English description below!

Barjanska okna laično poimenujemo vse izvire in manjše vodne površine na območju Barja in njegovem obrobju. Ta geološki zaklad pa vas bo podrobneje seznanil z vršajnimi izviri in okni.

BARJANSKA OKNA

NEKDAJ…

Valvazor je že v 17. stoletju barjanska okna zelo realistično opisal: » … je med Ljubljano, Igom in Vrhniko še dosti takih nadvse globokih mlak ali lukenj, ki so polne vode. Običajno jim pravijo okna. Tem nobeden ne more izmeriti globin, to pa kaže na to, da so te majhne mlake ali luknje spodaj prepustne in segajo do žrela globoko v zemlji skritega kanala ali bolje, da od tega segajo navzgor. …« Janez Vajkard Valvazor, 1689, Slava Vojvodine Kranjske.

V času pred večjimi izsuševalnimi deli je bilo Ljubljansko Barje povsem drugačno kot danes. Prekrivalo ga je visoko barje, ki je bilo zelo težko prehodno, saj na tem območju ni bilo utrjenih poti. V tako pokrajino so si upali le ribiči in lovci, pa še ti so morali biti zelo previdni, da niso padli v barjanska okna, kar je v podrobni naravoslovni študiji Ljubljanskega barja leta 1886 slikovito opisal Ivan Šubic: »Pri lovu pa je bilo treba veliko previdnosti in spretnosti. Časih se je noga pogreznila in možje so morali drug druzega vleči iz grezi in blata. Pri vsakem koraku se je površje treslo. Posebno nevarna pa so bila barjanska okna ali vretjà (Seefenster). Tako so nazivljali vodoshrambe, ki so bile časih odprte, časih pa goljufivo pokrite s tenko skorjo mahu in šašev. Gorjé samotnemu človeku, ki je zašel tjà! Zginil je bil brez sledu v oknu…« (I.Šubic, 1886, Jahresbericht des k.k. Obergymnasiums zu Laibach).

… IN DANES

Ljubljansko barje je v zadnjih tristo letih močno spremenilo svojo podobo. Na mestu nekdanjega težko prehodnega visokega barja so danes mokrotni travniki, prepredeni z izsuševalnimi jarki. Pokrajina na pragu glavnega mesta je znana po izredni biotski pestrosti in vsakoletnih poplavah. Njeno pomembno naravno dediščino predstavljajo morfološki in geološki pojavi, med katere spadajo izviri, imenovani barjanska okna ali retja.

Hidrogeologi barjanska okna delijo glede na območje nastanka:

• kraške izvire na kraškem robu Ljubljanskega barja,
• prava barjanska okna na visokem barju,
• vršajna okna in izvire na robu vršajev - stiku nanosov prodonosnih rek in barja.

Kraških izvirov ta EarthCache ne obravnava. Več o njih izveste v našem drugem EC »River with seven names - LJUBLJANICA – Retovje«.

PRAVA BARJANSKA OKNA

so odprte vodne površine, ki se nahajajo na visokem barju. Tako kot pravega visokega barja danes na Ljubljanskem Barju ni več, tudi pravih barjanskih oken na tem območju ne bomo več našli. Izginila so skupaj z visoko vodo, ki je odtekla po prvih večjih izsuševalnih delih. Mesta njihovega pojavljanja lahko razberemo iz Jožefinskih vojaških topografskih kart prve izmere za Srednjo Avstrijo, ki so bile na Barju opravljene v letih 1784 in 1785. V razlagi, ki je nastala istočasno s kartami, je za območje Notranjih Goric zapisano: »Odprte vode, ki so imenovane jezerska okna, so skoraj nedostopne.«

VRŠAJNI IZVIRI IN OKNA

nastanejo na robu vršajev (nanosov) prodonosnih rek - torej na stiku dobro vodoprepustnega prodnatega vršaja reke in slabo vodoprepustnih barjanskih usedlin (sedimentov).

V 19. stoletju so bila vršajna okna in izviri na Barju številnejši. Osuševanje Barja, črpališča pitne vode v Borovniškem in Iškem vršaju, pa tudi nekaj bolj sušnih let je povzročilo znižanje gladine podtalnice, zato so številni izviri in okna izginili.

