Geology of the Spannagel Cave
Detailed background information
Geology
History of Exploration
The cave at the Spannagelhaus and the smaller neighbouring caves are all relatively young subjects of speleological research. It was not until 1960 that Max H. Fink and Heinz Ilming surveyed the first 320 m of passage. In 1964, the cave at the Spannagelhaus was declared a natural monument; in 1994, the entrance passages were opened to the public as a show cave after appropriate development. In 1995, the northward-descending 95er System was discovered. Its prospection is still far from complete.
Location
The caves in the immediate and wider surroundings of the Spannagelhaus, located at 2,529 m above sea level, are reached by travelling to the head of the Tux Valley, the westernmost side valley of the Zillertal, and taking the glacier lift to the Tuxer Fernerhaus. From there, it is a descent of about 10 minutes to the Spannagelhaus. This is where the entrance to the largest cave in the area is located: the “Cave at the Spannagelhaus”, commonly known as the Spannagel Cave (cave register no. 2411/001). Its entrance, which is also the access to the publicly accessible show cave, lies on a broad, grass-covered ridge descending towards the north-northwest.
Geological Setting
Larger caves in the Central Alps are almost always associated with local occurrences of carbonate rocks. In the case of the caves in the upper Tux Valley, these are true calcitic marbles, locally tightly folded into crystalline rocks, mainly gneisses. In the shafts and caves of the Kitzsteinhorn area, which occur at comparable elevations, the relevant rocks are the calcareous mica schists of the Bündner Schists; in the high-alpine karst area along the Grossglockner High Alpine Road east of the Hochtor, they are dolomites and rauhwackes of the so-called Seidlwinkl Triassic.
View of the broad rocky ridge descending from the Spannagelhaus towards the north-northwest, beneath which a large part of the currently known cave system is located. The mountain in the central background is the Gefrorene-Wand-Spitze (3,286 m). To the left and right lie the rapidly retreating ice masses of the Gefrorene-Wand-Kees, the Hintertux Glacier. On the right, the sharp-edged lateral moraine from the glacier highstand around 1850 is clearly visible. The landform to the left of the lift line is the artificially constructed ski slope.
How Marble Forms
The age of marble is not easy to determine, because rock metamorphism has profoundly altered the original material. In the case of the upper Tux Valley, we know that these rocks were only exhumed again in geologically recent times after a phase of deep burial within the Earth’s crust.
The first process, the “downward journey” into the Earth’s interior, took place during Alpine mountain building, when nappes of crustal segments situated farther south were thrust over the present-day Central Alps. The second process, exhumation, occurred as a consequence of west-east extension in the Eastern Alps, during which the Hohe Tauern between the Wipptal and the Katschtal rose and continue to rise.
As part of this “upward lift”, fractures opened in the rock. Hot fluids penetrated them, forming the well-known Alpine fissure minerals of the Hohe Tauern as well as Tauern gold. Weathering and erosion remove almost as much material as has been supplied from below since the beginning of this so-called uplift around 20 million years ago. The former mountains of the Hohe Tauern and Zillertal Alps now lie, finely reworked, as sediments beneath southern Bavaria: the work of the precursors of the Ziller and Inn rivers.
The interaction of slow exhumation and erosion ultimately means that in the western Zillertal Alps one now walks over rocks that, around 20 million years ago, were situated at depths of approximately 25 km within the Earth’s crust. Under the conditions prevailing there, up to about 550 °C and up to 10,000 bar pressure, rocks already behave plastically. Their mineral composition is also transformed through metamorphism. In the case of the Spannagel Cave system, this process turned a marine limestone of Late Jurassic age into marble.
The so-called Central Gneiss, which today overlies the marble and forms, for example, the summit of the Olperer (3,476 m), was also affected by these processes. It has since acquired a weak foliation that it originally lacked when it was still granite.
Determining the Age
One consequence of the metamorphism of carbonate rocks is the extensive destruction of fossil remains. The age of such rocks can therefore usually only be determined by other observations and comparisons. In the case of the marble in the Spannagel area, chance also played an important role.
