Jeršanova dolina
Velika Jeršanova doline
m
8a- "^v,
i . i , i
~i~rr
TT
10 V, 11	12	13 O
150/90
15
16 IX» 17 B-
Fig. 3: Cross section of Velika Jeršanova doline with cross section of Pisani rov. Sl. 3: Profil Velike Jeršanove doline s prečnim profilom Pisanega rova.
color was at 2,35 m where the color changes from dark brown and brown to dark yellowish brown 10YR4/6 (after Munsell Soil Color Charts, 1975). On 2,5 m the color was between dark brown and brown 7,5YR4/4. That is the layer of sediments which can be followed to a depth of 5 m (7,5 YR 5/6 - strong brown) and more. It is a cross bedded layer of cave sediments which probably plug Pisani rov.
DISCUSSION
The actual morphological shape of Velika Jeršanova doline is not typical of collapse dolines. Velika Jeršanova doline is a very big karst object. It is genetically linked to different lithological and tectonic-structural elements. The thin bedded limestones rich in clays are one of the causes for less expressed morphological shapes and more gentle slopes of collapse doline. One of the ideas is
also that Velika Jeršanova doline can be a relic of a former cave passage which is today a roofless cave or collapse doline with gentle slopes. The roofless cave was once connected with Pisani rov, Zguba jama and Jama na poti cave.
Regarding the actual morphological shape of Velika Jeršanova doline we can conclude that it is not a typical collapse doline, nor a collapse doline with gentle slopes, nor a regular doline.
The difference between a collapse doline and collapse doline with gentle slopes (Car, 1982) is in the degree of the tectonically crushed rocks. The mechanism of the formation is for both examples the same - washing out and carrying away of the material. That is what causes the different shape of the slopes. Velika Jeršanova dolina is an old collapse doline, which today is not active any more. After its formation the landscape was erosionally lowered and today represents a relic of the former well shaped collapse doline.
CONCLUSIONS
Velika Jeršanova doline is one of the less expressed collapse dolines above the Postojnska jama cave system.
The end part of Pisani rov is closed with collapses and cave sediments. The lowest thickness of the cave ceiling is 30 m. Biologists found species from the surface in the cave. The connection must be through the fissures and collapse blocks.
At the northern end of Pisani rov the passage is filled with cave sediments which according the textural characteristics relate to loams from the rest of Postojnska jama cave system and Planinska jama. The age of such loam is younger than 730.000 years (Sebela & Sasowsky, 1999). In the loam we find collapse blocks which must be younger than the deposition of the loam. In this period also the final lowering of Velika Jeršanova doline to the level of Pisani rov and Jama na poti can be deduced. The lowering caused the disruption of the former connection between Pisani rov and Jama na poti.
The actual shape of Velika Jeršanova doline represents a relic of a former well expressed col-lapse doline. Processes of corrosion and erosion lowered the surface morphology. Steep slopes of the former collapse doline were eroded and the erosion even reached the underground passages of Pisani rov filled with cave sediments. That is what we find today at the depth of 2,5 m in the bottom of Velika Jeršanova doline.
The most intense formation of the collapse doline is due to lowering of the water table.
LITERATURE
Buser, S, Grad, K. & Plenicar, M. 1967: Osnovna geološka karta SFRJ, list Postojna 1:100.000.-
Zvezni geološki zavod, Beograd. Cave Register IZRK ZRC SAZU, Postojna. Cramer, H. E., 1944: Die Systematik der Karstdolinen.- Neues Jahrbuch für Mineralogie, Geologie, und Paläontologie, Beilage Band, Abt. B, 85, 293-382. Car, J., 1982: Geološka zgradba požiralnega obrobja Planinskega polja.- Acta carsologica 10 (1981), 75-105, Ljubljana.
Car, J. & Gospodaric, R., 1984: O geologiji krasa med Postojno, Planino in Cerknico.- Acta carsologica 12 (1983), 91-106, Ljubljana.
Gams I. et al., 1973: Slovenska kraška terminologija.- Zveza geografskih institucij Jugoslavije, 1-77, Ljubljana.
