Fold at Riedberg Pass, detail view. Stick measure for scale, length 1 m.

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Fold at Riedberg Pass, Full view.

Fold at Riedberg Pass, detail view (left of the traffic sign). Stick measure for scale, length 1 m.

Sadly, last summer I didn’t have much time for photographing this fold, and the pictures didn’t align nicely for a “panorama view”. (Thus the blanks in the images.)

Geologic setting

At this outcrop, the Ofterschwang Layers of the Rhenodanubian Flysch are exposed, which consist mainly of mudstones and marlstones from the Albian and Cenomanian. To the north, the Helvtic shelf deposits were formed in the Cretaceous and Paleogene. At the end of the Paleogene, the Flysch nappes thrust over the Helvetic. (Later, the Alpine Limestone („Kalkalpin“) nappes from farther south where thrust over this stack.) In the process, this north-vergent fold was formed.

In the area of the mountains Ifen, Besler and Schwarzenberg (in the south), the Helvetic is already exposed. But the fold outcrop lies just a bit to the north of the boundary between Helvetic and Flysch:

Geologic map for the fold at the Riedberg Pass (LfU Bayern¹, some missing symbols added).
fo = Rhenodanubian Flysch: Lower Coloured Marls or Ofterschwang Layers to Anthering Formation – claystones, siltstones, marlstones and sandstones, locally conglomerate, limestone.
ff = Feuerstätter Flysch – claystones, siltstones, marlstones and sandstones, locally conglomerate and breccia, with limestone and mafic volcanic rocks.
hm = Schrattenkalk (“grykes limestone”) to Garschella Formation – limestone and sandstone, partially marlstone.
ho = Seewer Limestone to Hachauer Layers – marlstone and limestone, partially sandstone.
W = Young Moraine (Würm Glacial) with end moraines, partially with glacial-advance outwash deposits – sandy to clayey-silty gravel.
WG = Outwashdeposits from the Würm glacial – sandy gravel.

The geologic development of the sediments in the surroundings is shown in the table below. North is to the left, south to the right. Vertical bars are the time spans for the sedimentation of the layers (labelled with the symbols from the map above).

This is a schematic depiction of the situation today, after thrusting, folding and exhumation of the Helvetic:

Block diagram of the Allgäu Alps (from Scholz, 1995). North is left.
KA = Main alpine limestone, F = Flysch, H = Helvetic (mainly karstic limestone), M = Molasse.

Slope stability

In this picture you can see that the clay-/mudstone layers are dipping parallel to the slope. Combine this with the generally quite moist conditions and you’ll understand that there’s always some repair work to be done somewhere on this road, as it slowly creeps downhill.

Slope to the right of the outcrop shown above.

Getting there

The entry in the Bavarian Geotop Inventory (number 780A006) contains a piece of the topographic map² Here is – more interactive – the location in Google Maps:

If you come from Obermaiselstein (in direction to Balderschwang), you pass a small tunnel, cross the brook Schönberger Ach und reach the first ascent of the pass road. The outcrop lies in a slight left curve. At the outcrop and shortly before, there are two openings in the railing with very short dead-end tracks where you could park a car.

On foot, you could go down from the Großer Herrenberg to the pass road which you’ll reach by a little bridge over the Schönberger Ach, roughly 300m below the outcrop. Be careful of the traffic; there’s no separate pedestrian sidewalk and some people drive like crazy on this road.

Literature:
Scholz, Herbert (1995): Bau und Werden der Allgäuer Landschaft. Schweizerbart.

1. The old website from where the map was taken doesn’t exist anymore. In their new PDF versions, the image quality sadly is lacking.
2. As already mentioned, by now in hideous quality.