Danes jih je največ na robu vršaja reke Iške, od Ižanske ceste pri Igu, mimo Iške Loke in Bresta do Podkraja. V popisu leta 2015 so jih hidrogeologi našteli kar 53, večina je povezanih v skupine, imenovane »roji«. Podobne izvire najdemo tudi med Pakim in Goričico pri Borovnici.

Slika 1 / Figure 1

Vršaj reke Iške je bogat s podzemno vodo, ki vanj pronica s ponikanjem reke Iške pri Iški vasi ter z napajanjem iz obsežnih kraških vodonosnikov Krima in Mokrca. Raziskave tal na vršaju Iške so pokazale, da se v globini izmenjujejo plasti proda in jezerskih (neprepustnih) usedlin. Takšna sestava je posledica geološke zgodovine Ljubljanskega Barja. Barje je nastalo v tektonski udorini, ki so jo skladno s spreminjajočo se klimo izmenično zasipale reke s prodnimi nanosi ali pa prekrivalo jezero in odlagalo jezerske usedline. Obdobja zasipanja in ojezeritve so se večkrat ponovila. Zaradi omenjene plastovite sestave imamo danes v vršaju reke Iške enega nad drugim dva vodonosnika. V zgornjem, plitvem vodonosniku je podzemna voda blizu površja, v spodnjem, globokem vodonosniku pa je voda v prodnati plasti ujeta pod slabo prepustno, zaglinjeno plast. Voda je zato v tem vodonosniku pod večjim tlakom kot bi bila na površini. Če v spodnji vodonosnik zavrtamo vrtino, se zato voda v njeh sama dvigne in prelije na površino.

Posledica obstoja dveh vodonosnikov so raznoliki izviri na robu vršaja. Razdelimo jih lahko na dva tipa.

Plitvi vršajni izviri in okna so vezani na stik med slabše prepustnimi in barjanskimi nanosi (glina, melj, droben pesek) in bolje prepustnimi nanosi prodonosnih rek (prod, pesek) na površini, ali blizu površine. Ob tem stiku na površino prihaja voda iz zgornjega vodonosnika. Za te izvire je značilno nihanje gladine vode in temperature. Največja skupina plitvih vršajnih izvirov je pri Iški Loki, in sicer so to Mareke in izviri Lošce.

Plitva vršajna okna od izvirov ločimo po tem, da nimajo naravnega odtoka in so zato običajno večja. Na meji vršaja reke Borovniščice je Anžutovo retje, ki z dolžino 20 m in širino 16 m sodi med največja popisana barjanska okna.

Slika 2 / Figure 2

Globoki vršajni izviri in okna se pojavljajo v rojih, v katerih je večje število med seboj povezanih izvirov, ki se napajajo iz spodnjega, globokega arteškega vodonosnika. Zanje so značilne do en meter globoke kotanje, ki se navzdol pogosto nadaljujejo v kanale, iz katerih izvira voda. Voda s seboj prinaša drobce usedlin z drobnimi lupinicami polžkov in školjk, ki jih spira med svojim prodiranjem skozi glinaste jezerske usedline proti površini.

Na sliki 1 je razvidna zelo jasna razporeditev izvirov ob robu vršaja reke Iške, kjer prodnate plasti dosežejo barjanske, zaglinjene, za vodo slabo prepustne ali neprepustne usedline. Voda iz spodnjega vodonosnika, ki zaradi povišanega tlaka »išče« pot na površje, to najde na »šibkejših« mestih in neenakomernostih v usedlinah. Te so nastale lahko kot posledica prelomov, ki so zrahljali nad vodonosnikom ležečo neprepustno plast, zaradi nezveznosti te plasti ali zaradi različnega odziva prodnatih in glinenih plasti na deformacije.

Voda v teh izviri ima ves čas zelo enakomerno temperaturo, med 10,5 in 11°C. Tudi izdatnost izvirov je v mokrih in sušnih obdobjih skoraj enaka.