Gospodaric, R., 1969: Speleološki procesi v Postojnski jami iz mlajšega pleistocena.- Naše jame 10 (1968), 37-46, Ljubljana.
Gospodaric, R., 1976: Razvoj jam med Pivško kotlino in Planinskim poljem v kvartarju.- Acta carsologica SAZU 7, 8-135, Ljubljana.
Habic, P., 1963: Udorne vrtace - koliševke in podzemski tokovi.- Treci jugoslavenski speleološki kongres, 125-130, Sarajevo.
Lowe D.J. & Waltham, T., 1995: A Dictionary of Karst and Caves.- Cave Studies Series Number 6, Editor Bryan Ellis, British Cave Research Association, 40 p., London.
Michler, I., 1954: Vrtace in doline.-Proteus 16, 8, 204-209, Ljubljana.
Munsell Soil Color Charts, 1975: Macbeth Division of Kollmorgen Corporation, Baltimore, Maryland.
Placer, L., 1981: Geološka zgradba jugozahodne Slovenije.- Geologija 24/1, 27-60, Ljubljana.
Placer, L. & Car, J., 1997: Structure of Mt. Blegoš between the Inner and Outher Dinarides.- Geologija 40, 305-323, Ljubljana.
Plenicar, M., 1960: Stratigrafski razvoj krednih plasti na južnem Primorskem in Notranjskem.-Geologija 6, 22-145, Ljubljana.
Plenicar, M., 1970: Osnovna geološka karta SFRJ 1:100.000. Tolmac za list Postojna.- Zvezni geološki zavod, 1-62, Beograd.
Rižnar, I., 1997: Geologija okolice Postojne.- 78 str., + 3 priloge (Magistrsko delo. NTF, Univerza v Ljubljani, Oddelek za geologijo, Ljubljana).
Sebela, S., 1994 a: Jama na poti in Zguba jama.- Acta Carsologica SAZU XXIII (1994), 233-243, Ljubljana.
Sebela, S., 1994 b: Vloga tektonskih struktur pri nastajanju jamskih rovov in kraških površinskih oblik.- 1-129 str.+19 prilog (Doktorska disertacija. Univerza v Ljubljani, FNT, Ljubljana).
Sebela, S., 1996: The dolines above the collapse chambers of Postojnska jama.- Acta Carsologica 25, 2nd and 3ld International Karstological School, Postojna 1994 and 1995, 241-250, Ljubljana.
Sebela, S., 1998: Tektonska zgradba sistema Postojnskih jam.- Založba ZRC 18, 112pp., Ljubljana.
Sebela, S. & Sasowsky, I.D., 1999: Age and magnetism of cave sediments from Postojnska jama cave system and Planinska jama cave, Slovenia.- Acta Carsologica 28/2, 293-305, Ljubljana.
Sušteršic, F., 1968: Nekaj misli o nastanku kraških udornih dolin.- Naše jame 9, 58-65, Ljubljana.
VELIKA JERŠANOVA DOLINA - NEKDANJA UDORNICA
Povzetek
Lega in Morfologija
Zgornji rob udornice zajema površino velikosti 325x300 m. Njen rob je v najvišji nadmorski
višini na 575 m, kar je 40 m višje kot je njeno dno. V JZ delu je pobočje prekrito s pobocnim
grušcem. Dno udornice je na debelo prekrito z rdeco glino. Oblikovanje Velike Jeršanove doline je
v neposredni povezavi tudi z nastankom podorne dvorane Velike gore in podornim preoblikova-
njem Čarobnega vrta.
Geodetska merjenja dna Velike Jer{anove doline in položaja Pisanega rova glede na udornico so bila opravljena v letih 1969 (Gospodari~) in 1993 ([ebela, 1994). Z laserskim teodolitom je izmerjeno dno na nadmorski vi{ini 535 m, medtem ko je na topografski karti v merilu 1:5000 dno na koti 539,9 m ([ebela, 1994).