In 1939, there was a small quarry near the confluence of the Tuxbach and Zemmbach streams at the hamlet of Hochstegen; it can still be seen from the road today. The rock quarried there is the same marble as that found above at the Spannagelhaus. From there it continues towards the northeast and is responsible, for example, for the steep gorge of the Tuxbach west of Finkenberg.
An attentive observer noticed an unusual spiral-shaped imprint in a large block of marble that had already been built into a roadside wall in Zell am Ziller. Specialists from the Institute of Geology and Palaeontology at the University of Innsbruck recognised it as the negative imprint of an ammonite. Although strongly deformed, it could still be clearly identified as such, and the block was recovered. It weighed around 120 kg. The preparation of this find was carried out by Georg Mutschlechner, later long-standing chairman of the Tyrolean Speleological Association.
Later investigations were even able to determine the fossil to genus level. This allowed the rock to be assigned to the uppermost Jurassic, around 150 million years before present. Further studies of samples from Hochstegen and from the clearly visible outcrop shortly before the lower station of the Penkenbahn in Finkenberg provided additional support for this age assignment through microfossils.
A Sea 150 Million Years Ago
These sparse remains of former organisms allow the cave-forming marbles of the Spannagel area, known in the literature as Hochstegen Limestone or Hochstegen Marble, to be dated. Their occurrence documents not only their age but also the environment in which the rock formed. It was deposited on the floor of a shelf sea whose deeper waters can still be shown, despite later metamorphism, to have been hostile to life, meaning poor in oxygen.
Several observations, some of which can also be made inside the cave, support this interpretation. When the dark layers are struck, a smell of hydrogen sulphide is often noticeable. Rock analyses also show the frequent occurrence of tiny pyrite crystals. Pyrite is an iron sulphide that can only form under conditions comparable to those found today in the depths of the Black Sea.
The regular bedding, which locally can be traced down to the millimetre scale, also indicates that the former marine basin was poorly ventilated and therefore had no significant benthic life on the sea floor. In modern open oceans, where deep waters are rich in oxygen, the upper centimetres of the sea floor are intensively reworked by various organisms. Fine lamination therefore cannot be preserved there.
A further indication of a hostile environment in the Hochstegen Sea is the predominantly dark colour of the rock. It derives from former organic matter, which after metamorphism is present as a very fine graphitic component. Even in very dark samples, its content hardly exceeds about 1 percent by weight. The rock therefore consists overwhelmingly of calcite, with some quartz.
Organic matter derived from dead microorganisms that lived in the upper, sunlit tens of metres of seawater is normally not preserved on today’s sea floor. Its preservation requires an almost oxygen-free environment, which evidently existed during the uppermost Jurassic.
In the Spannagel Cave, lighter layers can repeatedly be seen intercalated within the predominantly grey calcitic marbles. Their striking light colour probably has the following origin: along the margins of this restricted marine basin, shallow-water areas with light-coloured carbonates were able to form. From these elevated zones, submarine avalanches slid into the deeper parts of the basin and transported light material into the poorly ventilated environment. These light layers are therefore very probably allochthonous material that was introduced into the basin during short-lived events and then covered by the next layer of organic-rich sediment.
Another important feature of the Hochstegen Marble is the occurrence of chert. Visitors to the cave often notice the dark inclusions projecting from the walls; they consist of a material more resistant than calcite. They are composed of quartz and are broadly comparable to the dense siliceous rocks that were extracted from various carbonate rocks in the Stone Age and worked into sharp stone tools.
“Normal” cherts are usually rounded in shape. In the Spannagel area, however, they may be metres long, sword-like, spindle-shaped, or occur as plates only a few millimetres thick. Compass measurements show that these bodies are essentially oriented west-east and dip towards the west.
This strict arrangement already points to their formation and subsequent deformation. Through the crystallisation of silica, itself derived from siliceous microorganisms, “normal” cherts, also known as siliceous concretions, formed shortly after deposition of the Hochstegen Limestone. During the intense heating and extension of the area, their microscopic fabric coarsened, and the concretions were strongly stretched in accordance with the general west-east extension of the Tauern Window.
These structures therefore make the mountain-building deformation particularly tangible: they record processes that were active here between about 20 and 15 million years ago and contributed to the exhumation of the Alpine main ridge between the Wipptal and the Katschtal.
