Reef complex of Altenstein

In 1798, Duke GEORG I von Sachsen Meiningen (1761-1803) authorised the surroundings of his summer residence to be re-designed. This project included the foundation of the spa town Bad Liebenstein (before: Grumbach and Sauerborn) and the creation of the landscape parks on the Altenstein and Liebenstein reefs. The special feature of these parks is the deliberate integration of geological elements such as rocks and caves in the park design.

Altenstein Reef covers an area of about 1.6 km2, making it the largest Zechstein reef of the Ruhla Island and one of the biggest fossil single reef structures in Germany. Erosion processes have exposed the reef in its near-original form. Here it is possible to study the original morphology of the reef in near perfect condition (back reef, reef flat, reef crest, upper and lower reef face).

The different reef zones host different fossil faunal habitats. For example, the reef crest fauna consists almost exclusively of ocean floor sessile organisms living in highly agitated water. Organisms living on the soft sediments are found in the reef flat zone. The faunal diversity of the saltier waters of the back reef is very restricted. The actual main reef building organisms were calcium carbonate producing microorganisms that built the structures known as Stromaria (GEINITZ). In addition to these, colonial bryozoans also helped to build the reef.

Most of the Altenstein Reef sits on the Ruhla granite (295 ± 2 million years ago) inselberg. As the reef grew it encroached onto the Lower Permian sediments (Rotliegend). The rocks on the ocean-side of the reef are up to 130 m thick and are intensely dissolved showing many karst features. Although the Zechstein reef carbonates only cover an area of 36 km2 in Thuringia, some 140 of the known 400 Thuringian solutional caves are found here in these rocks!

Altenstein Cave was discovered during the construction of the road to Altenstein Castle in 1799. By 1802 it was developed as the first tourist cave in Thuringia. The cave can be seen as the underground part of the landscape park and was designed in the same early romantic style, including a dammed up navigable subterranean lake (see BRUST, 2002). At over 2 km explored and surveyed length, Altenstein Cave is currently the longest cave in Thuringia.

Altenstein Cave is the most important type locality for invertebrate fossils of the Zechstein period. Around half of the known and scientifically examined species of the Zechstein reef carbonates  were first described in the "cave limestone of Glücksbrunn“. Among these are such famous fossils like the reef bryozoan Rectifenestella ("Fenestella“) retiformis (SCHLOTHEIM, 1820a, emend. ERNST, 2001).

The palaeontologist Ernst Friedrich von SCHLOTHEIM (1764-1832) of Gotha was first to study the Altenstein reef complex. He recognized the nature of the limestone rocks of Schweina and Liebenstein as fossil reefs (two decades before Roderik I. MURCHINSON identified the fossil reefs of Gotland).

Large volumes of cave sediment were removed during the development of Altenstein Cave. This contained numerous remains of Pleistocene vertebrates especially of the cave bear, Ursus spelaeus (ROSENMÜLLER, 1794). ROSENMÜLLER (1804) published the first anatomical study of the cave bear using these bones.

The parts of the extensive system of the Altenstein Cave that are only accessible to speleologists also access fossil-bearing layers of the Upper Permian parent rock. These beds consist of loose fossil detritus from the early phase of reef growth. The material yields small and microscopic fossils that can be extracted and analysed without prior chemical treatment.