The Kössen strata are approximately 200-million-year-old rocks from the Late Triassic. Named after the Tyrolean town of Kössen, these fossil-rich layers were formed in a shallow, tropical sea (likely 20–30 meters deep). In the Rofan Mountains, the Kössen Formation is a key geological feature. It stands out as darker, softer rock layers contrasting with the lighter, harder limestones and often forms soft slopes or steps between steep limestone cliffs.
Composition and properties
The Kössen Formation comprises various rock types:
- Dark brown to black marls and claystones
- Calcareous marls and thinly bedded limestones
- Reef or coral limestones
Marl – A special rock mixture
At this station, a dark brown marl from the Kössen Formation is on display. But what exactly is marl?
- Marl is a composite rock containing both lime and clay. Depending on the proportions, it is classified as either calcareous marl (lime-rich) or clayey marl (clay-rich).
- Its soft texture means it weathers easily, contributing to gently rolling landscapes.
Reservoir layers – Why do springs emerge here?
Due to its high clay content, marl is poorly permeable and acts as a water-retaining layer. In many mountain regions, including the Rofan Mountains, the Kössen layers prevent water from draining away, leading to the emergence of springs at the interface between permeable limestones and impermeable marls.
The smell test – Can you smell the marl?
If you breathe on the marlstone of the station and smell the moistened, warmed area, you will notice an earthy, loamy smell:
The earthy, loamy smell of the marl is caused by the chemical property of the clay to bind water and microorganisms in the moist rock.
- Clay minerals bind water and increase moisture in the rock.
- Microorganisms settle in the moist clayey rock and decompose organic matter, creating the typical earth smell.
- The earthy smell of marl is therefore a combination of chemical and biological processes in the rock. In contrast, limestone is odorless.