Typical view of the beautifully banded Hochstegen Marble, photographed in the so-called Marble Hall. The very sharp alternation between light-grey and dark-grey layers is clearly visible.
Brown quartz plates and spindle-shaped rock inclusions, known as cherts, protrude from this wet cave wall. Unlike the surrounding calcitic marble, they are not dissolved and are therefore slowly etched out of the wall. In this image section, the cherts project up to 15 cm.
The Gneisses
Rocks younger than the Hochstegen Marble are not present in the immediate area of the Spannagel Cave system. Above the marble lies the Central Gneiss as an entirely allochthonous “cap”. It is much older and, according to radiometric age determinations, belongs to the Upper Carboniferous, making it about 300 million years old.
Its present position above the younger Hochstegen Limestone is explained by the complex tectonic situation of this area. A characteristic feature of the Central Gneiss is its light colour. At its base, directly at the tectonic contact with the underlying marble, it is strongly altered and usually dark in colour. This razor-sharp, folded contact is one of the important structural elements of the Spannagel Cave. In many places, this rock boundary can be followed for long distances inside the cave.
Compact, light-coloured Central Gneiss also occurs locally and frequently within the caves of the Spannagel area, mostly as well-rounded pebbles and boulders. These were evidently transported into and through the cave by high-energy glacial waters. One impressive example is found near the entrance of the Kleegruben Cave. Today it lies above a rock step approximately 15 m high, yet at the surface several rounded Central Gneiss boulders up to around half a metre in size are wedged into niches in the Hochstegen Marble. They were, so to speak, left behind there by the Ice Age glacier.
Much less is known about the dark rock that forms the footwall of the marble. It bears the rather technical name “phengite arkose gneiss” and forms, for example, the prominent Höllenstein (2,873 m) east of Hintertux, which should properly be called “Höhlenstein”. To explain the name: phengite is a light mica, and arkose denotes a sandstone with a high content of feldspar grains.
This coarse-grained, partly nodular gneiss formed through the metamorphism of originally conglomeratic, gravelly and sandy, but carbonate-free sedimentary rocks. It was therefore not a granitic melt like the Central Gneiss. A precise age for the formation of these sedimentary protoliths has not yet been determined. Most researchers interpret it as a Palaeozoic, possibly Permian sediment. The Permian spans roughly 290 to 250 million years before present.
What is clear, however, is that the Hochstegen Limestone was deposited on this rock. The now mostly folded lower boundary of the marble, locally cut by quartz veins up to half a metre thick, therefore represents an original boundary. Directly above this basement lie a few metres of brown-weathering, mica-rich rock, the so-called basal marble. The boundary and the basal marble are clearly visible in many parts of the cave.
Above this follows the medium-grey, banded calcitic marble, 20 to 30 m thick in the Spannagel area, which also locally shows distinct karst features at the surface. The boundary from the underlying gneiss to the basal marble is well exposed directly behind the Spannagelhaus and to the north in the area of the ski slope.
Rounded pebbles and boulders of white Central Gneiss, which in the caves often have a dark-brown coating of iron and manganese oxides. The lower specimen was broken open to show its interior. Scale in centimetres.
A geologically important boundary zone is well exposed along the ski slope below the Spannagelhaus: above the underlying gneiss follows, with a sharp boundary, a brown-weathering rock forming the base of the Hochstegen Marble. It was deposited as a sandy, mica-rich sediment directly on the gneiss basement and was later slightly folded. Image width approximately 3 m.
White Central Gneiss, which forms the crystalline cores of the western Zillertal Alps and, for example, the summit of the Olperer. This hard rock is commonly found in the cave as rounded pebbles and boulders. Long diameter of the sample: 8 cm.
Important tectonic boundary, as seen in many places in the Spannagel Cave: above the grey Hochstegen Marble follows the Central Gneiss along a razor-sharp, often folded boundary. At this thrust contact, the gneiss is conspicuously dark in colour. Locality: Tropfsteingang. Glove for scale. The Central Gneiss is much older than the marble and was thrust over it only during Alpine mountain building.
Karstified Hochstegen Marble exposed on the ski slope in the area known as the Berger Seite, north of the Spannagelhaus. Formerly very narrow fractures have been significantly widened by slow dissolution, that is, by karstification.
