Around Basel: The Rheintalflexur – some background information and observations

Let me now follow up on the Rheintalflexur or “Rhine Valley flexure”, reading more about it and observing it in nature.

Let us do so after having thought about the earthquake of 1356 and the emergence of the Rhine Rift Valley (Rheintalgraben).

 

Gathering information about the geology of the Rheintalflexur 

In the lecture notes provided by the Basel university, I found this geological cross section of the Rheintalflexur from the Wartenberg (via Muttenz) to the Bruderholz. The sediments bent are shown here, and they flexed, when the crystalline basement cracked down.

The Rheintalflexur is rather exotic in the Rhine Rift Valley. At the flexure, the fractures are about 1000m deep, whereby farther north the rift slid down up to 4000m. While extension had caused the cracking of the rift valley, compression seems to act now at the Rheintalflexur (Laubscher, 1971, p. 157). We are at the southern end of the rift valley, where it turns to the west to form the Belfort gap, and somehow the rift had to “find a way” to complete the “turning” which lead to a complex pattern of small cracks. The Tafeljura echoed that with the fractions of the Wartenberg and the Adler.

 

The “Schänzli”: Building the highway tunnel allowed to explore the “Rheintalflexur”

This often cited graph of Buxtorf illustrates, how steeply the layers bend under the Schänzli (he assumed the fault to be reverse). The tunnel of the highway T18 going south to the Birseck was built exactly here, at the Schänzli.

Source: Hans Laubscher, 1971

Below we see the cross section of the highway tunnel  “Schänzli” that «sits» on the fault line and reaches into the layers of lime stone that are almost vertical here. Stairs allow now to enter and observe the geological activity (left). 

Source: Website “Heimatkunde Muttenz

During the excavation for the tunnel, the almost vertical layers of lime (Hauptrogenstein) were temporarily uncovered (right).

Let us now go out and shoot some photos to illustrate the tracks of Rheintalflexur. 

 

The Rheintalflexur at the Birs

The concrete pier of the wooden bridge at Münchenstein crossing the river Birs sits on the Rheintalflexur. Between the two pillars, we can see, how the  sediment layers are inclined. 

Just above the bridge there is the cataract formed by the slanting sediment layers of the flexure. 

I remember that I stood here with our geography teacher more than fifty years ago and he talked about the flexure.

Before taking these photos, I looked for the flexure walking up and down along the Birs; people kept on asking me, whether I had seen the kingfisher. The kingfisher? “No”, I replied, “I look for the “Rheintalflexur””. “You look for what?” – and  then I had some long conversations. May be, one day I should also look for the kingfisher.

Beyond the Rheintalflexur the Tafeljura cracked into “pieces”, and the Wartenberg was tilted (see the geological cross section above). The tilted layers of the Wartenberg “ditch” can be seen at various places. This is a photo from the quarry above the vineyards. Or is it here, where part of the landslide broke off in 1952? 

A dog sniffs at my shoes. The lady asks me: “Do you  take a photo, before it all slides down?”. And then she continues: “The slopes are very unstable here, a gap keeps on appearing that is being closed again and again to “hide it”. We have built our house on rocks, after having consulted a geologist. At the Wartenberg, you should never build a house without consulting a geologist.” Okay, well I had never planned to build a house here, my mum had always warned me about the Wartenberg…

 

More signs of the Rheintalflexur: The quarry above Münchenstein

Above Münchenstein, in the quarry “Blinden”, the bending sediments of the Rheintalflexur are also uncovered. 

The quarry is no longer in use. It has become a romantic nature reserve, even with a picnic place (as there are lots of them around Basel, some even providing free firewood).

During my next walk on the Bruderholz, I discover the quarry across, above Münchenstein. I have never noticed it from here, though having been here hundreds of times. Yes, we notice, what we have read about and thought about before! Not only, when travelling to other countries, but also, when walking around home.

 

Looking for the”Rheintalflexur in the landscape park “Ermitage” – not really successful

The Ermitage of Arlesheim is a marvellous landscape park, first established in 1785 and reconstructed in 1811. I have blogged about it in 2015 at the occasion of the park’s 230th anniversary.

The heart of the landscape park is under the Birseck castle. This castle is a ruin, integrated into the Ermitage, as every “proper” landscape park must have a ruin. A maze of footpaths zigzags up and down the rocky slope under the castle. The wall of the so-called “carousel square” stands out at the foot of the hill. 

Some 150 million years ago, this hill was part of a riff at the shore of the former Tethys sea, long before the Rhine Rift Valley emerged (40 million years ago). Many natural caves emerged later that create an exceptional atmosphere to the park.

I could not find identify the flexure around the Ermitage. May be the rock above the “carousel square” is part of it? Erich Plattner’s “Höhlen der Ermitage” does not help either, as what he calls “geological cross section of the Ermitage” seems to be farther north (from the Predigerhof across Arlesheim to the Hinteri Ebni, p. 47).  

Not really successful. Anyway, I do love to stroll through this magnificent landscape park that hides many surprises such as the hut of the “Waldbruder” or “brother of the woods”. Now, in winter, his hut can be seen from far.

The architects of the Ermitage created their landscape park with a lot of phantasy, and I keep on discovering new details.

 

The “Rheintalflexur” at the Hornfelsen (Dinkelberg)

The Rheintalflexur can be seen above Grenzach (already in Germany) at the Hornfelsen,…  

… where in particular the “lowest” rocks are bending clearly downwards. 

Also here I have been so many times before and have never noticed the flexure. 

I would love to find a book about the geological phenomena around Basel explaining them and informing, where they can be seen in nature. A book for interested hikers such as our group of Nordic Walkers. 

 

Hans Laubscher, Das Problem von Rheintalflexur und Tafeljura, Eclogae Geologicae Helvetiae, 64 (1971), Heft 1; http://doi.org/10.5169/seals-163976 

Peter Huggenberger, Lecture Notes “Jura Tektonik”, Department of Environmental Sciences, University of Basel (Überblick (Di ) Teil 2 Tafeljura (Do ) – PDF Free Download (docplayer.org)

Erich Plattner, ” Die Höhlen der Ermitage bei Arlesheim”, Speleo Projects 2014.

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