STRAHOMERSKA OKNA IN EARTHCACHE TOČKA

Ta geološki zaklad, oziroma EarthCache točka je postavljena k Strahomerskim barjanskim oknom, ki so značilen primer globokih vršajnih oken. Voda v kotanjah se neprestano vrtinči ter s seboj prinaša drobne delce belih lupinic. Občasno se na površini pojavijo mehurčki zraka, kot da voda brbota in vre, od koder iz besede »vretje« izhaja skrajšano poimenovanje retje.

Okna so vključena v učno »Pot barjanska okna«, ki poteka od Podkraja do Iške. Pri Strahomerskih oknih je v okviru učne poti urejena opazovalna ploščad z dvema balkonoma, ki segata skoraj nad vodno gladino okna. S ploščadi lahko barjansko okno in dogajanje v njem varno opazujemo iz neposredne bližine. Na ploščadi sta dve informacijski tabli, ki z risbo in besedo predstavita najpomembnejša dejstva o barjanskih oknih. Otroci lahko s pomočjo vrtljive sestavljanke poskusijo sestaviti pravilen prerez terena na barju in na vršajnih oknih.

DOSTOP

Avto pustite pri gostilne Anzeljc v Podkraju, točka poti P1. Do Strahomerskega okna se odpravite peš po označeni poti, ki vodi sprva po asfaltni cesti, nato po boljšem kolovozu. S kolovoza po nekaj manj kot 600 metrih pripelje do izvira.

Držite se urejenih poti in ne plezajte čez ograjo do izvirov. Na Barju se obnašajte prijazno do okolice in njenih naravnih prebivalcev. Če ste z avtom, ne vozite po kolovozu in ne parkirajte na travnatih površinah! Te so v zasebni lasti in na njih parkiranje ni dovoljeno.

NALOGE

Voda v spodnjem vodonosniku Iškega vršaja je »ujeta« pod za vodo neprepustno ali slabo prepustno plast. Skoznjo se na površino prebije skozi vrzeli / razpoke., nezveznosti.

1. Kako se imenuje plast skozi katero se mora prebiti voda globokih vršajnih oken? Zakaj je tako poimenovana? Odgovor lahko poiščete tudi na informativnih tablah pri Strahomerskih oknih.

Opazujte dno Strahomerskega okna pod opazovalno ploščadjo.

2. Kakšne delce vidite v okolici odprtin iz katerih »vre« voda? Kaj se tam dogaja s temi delci? Ali mirujejo?

Med ogledom okna z razgledne ploščadi poiščite del okna kjer je globina vode največja.

3. Ali je to pred desnim delom ploščadi, pred sredino ali pred levim delom, če ste obrnjeni proti oknu?
4. Ocenite širino globljega dela dna
5. Ali v njej vidite kakšne živali? Če jih, katere so in približno kolikšna je njihova velikost?
6. Zapišite do katere številke na merilni levi sega voda med vašim obiskom.

>>>>>

Odgovore pošljite preko GC profila preden vpišete najdbo. Po pošiljanju odgovorov ne potrebujete dodatnega dovoljenja za vpis. Prosimo, naložite fotografije, če ste jih posneli.

Vpisi brez predhodnega sporočila s pravilnimi odgovori bodo šifrirani in kasneje izbrisani!

Vpisi z odgovori bodo izbrisani!

VIRI:

Šišić, Sonja et. al., 2016, Reka, Lokna, kraški kali in barjanska okna;

Brošura, Pot barjanska okna, Maša Bratina, Mihael Brenčič et. al.;

Vahtar, Marta et. al., 2013, Domžale, Ljubljansko barje – moj navdih VODA, ICRO Domžale;

English description

Marsh springs are a lay term for all the springs and smaller water areas in the territory of the Ljubljana Marsh and its periphery. This EarthCache however, will introduce you in more detail to the alluvial fan sources and springs.

