planetwater

ground- water, geo- statistics, environmental- engineering, earth- science

Geology of the Lechtal

without comments

Outcrop at the Gorge of the Bernhardsbach, Ebigenalb, Lechtal, Austria
Outcrop at the Gorge of the Bernhardsbach, Ebigenalb, Lechtal, Austria

These wonderfully folded geological layers can be found directly behind the open-air theatre where “Geierwally” is played in Ebigenalb, Austria (Lechtal). Behind that theatre, the creek “Bernhardsbach” forms a george for few hundred meters, and offers this spectacular view on an outcrop. This outcrop is one of the best windows into the history of the Alps.

Field Trip

Peter Nasemann guided us this summer to glance through some of such windows on a walking tour organized by the OEAV Lechtal. Peter Nasemann has written a book on the geology of the “Lechtal” and is considered an expert on the geology of the Alps. During the walk we stopped at three spots (see map).

Map locations
Locations of site-visits during the geological field trip

1) Gravel Bed next to Klimm Brücke, Elmen

At the first stop, Peter Nasemann introduced us to local rocks and their relation into the geologic sequence (main types are set in bold, youngest one are listed on top):

  • (Aptychenkalke)
  • Radiolarit (Elmer Mutterkopf, Mittagspitze) youngest that we looked at (Jurassic), red-ish, not prone to erosion
  • three similar types:
    • Allgäu Schichten: old, middle, and young (in the sequence right after Radiolarit), all still Jurassic),
    • Lechtaler Schichten: not entirely sure about this layer anymore.
    • Kössener Schichten (Triassic; The Stablalm is located on Kössener Schichten but everywhere around in its vicinity Radiolarit can be found, which consists of black / dark clay of light grey colour, sometimes yellow chalks with many fossils. Sometimes the annual cycles of the organic material are visible. At the bottom of each layer, remnants of animals can be found that sank into the deposits. Stones of the Kössener Schichten are somewhat “mudstones”… if you place them in your garden and wait for a couple of years, they will expand due to the water added over time, and the remnants can serve as a good addition to the soil in your garden.
  • Hauptdolomit (Triassic, Klimmspitze): prone to erosion, not nice for climbing; some sections of the Hauptdolomit, predominantly dark ones (“Seefelder Schichten”) are considered building material for mineral oil, and can make blotting paper oily; these are also used for Steinöl
  • (Raibler Schichten)

Theoretically, the mesozoic sequence of the East-Alpine of the Central Alps has been summarized by Adrian Pfiffner in his book “Geologie der Alpen” (figure below).

Mesozoische Schichtreihe Ostalpins small
The mesozoic sequence of the East-Alpine of the Central Alps has been summarized by Adrian Pfiffner.

Generally, the Lechtal can be divided into dominantly Hauptdolomit in the lower valley and dominantly not-Hauptdolomit (the three similar types) in the upper valley. The divide is located near Häselgehr (Otterbach). Traditionally, the upper valley has a richer agriculture, the lower valley richer forrestry (Hauptdolomit sustains trees, but not agriculture). In fact, the richness of the diocese of Augsburg partly originated in its forests around Füssen and in the lower Lechtal (there is still a Flurbezeichnung “Füssener Wald” in the Lechtal)

The Walser were experts on agriculture, and they found the patches where the “Abtychen-Schichten” (and “Kössener Schichten”) appeared on ground surface. In the case of the Lechtal, these areas are generally higher up in the mountains. Coincidentally, the Walser came over the saddles or ridges of the mountains, not via the valleys (which one would normally think of being the connecting features).

Since we were standing on the banks of the river lech, we had a closer look on the gravel bed. It served as an excellent example for how a river builds its bed, and how the river bed gets stronger after a high flow event. Behind each stone in the river bed, sediment gets accummulated. This can form solid patches, depending on the flow rate and hence stone size at a given location.

Rocks Bottom Lech
Rocks at the bottom of the river Lech. When wet, flow direction from right to left. The deposits “behind” the larger rock are visible next to the black part of the photo bag.

2) First Bend on the Road to Bschlabs

After having been introduced to the rocks occurring in the area, based on samples from the river bed, and to the sequence of their occurrence in the geological sequence, we drove up to higher ground along the road to Bschlabs, to get an overview over the area.

Sometimes, the sequence of the rocks is not occurring according to the order of deposition described above. This can be seen at the Mittagsspitze (on the far right on the picture below), where Radiolarit is not at the top.

Lower Lech Valley
View from spot 2 along the river Lech towards north (downstream, “U-valley”). At the right edge of the valley, younger terraces can be seen.

Another example for an “inverted” sequence of rocks is the Grießtalerspitze, where Hauptdolomit lies above Radiolarit and above Aptychenkalk.

Otto Ampferer was the first to recognize that such “mixed up” sequences can occur when one layer is pushing against other layers. This phenomenon is called Überschiebung / overthrusting. This is also why the picture of the outcrop at the top of this post, is so important.

3) Gorge of the Bernhardsbach behind Geierwally Theatre

The last spot was the Gorge of the Bernhardsbach (introductory picture), which served Otto Ampferer as a basis to establish the theory of overthrusting in the 1920s (english article with interesting picture of Sölden and bautiful sketches, paper in German).

Further Information

Joachim Kuhlemann has assembled a geologcial trail in the Sulzetal, a side valley of the Lechtal. His homepage also has some further information.

Written by Claus

September 28th, 2014 at 10:47 am

Posted in