Hydrogeology
Cave Streams
Today, most of the Spannagel Cave system is hydrologically fossil. This refers to the rapidly flowing water component that, in earlier phases and under entirely different conditions, created passages and chambers with large discharges. Entering the cave during its active phase would have required full diving equipment.
Traces of these considerable watercourses can still be seen after thousands of years in the form of scallops, from which the direction and approximate flow velocity of the water can be inferred. The cave streams encountered in the cave today are all mere rivulets compared with what once took place in this system.
The Gneisbach, named because it closely follows the upper surface of the underlying gneiss, and the Kolkgangbach reach only a few litres per second even during snowmelt. In late winter their discharge drops to a fraction of this, and the Kolkgangbach may disappear entirely. The origin of the waters is well understood: the Gneisbach mainly carries snowmelt water from the Schneefleckhöhle, while the Kolkgangbach originates primarily in the watercourse of the Wassergang in the show cave section.
Where does the water flow within the cave, and where does it emerge again? This question was investigated by Ernest Jacoby, who studied the Spannagel Cave in his dissertation under the supervision of Georg Mutschlechner (1978). Dye-tracing experiments carried out between 1975 and 1977 showed, among other things, that a continuous flow path exists from the uppermost cave sections, specifically from the Halle der Vereinigung, to the Spannagel-Mündungshöhle, located more than 500 m lower. The water took 15 hours to cover this distance.
The Spannagel-Mündungshöhle was in fact discovered only during this tracer experiment. One main drainage direction of the cave system is therefore defined, closely following the tectonic attitude of the Hochstegen Marble. However, exploration still shows a considerable spatial gap to the Spannagel-Mündungshöhle, approximately 900 m horizontally and 250 m vertically. Whether a passable connection between the two caves will ever be found remains uncertain.
A second drainage route occurs through the currently known Westsystem. At its lowest point, the Bauchbad, a small cave stream sinks underground. A further drainage route towards the north is assumed but has not been proven.
Seepage Waters and Speleothems
In addition to the rapidly draining water component, there is a broad spectrum of seepage waters that appear as drip points in the cave and are responsible for its speleothem decoration. They range from corrosive drip and splash waters to speleothem-forming waters and very slowly dripping sites where gypsum even crystallises through slow evaporation.
Good examples of the first type, namely actively limestone-dissolving waters, are found in the lower part of the Hermann-Gaun-Halle. In the overburden of the Spreizschlucht and on the ascent to the Sintertor, there are also places where present-day seepage waters “gnaw” at older dripstone forms, thus actively dissolving calcite. Even more impressive are those places where the water has dissolved the marble, while the insoluble chert inclusions protrude from the cave wall like blades.
In this context, hydrochemical data collected by Ernest Jacoby in 1977 must be corrected (E. Jacoby and G. Krejci, 1992). He reported that the pH values of the cave waters were acidic, between 6.0 and 6.5, and attributed this to similarly acidic, aggressive glacial meltwaters. Systematic water measurements have since shown that this was probably a methodological or analytical error. The pH values of the cave waters range between 7.9 and 8.4 and are therefore alkaline. Even during snowmelt, the pH value of the Gneisbach never falls below 7.9.
A wall covered with scallops in the now dry Nordsystem of the Spannagel Cave. These asymmetrical forms, produced purely by chemical dissolution, indicate the former flow direction of the cave stream; in this case, to the right.
Speleothem-forming drip waters are rather rare in the Spannagel Cave and are usually absent altogether in most neighbouring caves. This is not surprising given the elevation and low temperatures. Active speleothem formation at slowly dripping sites occurs mainly in the Nordsystem and at a few places in the Westsystem.
Long-term observations in the Märchenwelt area showed a high constancy in both chemical composition and drip rate. The waters dripping into the cave there must therefore have travelled for certainly more than one year.
Conditions are different at another site studied in detail in the Nordsystem, at the branch to the Porzellanladen. There the drip rate varies considerably, between 6 seconds and 5 minutes. The composition of the water is also subject to large, mainly seasonal fluctuations. Overall, these are clear indications of a more direct connection to the surface, which in this specific case is also confirmed by the shallow overburden of about 10 m. By comparison, the overburden above the Märchenwelt is about 55 m. Hydrochemical calculations indicate that active speleothem formation at this site takes place only during late winter.