MARSH SPRINGS

IN THE PAST…

Already in the 17th-century Slovenian polymath and Member of the Royal Society of London, Janez Vajkard Valvasor described the marsh springs very realistically:

"... between Ljubljana, Ig and Vrhnika there are still many such extremely deep pools or holes that are full of water. They are usually called ‘windows’. Of these, no one can measure the depths, and this indicates that these little pools or holes are permeable below and reach to the hidden canal deep in the earth, or rather, that they extend upwards from it. ..." Janez Vajkard Vavasor, 1689, The Glory of the Duchy of Carniola.

Prior to major draining works, the Ljubljana Marsh was completely different from today. It was covered in a high marsh, which was very difficult terrain, as there were no well-trodden paths. Only fishermen and hunters dared to venture out into such a landscape, and even they had to be very careful not to fall into the marsh ‘window’, which was vividly described by Ivan Šubic in a detailed nature study on the Ljubljana Marsh in 1886:

"Hunting required a lot of caution and skill. Sometimes a foot sank, and men were forced to drag each other out of the sludge and mud. The ground shook with every step. Marsh windows of bubbling springs (Seefenster) posed the biggest threat. These springs were sometimes open but sometimes fraudulently covered in a thin crust of moss and sedge. Woe betides any lonely man who lost his way there! He vanished without a trace in the window." (I.Šubic, 1886, Jahresbericht des k.k. Obergymnasiums zu Laibach).

… AND TODAY

In the last three hundred years, the Ljubljana marsh has changed significantly. In the place of the former difficult-to-pass high marsh, today there are wet meadows interspersed with draining ditches. The landscape on the doorstep of the capital is known for its extraordinary biotic diversity and annual floods. Its important natural heritage is represented by morphological and geological phenomena, which include marsh springs also called marsh windows or bubbling sources.

Hydrogeologists classify the marsh springs according to the area in which they occur, i.e.:

• real marsh springs in high marshes,
• karst springs on the karst edge of the Ljubljana Marsh,
• alluvial fan springs on the edges of alluvial fans - the contact of sediments from productive rivers and bogs.

Karst springs are not covered by this EarthCache. Find out more about them in another EC "River with seven names - LJUBLJANICA - Retovje".

REAL MARSH SPRINGS

are open-water surfaces located in high marshes. As real high marshes, today no longer exist in the area, real marsh springs can also no longer be found. They vanished together with the high water that was drained after the first major draining works. Their locations can be discerned on the Josephian military topographic maps of the fits survey for Central Austria, which was concluded in the Marsh from 1784 to 1785. The following was recorded in an explanation for the area of Notranje Gorice: “The open waters called lake windows are almost inaccessible.”

ALLUVIAL FAN SOURCES AND SPRINGS

are situated at the edge of the alluvial fans (the gravel deposits of rivers) - i.e., at the contact of the well-permeable gravel fan of the river and low permeable marsh sediments.

In the 19th century, there were more sources and springs on the Marsh. The draining of the Marsh, the tap water pumping stations in Borovniščica and Iška alluvial fans, as well as several drier years, caused the groundwater level to drop, and many sources and springs to disappear.

Today most of them are situated at the edge of the alluvial fan of the Iška river - from Ižanska cesta near Ig, past Iška Loka and Brest to Podkraj. In the 2015 census, hydrogeologists counted as many as 53 of them, most of them interconnected in groups, so-called swarms. Similar springs can also be found between Pako and Goričica near Borovnica.

Figure 1: See the Slovenian description.

The alluvial fan of the Iška river is rich in underground water, which seeps into it by the sinking of the Iška river and by the supply from the vast karst aquifers of Krim and Mokrc. Ground composition research at the alluvial fan of Iška revealed vertical cross-sections with alternating layers of gravel (permeable) and lake sediments (low permeable). This composition is the result of the geological history of the Ljubljana Marsh. The marsh was formed in a tectonic depression which was alternately filled in by rivers’ gravel deposits and then covered by lake sediments – depending on the prevailing climate. The periods of filling up with gravel and lake formation were repeated several times. Due to the above-mentioned layered composition, today we have two aquifers one above the other in the alluvial fan of the Iška river. In the upper, shallow aquifer, the groundwater is close to the surface, while in the lower, deep aquifer, the water is trapped in the gravel layer under a low permeable, silty layer. The water in this aquifer is therefore under higher pressure (called artesian pressure) than it would be on the surface. If we drill a well into the lower aquifer, the water under higher pressure flows to the surface by itself.

