STOPPING POINT 1 The gorge – rubble not diamonds
From the very first glance at the Garnitzenklamm gorge, the typical features of a gorge are evident on both sides of the Garnitzen brook: the almost vertical cliff walls.
The cliff walls are made up of light grey banded limestone, which is the dominant rock type along the Geotrail. The second main rock can be seen above the brook: dark grey schists.
The formation of the gorge undoubtedly dates back to the last Ice Age, the Würm glacial Period. Of course, there were river systems here before this but any ideas as to their course are purely speculative. Following the melting of the last glacier approximately 19,000 years ago, unimaginable quantities of rubble and debris began to accumulate, having been produced by frosts of the Ice Age. This rubble was transported into the Gailtal valley by the Garnitzen brook, giving the water substantial carving powers on the rocks (erosive power).
This erosive power of the water continues today, although in a weaker form, and as a result the gorge is slowly becoming deeper over time.
STOPPING POINT 2 Banded limestone – an enormous driving force
An imposing cliff of banded limestone with extremely polished cliff faces can be seen here. These striking surfaces developed because banded limestones and schists were once pushed against one another by unimaginable forces from inside the Earth – so-called endogenous forces. These displacement processes were responsible for the close proximity of these two tectonic units. However, only the impressive banded limestone can be seen today; the softer schists have been eroded away.
The name ‘banded limestone’ refers to the alternating layers of different colours within the rock. This is caused by minerals contained within the limestone (e.g. green chlorite and mica). Few fossilised finds have indicated that the banded limestone was deposited in a sea some 380 million years ago during the Devonian Period.
Younger rocks were formed on top of the limestone and the limestone underwent two mountain formation processes. The resulting pressure transformed the limestone into a coarse-grained marble. However, this is not reflected in its name.
STOPPING POINT 3 The common beech – rock fractures caused by root forces
A landscape is the result of the impact of various forces. The roots of the common beech highlight one type of forces which have an effect on shaping the Earth’s surface. These are known as exogenous forces.
As they grow thicker, roots develop such high pressure that they can split giant boulders or layers of asphalt apart. This process is known as root wedging and is a kind of weathering. The rock is broken down into smaller pieces but its composition remains unchanged.
However, this breakdown of the rock gives rise to additional weathering processes as it increases the surface area of the rock and exposes more surface area to attack by water and acids. These processes alter the composition of the rock and form new minerals.
Thus the erosion of rocks, their removal and the change in their composition cause whole mountains and landscapes to disappear over millions of years.
STOPPING POINT 4 Schists – a weak structure
The erosive forces of the water are responsible for exposing the second main rock type present along the Geotrail – grey schists.
The schists contain sand and were formed in a shallow sea. Rivers washed fine-grained erosion material from a long-gone island or continent into the primordial sea. These conditions are thought to have existed on Earth 450 million years ago and 330 million years ago respectively. However, it is not possible to date the schists exactly because they do not contain any fossils.
As with the banded limestone, these schists were also covered by younger rock deposits and were subject to two mountain building processes. The resulting pressure compressed the schists and changed their composition (metamorphosis). The cleavage of the schists is much more distinct than that of the banded limestone because unlike the granular limestone structure they are composed of flake-like minerals (sericite and chlorite). The schists are therefore less resistant to the erosive forces of the Garnitzen brook than the compact limestones.
STOPPING POINT 5 The course of the gorge – a show of strength
At this stopping point, from the Ida look-out point a waterfall can be seen rushing down over the light grey banded limestones. Below the waterfall, the limestones come into contact with the much weaker schists along a fault line. Such a fault results in a weakening of the rock structure as the rocks are moved against each other. The schists and the fault run in an east-west direction. They thus force the water flowing from the south to turn eastwards as the schist rock possesses less resistance to water erosion than the compact limestone.
Above the Ida look-out point, the water used to flow from south to north diagonally through the schists. Higher up it took an east-west course, following the schists and the fault line, just like it does below the Ida look-out point. How can we now explain these two different directions when the rock types are all the same?
In addition to the rock and its inclination, fissures in the rock also influence the flow of water. In the Garnitzenklamm gorge, the banded limestones reveal densely spaced fissures running north-south. They may well have been responsible for the first striking shift in direction from east-west to north-south.
STOPPING POINT 6 Kolks – deep whirlpools
At this stopping point, you can see the narrowest section of the Garnitzenklamm gorge. Here, the brook has carved its way deep down into the green-red-grey banded limestones and sculpted out some impressive kolks.
Kolks are cylindrical to trough-shaped indentations which develop when flowing water on a stream bed encounters an obstacle and is forced to rotate in one and the same place. The water in a kolk rotates with increased speed, carrying its load of debris with it. This debris increases the power of the water and its abrasive action on the rock. The increased speed also means that any eroded material is quickly transported away.
Similar hollow formations are also caused by melt water forming beneath glaciers. In this case, they are known as glacial kolks.
STOPPING POINT 7 Rock folds – in the vice of the mountain formation
Nowhere else does the gorge offer such striking insights into the interior of a mountain and rocks as on the opposite cliff wall. The exposed folds in the rock are the expression of the enormous forces involved in forming mountains. The rocks are laterally compressed as if they were in a vice, causing them to bend and fold.
On the left side of the wall, the water has polished the surface of the banded limestone resulting in a smooth appearance. The different coloured rock layers clearly demonstrate the folding process. To the left, you can see just how inconspicuous the rock looks without this polishing effect from the water.
The water has exposed large wave-like folds in the central part of the cliff. They extend from the black quartz-rich schists at the base of the wall to the light banded limestone above. The fold structure is less clear in the schists because they react differently under pressure to the banded limestones.
On the right of the cliff wall, an extreme fold can be seen just above the brook. Here the rock layers have been bent some 180 degrees!
STOPPING POINT 8 Erratic rocks – a gift from the Ice Age
A large block of red-grey laminated limestone can be seen near the bridge on the banks of the brook. This is one of the most beautiful rocks in the Carnic Alps. Looking more closely, you will see a network-like structure on its surface. This consists of darker clay particles surrounding lighter limestone cores. Rocks with this structure are known as nodular limestone. Just how they developed is still not clear.
In addition to its beauty, this 400 million years old nodular limestone has another special feature, the fact that is has been deposited at this location. Rocks of this kind do not naturally occur in the catchment area of the Garnitzen brook. Hence, this boulder has not been transported to its present position by water. But there is another explanation.
During the last Ice Age, the entire Gailtal valley was covered by glacial ice up to around 2,000 m. Unlike water, glaciers can flow over passes or water sheds carrying rock debris over great distances. Since such rocks are only exposed west of the Nassfeld area, the occurrence of this rock in the Garnitzenklamm gorge can easily be explained by glacial transport from a region several kilometres to the west. Rocks transported by glaciers in this way are known as erratic blocks or boulders.
STOPPING POINT 9 All sorts of stones – a joint show of strength
The striking rock debris in the stream bed of the Garnitzenklamm gorge is endlessly fascinating and is the result of various natural forces. The colours of the dominating limestones and schists are multiplied by those rocks which are exposed in the upper part of the gorge and were transported from the rear section of the gorge by the Garnitzen brook to this point.
There, the outcrops comprise different rocks in comparison to those occurring along the Geotrail. These are red sandstones, colourful limestone conglomerates, highly porous ochre-coloured calcareous tuffs, almost white dolomites and various different coloured limestones and schists. Many of these rocks contain fossilised sea creatures. All the different types can be studied at this stopping point.
Most of the stones were rounded during transportation. There are, however, some more angular examples. These angular rocks have travelled less far and are derived from the adjacent cliff walls.