Soda straws and small stalactites grow from the ceiling. On them sit twisted calcitic forms known as excentriques, which have also been known for centuries from ore mines. There they consist of the mineral aragonite and are called “Eisenblüte”.
Snow-white speleothem formations on the ceiling of the so-called Kristallgänge, consisting of soda straws and incipient draperies. Image width approximately 1 m.
The extreme third form of seepage water is also of interest: water from which gypsum crystals precipitate. In the Kristallgänge and in the Schatzkammer, but also on a smaller scale at several other sites, snow-white crust-like formations occur, more rarely also larger regularly formed crystals.
Significantly, gypsum occurs only where the boundary to the overlying Central Gneiss is immediately present. The waters evidently dissolve sulphide sulphur present there, oxidise it and thereby introduce sulphate-bearing waters into the cave. For such a readily soluble mineral as gypsum to precipitate, however, an additional process is required: slow concentration of the solution by evaporation.
This can be clearly observed in the Kristallgänge, through which a noticeable air current passes for most of the year. Although measurements there always showed values of at least 97 percent relative humidity, even this slight evaporation is evidently sufficient to concentrate the seepage waters to the point where the solubility of gypsum is exceeded.
Coloured speleothems also occur locally. In this example near the Sintertor, active dripstones and wall sinter are slightly brownish due to humic substances.
Special Features in the Cave
- A young stalagmite from the Spannagelhalle that began growing after the last Ice Age and is about 13,000 years old at its base.
- A snow-white crust of fine gypsum crystals covers ceilings, walls and older speleothems, indicating that the drip waters have a high sulphate content.
- Mineralogical rarity: a stalactite in the Kristallgänge consisting of calcite and encrusted by a second mineral, gypsum. Length of the stalactite: approximately 15 cm.
Geological Cave Description
A detailed description of the individual cave sections is not the subject of this article. Only an outline of the main passage systems is given here. It is also in the nature of the matter that this description may soon become outdated, as exploration of the cave continues.
Spatial Layout of the Cave System
Even a brief look at the plan of the currently known cave network shows that the passages clearly follow two main directions: a WSW to westward direction and a northward direction. The spatial arrangement of the cave system can be visualised by imagining the rock layer of the Hochstegen Marble dipping both westward and northward, approximately towards the NNW.
The entrance directly at the Spannagelhaus is therefore the highest point of the system. From there, the passage systems descend westwards towards the ÖTK-Schacht. Parallel to this, there is a second entrance, the Schneefleckhöhle, whose continuation, the Gneisbach, also descends beneath the prominent lateral moraine of the 1850 glacier stand.
Both streams, the Kolkgangbach and the Gneisbach, carry water seasonally and expose the base of the marble. The ÖTK-Schacht, currently the largest shaft in the system with a depth of 22 m, is bound to the steep, flexure-like bending of the strata.
Westsystem
The ÖTK-Schacht is followed by the Hermann-Gaun-Halle, which meets a conspicuously strongly sintered transverse passage in the area of the Spreizschlucht. At the foot of this canyon, a large pothole was excavated and now serves as a water reservoir for the Spannagelhaus. It is water intake III; the other two intakes are located in the Kolkgang.
From there, a main branch of the cave system descends westwards, dominated by canyon passages. In the area of the Tunnel der Hoffnung, a beautiful, almost circular tube has been preserved, which is important for cave genesis. Locally, smaller and variably active speleothem formations occur, for example in the Märchenwelt.
The westernmost point, the Bauchbad, lies 871 m west of the entrance and 326 m lower. The overburden of Central Gneiss at this point is already about 190 m.
Northern Passage Systems
To the north, three further, more or less parallel passage systems connect to the Westsystem: the narrow Gneisbach, the Plattengang and the only recently explored Millenniumsystem.
The Plattengang is a fossil system in an advanced state of collapse and descends from the 95er-Fenster towards the Hexenküche. The Millenniumsystem, discovered only in 2000, forms a WSW-trending connection between the northern Spinnengang and the 95er System.
