Fig. 2.3.1: Flat-lying sedimentary layers from the Triassic, Sella Group, Dolomites, Italy (photo: Meschede 2012).

The Sella Group in the Dolomites (Southern Alps, Italy) is a plateau-like mountain range with an average elevation of over 2,500 meters. The rocks were originally deposited in a shallow ocean basin and were later uplifted by mountain-building processes.

The layers here are consistently more or less horizontally layered. They are occasionally slightly tilted, and there are also occasional thrust faults, but large folds are not found in this part of the Alps.

In the flat-lying strata of the Sella Group, small overthrust structures can occasionally be observed, such as the ramp/flat thrust structure shown in Fig. 2.3.2. They indicate a relatively moderate conversion in the region of the Southern Alps.

In the Northern Calcareous Alps, the sedimentary layers are often significantly more steeply inclined and are sometimes embedded in large fold structures. This is because the Northern Calcareous Alps, as part of the large Eastern Alpine nappes, were constricted much more than the Dolomites while being transported far north. These layers can also be found today at elevations of over 2000 m.

Another example of an elevated plateau is the Sint Plateau in the Jebel Akhdar Mountains in northeastern Oman on the Arabian Peninsula. The rocks here, like those of the Dolomites or the Northern Calcareou Alps, were formed at the same time in the Triassic period and in the same ocean, the Neotethys. Only this part of the shallow sea lay further south.

Large assemblages of megalodonts can be observed on the Sint Plateau in Oman. These shells indicate that the calcareous sediments were deposited in a shallow marine basin. However, the Sint Plateau is now located at elevations of more than 1,000 or 2,000 meters. Megalodonts also occur in the Dolomites and the Northern Calcareous Alps. They are often called cowprint shells because, in cross-section, they resemble a cow’s footprint.

Evolution of the Alps

In a digital terrain model of the Alps (Fig. 2.3.6, created with GeoMapApp, 2025), which was created using satellite data, the mountains and valleys are particularly clearly visible. Some striking lines are immediately apparent, such as the Periadriatic Lineament.

This line has a tectonic origin, as it is the junction between the two continental plates that collided with each other: the European and the Apulian-Adriatic Plates. The term suture comes fromt the Latin word sutura, meaning “seam.”

Fig. 2.3.7 shows the division of the Alps into plate tectonic units superimposed over the digital terrain model of Fig. 2.3.6. The Periadriatic Lineament borders the Apulian-Adriatic Plate to the north. This is followed by the units belonging to the Penninic Ocean. Further north, the European Plate follows. The Eastern Alpine Nappe belongs to the Apulian-Adriatic Plate and was thrust onto the Penninic Ocean as a tectonic nappe. Along the Periadriatic Lineament, two formerly separate continents, Europe and the Apulian-Adriatic Plate, lie directly adjacent to each other. The remnants of an ocean that once existed between these two continents are imbricated and folded between them.

Process of Mountain Formation

The animation in Fig. 2.3.8 (Meschede, unpubl. 2025) schematically depicts the mountain formation process, beginning with the plate drift of two continents, their collision and nappe formation, followed by plate separation and the final uplift of the mountain range through isostatic adjustment.

Fig. 2.3.9 summarizes the most important stages of the mountain building process shown in the animation of Fig. 2.3.8: The continents move towards each other, resulting in a collision whereby one continental plate is pulled beneath the other. Initially, the entire stack, including both continents, is pulled downwards because the oceanic lithosphere is still connected to the continent. During this time, the initial further constriction in both the upper plate and the underlying plate leads to nappe thrusting and stacking of the rock units. At some point, the force is no longer sufficient for such thrusting, and the constricting movement stops. This is the point at which the oceanic lithosphere breaks away and descends into the mantle. This process is called the slab breakoff. The oceanic lithosphere behaves differently from the continental plate because it is heavier than the asthenosphere beneath it. It therefore literally falls downwards into the mantle. Only after the slab breakoff does the actual uplift of the mountain range begin as a result of isostatic adjustment.