Že v italijanskih na~rtih iz let 1933/34 (Kataster jam IZRK ZRC SAZU) sistema Postojnskih jam je za osnovni nivo vseh vzdolžnih in pre~nih profilov vzeta nadmorska vi{ina na umetni plo{-~adi pred vhodom v Postojnsko jamo (529,5 m). Zato smo to nadmorsko vi{ino vzeli tudi za osnovo na{ih dveh profilov, s katerima smo zajeli Pisani rov, udornico Veliko Jer{anovo dolino in Jamo na poti (Sl. 2 in 3). Nivo podorne dvorane Jame na poti (537,5 m) in Pisanega rova (to~ka 25 = 535 m) je skoraj povsem enak, med rovoma je 475 m horizontalne razdalje.
Litostratigrafski podatki
Po Rižnarju (1997) je južni del Velike Jer{anove doline zgrajen iz svetlorjavega mikritnega apnenca s tankolupinastimi {koljkami in pelagi~nimi mikrofosili (33K12). Severni del Velike Jerša-nove doline je zgrajen iz svetlorjavega dolomitiziranega apnenca v menjavanju z rudisti in rudistno lumakelo (23K12). Po Rižnarju (1997) je Velika Jer{anova dolina nastala v kamninah Zgornjega Cenomanija.
Velika Jer{anova dolina je razvita v 3-eh litolo{ko razli~nih ~lenih (Sl. 2). V zgornjem delu litolo{kega stolpca je tanek (d = 1-8 cm), laminiran apnenec. Je bolj glinast oziroma laporast. Navzdol sledi siv apnenec, plastnat (od 1 dm do 3 dm) s foraminiferami in intraklasti (organogen, s fosili, drobir s foraminiferami, z intraklasti ali peleti, biointra mikrit ali sparit). V spodnjem delu je apnenec z roženci, ki smo ga dolo~ili glede na razmere v jami. Na površju ga omejujemo s ko{~ki roženca v preperini.
Strukturni podatki
Na obravnavanem terenu so plasti povite v Postojnsko antiklinalo (Gospodari~, 1976). Rižnar (1997), Gospodari~ (1976) in [ebela (1998) so teme Postojnske antiklinale dolo~ili ob severnem robu Velike Jeršanove doline. Z geološkim kartiranjem v letu 2000 (Slika 1) smo os antiklinale dolo~ili skozi Veliko Jeršanovo dolino. Vpad in smer tankoplastnatih turonijskih in cenomanijskih apnencev sta v temenu antiklinale zelo neurejena. Apnenci vpadajo pod kotom 5-25°. Vpad plasti je na SV krilu Postojnske antiklinale manjši kot na JZ krilu. V temenu antiklinale gre za ve~ manjših gub, ki jih skupno združujemo v enotno antiklinalo. Nastanek in oblikovanje antiklinale moramo povezati z obdobjem narivanja od SV proti JZ, ko se je tektonska enota Hrušice narinila na Javorniško-Snežniško narivno grudo najverjetneje v zgornjem eocenu (Placer & Car, 1997). V miocenu in pliocenu je narivanje spremljalo gubanje. Osna ravnina Postojnske antiklinale je nagnjena proti JZ za 7-14°. Prvotno so bile osi gub bolj ali manj horizontalne. Današnje tonjenje je posledica regionalnega dviganja JV dela Slovenije, ki je odraz dviganja Dinaridov in hkratne erozije (Placer, 1981).
Raziskovano ozemlje prištevamo Zunanjim Dinaridom. Prelomne deformacije uvrš~amo v neo-gen in kvartar. Glavne tektonske strukture so dinarsko usmerjene (SZ-JV). Prelomi prve generacije (SV-JZ) so bili aktivni pri vseh tektonskih premikanjih, lahko celo prav do holocena. Vpliv regene-riranih prelomov prve generacije se kaže v legi udornic okrog Vodnega dola in doline Risovec ter v poteku vodnih rovov Črne in Pivke jame (Car & Gospodari~, 1984). Mlajša zmi~na tektonika ima
značilno dinarsko usmerjenost (SZ-JV), kar je splošna značilnost Zunanjih Dinaridov v Sloveniji. Cez Veliko Jeršanovo dolino, v enaki smeri kot teme Postojnske antiklinale, poteka Jeršanov prelom v smeri SZ-JV (Slika 1). V njegovi prelomni coni opazujemo zelo goste razpoklinske in porušene cone. Nemčji vrh JZ od tod sekata še dve enako usmerjeni, vendar manj izraziti, prelomni coni. Dinarske zmike povezujejo šibkejše vezne prelomne cone.