95er System and Nordsystem
The 95er System was the major breakthrough in the exploration of the Spannagel Cave. This 95er System, or Nordsystem, begins at the 95er-Fenster, where the Plattengang meets the Christine-Kapfinger-Dom.
It is followed by winding, well-formed passages interrupted by the Sandschlüfe. Branches to the west or east are limited, with the exception of the Porzellanladen. South of this point, one can descend from the main system into an underlying system at the base of the marble. Its southern parts have not yet been completely surveyed and include three short shafts.
A larger branch from the higher passage system is the Tropfsteingang, which also winds through the rock and ends in a sand crawl. Beautiful and partly clearly active speleothem formations occur in the Tropfsteingang. A passage branching south from there leads to the Sintertor, which is considerably smaller and narrower than its name suggests. From there, a connection back to the underlying main system has already been traversed.
Kristallgänge and Spannagelhalle
From the branch to the Tropfsteingang, the level of the Nordsystem descends stepwise towards the Kristallgänge and remains close to the boundary with the overlying, strongly foliated basal Central Gneiss.
East of the Kristallgänge, a series of new passages was discovered in 2002. Some dip westwards in a manner analogous to the Westsystem, while others connect back southwards to the underlying Nordsystem.
From the Kristallgänge, named after the glittering gypsum coatings, one can reach the Spannagelhalle through the Klufttunnel. Another longer branch descends farther towards the northwest.
Before the Spannagelhalle lies the inclined Blockhalle, filled with large marble blocks that have detached from the ceiling. The rock boundary runs there. The Spannagelhalle itself is the largest chamber in the cave; in its rear part, the floor is covered with sand and pebbles.
In the front part stands a large, blunt conical stalagmite, probably the largest in the cave system. Several continuations lead away from the chamber, some of them reaching close to the surface.
Cave Climate
A characteristic feature of this largest cave in Tyrol is the fact that the entrance directly below the Spannagelhaus is also the highest point of the cave system. It is very unlikely that passable continuations exist farther upward, because the Hochstegen Marble visibly wedges out immediately south of the Spannagelhaus.
Little is known about the various routes by which air enters the lower parts of the cave, especially the Nordsystem. It is clear, however, that air movement is particularly noticeable in the Nordsystem. In narrow passages such as the Klufttunnel or the Kolkgang, and even more clearly in the area of the Sandschlüfe or the Nadelöhr, this airflow can reach unpleasant wind speeds.
Ventilation in the Spannagel Cave follows the chimney effect. If the outside air temperature is lower than that of the cave air, that is below about +2 °C, the warmer cave air rises within the system and relatively warm air flows out at the entrance. Under the opposite temperature conditions, the circulation reverses.
This seasonal reversal of ventilation can be clearly recorded using an automatic temperature logger installed about 50 m behind the entrance in the entrance labyrinth of the show cave.
During the cold season, temperatures in the show cave section are fairly uniform and low, ranging between +1.0 and +1.5 °C. On its way through the branched cave system, the air has had enough time to approach the temperature of the surrounding rock. The measured values also correspond very well to those in more remote and deeper parts of the cave.
The peaks superimposed on this subsurface temperature spectrum mark those periods when outside air from above was able to penetrate at least as far as the show cave.
The first “tipping” of the inward, upslope-directed circulation occurred on 27 May in 1999, as early as 11 May in 2000, and on 22 May in 2001. The records show, however, that this first “tipping” was not permanent and that ventilation almost returned to “winter mode”: on 20 June 1999, on 18 and 29 May 2000, and between 3 and 21 June 2001.
Immediately afterwards, the air finally begins to flow into the cave from above. At the measuring point, located a considerable distance from the entrance and at about 2,510 m above sea level, notable positive temperatures of up to +7 °C were recorded, for example from 16 to 21 August 2000 and on 2 August 2001.
The resulting temperature pattern is a direct reflection of the temperature course above ground. Even several tens of metres inside the show cave, the distinct ventilation makes it possible to follow the temperature development of the summer half-year in detail.
In general, summer shows a trend of rising temperatures up to a maximum in August, followed by a gradual decrease and the transition to rising circulation, usually from early November. That this temperature course is not always uniform is shown by the year 2000, with a pronounced low in the first half of July. The year 2001 also deviated from the pattern: after high August temperatures, values dropped sharply in early September and recovered only somewhat in the first half of October.