The existence of two aquifers results in diverse springs on the edge of the alluvial fan. We can group them into two types.

Shallow alluvial fan sources and springs depend on contact between the low permeable marsh deposits (clay, clayey silt, and fine sand) and the more permeable gravel river deposits (gravel and sand) close to the surface. In these conditions, the water from the upper aquifer reaches the surface. These springs are characterized by fluctuations in water level and temperature. The largest group of shallow alluvial fan springs is located near Iška Loka, namely the Mareke and Lošca springs.

Shallow alluvial fan springs are distinguished from sources by not having a natural run-off and are therefore usually larger than sources. At the boundary of the alluvial fan of the Borovniščica river lies the powerful bubbling spring Anžutovo retje, which with a length of 20 m and a width of 16 m ranks among the largest listed.

Figure 2: See the Slovenian description.

Deep alluvial fan sources and springs form swarms that contain several interconnected springs recharged with water from the underlying, lower section of the aquifer, where the water comes to the surface through channels. These sources and springs are characterized by depressions of up to one meter deep. As the water coming to the surface is under pressure, it washes away different layers of marsh soil on its way from the lower section of the aquifer to the surface - numerous mollusc shells among other fragments.

Figure 1 shows the position of the springs along the edge of the alluvial fan of the Iška River very clearly. Gravelly layers reach marshy, silty, poorly permeable or impermeable sediments there. Water from the lower aquifer, which "seeks" its way to the surface due to increased pressure, finds it in "weaker" places and unevenness in the sediments. These may have been a result of faults that fractured the low permeable layer lying above the aquifer or due to the different responses of gravel and clay layers to deformations.

In contrast with shallow alluvial fan springs, the temperature in the deep springs remain constant throughout the year, at 10.5 to 11 degrees Celsius, and the outflow doesn’t fluctuate much either.

STRAHOMER ALLUVIAL FAN SPRINGS

This EarthCache is located next to the Strahomer alluvial fan springs, which are typical representatives of the deep alluvial fan springs. The water in the spring is constantly swirling, bringing up tiny particles of white shells. Air bubbles appear on the surface from time to time as if the water is bubbling and boiling. The springs are included in the educational Marsh Springs Trail (Pot barjanska okna)".

There is an observation platform at the Strahomer springs, with two balconies that almost reach above the water of the spring. You can safely observe the spring from a close range there. Drawings and explanations on the two information boards present the most important facts about alluvial fan springs. There is also a kind of rotating puzzle, where you can try to assemble the correct vertical cross-section of the marsh and of the alluvial fan.

ACCESS

Park your car at the Anzeljc inn in Podkraj, waypoint P1. Continue to the Strahomer springs on foot - initially on a paved road, then on a carriageway. After a little less than 600 meters of the carriageway, you reach the springs.

Stick to the maintained paths and don’t climb over the fence to the springs. Be kind to the surroundings and natural inhabitants of the Marsh. If you have a car, leave it in the settlement, do not drive on the carriageway and do not park on grassy areas! These are privately owned, and parking is not permitted there.

LOGGING TASKS

The water in the lower aquifer of Iška river alluvial fan is "trapped" under the impermeable or low permeable layer. It penetrates to the surface through this layer through gaps / cracks, discontinuities.

1. What is this layer called? You can find the answer on the information boards at the Strahomer springs as well.

Observe the bottom of the Strahomer Spring below the observation deck.

2. What kind of particles do you see around the openings from which the water "boils"? 3. What is happening there with these particles? Are they idle?

While looking at the spring from the viewing platform, identify the part of the spring where the water depth is greatest.

3. Which part of the platform faces the deepest part of the spring? The right / the middle / the left part?
4. Estimate the width of the deeper part of the spring bottom.
5. Do you see any animals in it? If so, what are they and how big are they?
6. How high is the water level during your visit? Mark down the number on the water level measuring stick reached by the water.

>>>>>

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References:

See the end of the Slovenian description.