Diskusija in zaključki
Današnje stanje - predvsem morfološke posebnosti - Velike Jeršanove doline neposredno ne odgovarja, niti značilni vrtači, kot tudi ne udorni vrtači (Car 1982) oziroma klasični udornici.
Velika Jeršanova dolina je izredno velik, značilen kraški objekt. Genetsko je dolina vezana na različne litološke in tektonsko-strukturne elemente. Zaradi tega ima zelo nepravilno obliko zunanjega roba. Gre za položna in morfološko zapletena pobočja, kjer opazujemo grbine, zajede in manjše vrtače. JV in V obrobje doline je nasuto z velikimi podornimi bloki. Dno velike Jeršanove doline je obsežno in nepravilno ter prekrito s sedimentom (preperino). Naštetemu je potrebno dodati še povezavo doline s Pisanim rovom sistema Postojnskih jam.
Iz opisa je razvidno, da Velika Jeršanova dolina ni vrtača ali morda "velika" vrtača. Na morebitno udorno vrtačo bi sicer kazalo več značilnosti kot tlorisna velikost, položna pobočja, itd., vendar za udorne vrtače ni značilna navezanost na različne geološke genetske elemente.
Ostaja še, da pretehtamo ali je Velika Jeršanova dolina morda "klasična" udornica. Glede na klasične udornice Veliki Jeršanovi dolini manjka predvsem globina, zelo strma ali celo vertikalna pobočja ter pokritost dna z velikimi podornimi bloki. Vse ostale zgoraj omenjene značilnosti pa lahko brez težav pripišemo udornicam. Iz tega izhaja ugotovitev, da Velika Jeršanova dolina danes nima značilnega izgleda udornice.
Vzroki za današnji netipični izgled Velike Jeršanove doline je v njenem nastanku v temenskem delu Postojnske antiklinale, oblikovanje v tankoplastnatih laporastih apnencih, ki so ob Jeršano-vem prelomu precej pretrti ter zaradi intenzivnega korozijskega znižanja terena. Glede na današnje morfološko stanje menimo, da je Velika Jeršanova dolina ostanek nekdanje udornice.
Posebej zanimiva je povezava Velike Jeršanove doline s Pisanim rovom sistema Postojnskih jam. To dodatno dokazuje podorni značaj Velike Jeršanove doline. Današji zaključni del Pisanega rova je podorno preoblikovan. Najnižja debelina stropa na koncu Pisanega rova je 30 m. Biologi so v tem delu jame večkrat našli živali, ki so v jamo prišle s površja. Glede na razdaljo 30 m to lahko razložimo le z odprtimi razpokami, saj je sicer površje precej daleč, da bi lahko sklepali na podor med površjem in jamo.
Na skrajnem severu je rov zatrpan z jamsko ilovico, ki po teksturi in barvi močno spominja na jamske zapolnitve iz ostalih delov sistema Postojnskih jam in Planinske jame. Starost teh rumeno rjavih ilovic je bila s paleomagnetnimi analizami določena za mlajšo kot 730.000 let. Ker ležijo podorni bloki na tej ilovici je podor prav tako mlajši kot 730.000 let. V to obdobje lahko uvrstimo tudi poglobitev Velike Jeršanove doline do nivoja Pisanega rova in Jame na poti, kar je prekinilo njuno domnevno povezavo.
Najbolj intenzivno oblikovanje udornic se pojavi ob znižanju gladine podtalne vode.