How far can these temperature anomalies be detected towards the cave interior? The next automatic measuring station lies already well inside the mountain, in the Hermann-Gaun-Halle, about 365 m in a straight line from the entrance. There, no weather-related fluctuations can be detected within the resolution of the instrument; the temperature remains constant at about +1.3 °C.
Presumably, already in the upper Kolkgang, the air temperature has adjusted so far to that of the mountain that surface variations can no longer be recorded. Measurement series from these cave parts show monotonously stable values, interrupted only by small, very short-term peaks that indicate slight temporary warming caused by passing cave explorers.
Measurements of relative humidity showed that in all parts far from the entrance, humidity is close to saturation. During the winter half-year, it may fall slightly to 96 to 97 percent as dripping activity decreases.
The carbon dioxide content of the cave air, an indicator of the nature of the soil above the cave, is very similar to the atmospheric value at this elevation and shows no systematic fluctuations. This is probably the result of the initially low concentration, caused by the limited thickness of alpine soils, as well as good air circulation and mixing.
Cave Formation
The history of the Spannagel Cave system has only recently begun to emerge. While the first researcher, Ernest Jacoby, still held the view that the system was hardly more than about 10,000 years old and therefore largely formed after the last Ice Age, modern analytical methods now allow this figure to be increased by at least a factor of fifty.
Ongoing investigations and age determinations on calcitic dripstones have yielded a series of ages, some older than about 400,000 years. This is the measurement limit of the thorium-uranium method used. Measurements using the uranium-lead method, possible because the samples are very rich in uranium, have even produced an age of well over half a million years for one sample.
Younger speleothem samples were of course also found. For the formation of the cave, however, the highest ages are the most relevant. It can therefore already be stated that the Spannagel Cave system has a long history and has been strongly shaped by the effects of major glacier fluctuations, that is, Ice Ages. During the last 10,000 years, by contrast, the form of the cave probably changed only slightly, apart from processes such as ceiling collapse or frost shattering.
Glacial Imprint on the Cave System
When asking which processes led to the formation of the branched cave system near the Hintertux Glacier, the main agent has already been named: the glacier, with its considerable water discharge. It has left unmistakable traces inside the cave.
First, the numerous, usually very well-rounded pebbles and boulders of Central Gneiss should be mentioned. They are foreign to the cave and can be traced to the far ends of the system, for example to the Bauchbad and the Spannagelhalle. Present-day cave streams are not capable of moving such coarse sediment, not even during snowmelt.
Entirely different streams must therefore have found their way through the marble. It was also these meltwater streams that scoured out the potholes, which cut several metres deep into the underlying gneiss. Today filled with sand and gravel, they are clear evidence of former powerful cave streams. One such excavated pothole, an underground glacier mill, can be seen in the show cave.
It is easy to imagine that glacial meltwaters used the permeable Hochstegen Marble as a convenient drainage route. Here too, however, caution is required with regard to timing. An age determination of a speleothem that formed within a pothole in the Kolkgang yielded an age of more than 400,000 years. Since the speleothem is younger than the pothole, the pothole must have formed earlier.
The data therefore suggest that powerful erosional forces were at work long before the last major glaciation. The glacial meltwaters of the last Ice Age, whose maximum occurred around 20,000 years ago, thus used an underground drainage system that had already existed for a long time.
Loose Central Gneiss pebbles and boulders, or those embedded in gravelly-sandy sediments, testify to the turbulent activity of these streams. They also carried large amounts of sandy material from the glacier area into the cave, filled the potholes and caused blockages at narrow points. The Sandschlüfe are therefore largely clogged with only weakly consolidated gravels and sands, representing traces of the last Ice Age in the Spannagel Cave.
These sediments are locally overlain by the youngest, often still active, white speleothem generation. Its basal ages range between about 13,000 and 9,000 years, meaning that it formed after the last Ice Age.
Evidence of Rapid Erosion
There is also clear evidence of relatively rapid erosional processes. During mapping work at the foot of the Lärmstange, in the western forefield of the retreating Hintertux Glacier, remnants of dripstones abraded by the glacier were found. They yielded an age of about 100,000 years; no cave is visible there today. This indicates that the glacier carried out substantial erosional work in this area.
