A distinctive feature of the Saale valley between Jena and Dornburg is the characteristic shape of the valley slopes. The foot of the slope rises moderately above the Saale plain. This is where settlements, fields and orchards are located. In contrast, the upper parts of the Saale valley slopes are steep and mostly wooded, with beech forest predominant on the north-facing slopes, while open black pine forest defines the dry chalky soil on the southerly slopes. Orchids can be found both on the sun-drenched, nutrient-poor grassland of the black pine slopes and under the dense leaves of the beech forests, but there mostly at the edge of paths, clearings and the forest itself.
The Saale valley owes its characteristic shape to the prevalent rock. While the foot of the slopes is geologically a type of variegated sandstone, the upper slopes and the hilltops are shell limestone formations. Both variegated sandstone (251–243 million years old) and shell limestone (243–235 million years old) belong to the geological system of the Triassic (251–199 million years old). The Triassic, which also includes a third lithostratigraphic group, Keuper (which can be found on the Saale-Ilm plateau towards Apolda), has shaped the landscapes of the Thuringian basin.
During the Tertiary period, when the Thuringian basin was formed, the Saale river was able to cut into the eastern edge of the basin and expose first the limestone strata and then the variegated sandstone strata below.
This geological structure resulted in fascinating terrain formations.
The variegated sandstone to the east of the Saale river lies higher, because to the west the rock strata slope down towards the central Thuringian basin. This allowed additional valleys (Pennickental, Gembdental and Gleistal) to be formed in the soft rock. On the west bank of the Saale river, ravine-like valleys predominate, which have been cut into the harder shell limestone (Gönnatal, Rosental, Rautal and Mühltal valleys).
Shell limestone tends to develop fissures and cracks that allow surface water to seep away quickly. By contrast, Röt, the geological formation of the upper layer of variegated sandstone, is impermeable, so water cannot pass through it. This is the reason why springs appear at the boundary between the variegated sandstone and shell limestone layers in the side valleys of the Saale river. The most impressive example along the SaaleHorizontale trail is Fürstenbrunnen spring in the Pennicken valley.
Although the Röt layer itself is impermeable, the clay contained within it can absorb water, causing it to swell up and making the Röt malleable. The rigid chalk layer above then breaks and slips slowly towards the valley on top of the Röt. This process is most evident at the Diebeskrippe formation in the Pennicken valley, where large boulders have become detached from the hillside, and the SaaleHorizontale leads through the gap between the slope and the rocks.
Rainwater combines with carbon dioxide to form a weak acid, which dissolves the limestone contained in the shell limestone layer. The colder the water, the more limestone can be dissolved. When highly calciferous water rises in springs and travels above ground as a stream, the chalk precipitates and is deposited at the bottom of the valley. Plant particles and air bubbles are locked away in the sedimentary limestone layer that is formed in this way. This travertine used to be quarried in the side valleys of the Saale river and was used as a building material and as a raw material in the local glass-making industry and in the manufacture of toothpaste.