Phreatic Tubes and Older Passage Forms
In addition to the activity of cave streams, which left clear traces in the form of erosional features, especially canyons, older forms can also be recognised along certain stretches. These are circular to elliptical passage forms, often later modified into keyhole profiles, which can clearly be assigned to phreatic conditions.
In plain terms, such passages were formed by the slow dissolving action of water that completely filled them. Evidence includes not only the approximately round cross-section, indicating dissolution on all sides, but also the common occurrence of large scallops, especially on the ceiling. The asymmetry of the scallops shows that in some of these pressure tubes, water even flowed upwards for several metres. This too is evidence of phreatic conditions.
Examples of such old phreatic tubes include the Tunnel der Hoffnung below the Märchenwelt and the passages before and after the Sandschlüfe in the Nordsystem. Significantly, these probably oldest parts of the cave system are almost always located near the upper boundary of the marble. From there, later vadose canyons cut downwards to the lower boundary, causing the tubes to fall dry.
When the Spannagel Cave was at least seasonally water-filled is not yet known. What is certain is that these are the oldest parts of the cave, and an age of at least several hundred thousand years is therefore very likely.
Because of the many water routes and numerous blockages, for example in the Sandschlüfe, it is quite possible that such phreatic tubes later became active again. In 2002, during an exploration west of the sand crawl near the Gothischer Gang, a westward-descending pressure tube was discovered. Its walls are coated with a thin, fresh-looking layer of clay, making it very likely that water backed up here for several tens of metres.
Since this situation lies beneath the present glacier forefield, it is probably connected with the last major glacier advance around 1850, or may even be more recent. In this context it should also be recalled that a large part of the currently known cave system lies beyond the prominent 1850 moraine and therefore occupied a subglacial position not very long ago.
There is also an observation passed down by a former hut keeper that, around the middle of the 20th century, a fairly substantial glacier stream disappeared into a fissure in the Central Gneiss in the glacier forefield. The exact position can no longer be reconstructed.
Structural Control and Karst Dissolution
Overall, this paints a truly turbulent picture of the earlier Spannagel Cave and its smaller neighbouring caves, most of which were very probably connected to the main cave. High-energy meltwaters, as well as slowly draining phreatic waters, left their traces in the rock over hundreds of thousands of years.
The rock itself contains the basic layout of the cave system: on the one hand in the form of its pronounced bedding and the associated bedding-parallel water permeability, and on the other hand in its dominantly west- and north-oriented jointing.
The Hochstegen Marble, enclosed between gneisses like a sandwich, therefore functioned in the past like a porous lamella through which water could drain rapidly and act as a cave-forming agent.
In addition to the obvious erosion, genuine karst dissolution has also played an important role in speleogenesis and continues to do so today. This is shown not only by the phreatic tubes but also by the brown chert bodies that protrude spectacularly from the cave wall in some places. Where stream erosion prevailed or still prevails, they are abraded, similar to the surface above the cave. Where, however, splash or drip water flows over a wall, for example in the Hermann-Gaun-Halle, it dissolves the marble, leaving the siliceous cherts behind.
Large dissolution forms, known as scallops, on the ceiling of this large tubular passage, the Tunnel der Hoffnung. It was formerly a pressure tube completely filled with water in the Westsystem.
The 95er-Fenster, the beginning of the Nordsystem discovered in 1995, is a fine example of an originally preserved phreatic passage system, that is, a tube completely filled with water. The large scallops, indicating slow water movement, are particularly noteworthy.
Regional Geological Context
One geological feature of the Spannagel Cave is the Hochstegen Marble in which the cave system is developed. This marble belongs to a larger marble belt within the Tauern Window and is exposed at the surface in several places across the Central Alps, from the Brenner area towards Gerlos.
In the area of the Spannagelhaus, this marble continues underground towards the north and forms the geological basis of the cave system. The cave therefore makes accessible a geological structure that is otherwise mostly visible only in natural outcrops, geological profiles or in the context of major engineering projects such as the Brenner Base Tunnel.
Source: Festschrift 50 Jahre Landesverein für Höhlenkunde in Tirol.