“Fossilised Lake” near Börtewitz

  • Geotope type: Exposed prospect
  • Rocks: Alternating layers of fossil-rich silicified lake sediments and volcanic ashes
  • Geological period: Permian (Rotliegend)

The fossilised Börtewitz lake, today a large-size protected natural monument, is an open trial pit at the eastern slope of the Kemmlitz stream to the north of Börtewitz. This is where the locals would look for whetstones at a place called “Rummeliche” which they used to sharpen their scythes. This is also where the Freiberg geologist Harald Walter found the first fossils on kaolin during geological reconnaissance work.

location
04703 Leisnig district Börtewitz

coordinates
N 51° 12′ 56.0; E 12° 59′ 9.3 (WGS 84)
4568993; 5676085 (Gauß-Krüger)
359335; 5675722 (UTM 33)

contact
Stadtverwaltung Leisnig
Markt 1, 04703 Leisnig
Tel. 034321/ 666-0
Internet: www.leisnig.de

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The approximately ten metres thick deposits exposed during the prospecting consist of more or less layered, yellowish to light brown claystones. These are partly fossilised or chalky. Along with these lake deposits various products of volcanic deposits such as tuff can be seen.

Abundant fossil material was salvaged during several geo-scientific excavations conducted by the Freiberger Bergakademie (Mining Academy) and the Schleusingen Museum of Natural History. This included plant remains, ostracods, clam shrimps, insects, myriapods, fish and amphibians. Along with the examination of the rocks, informed statements could also be made on the ecological development of the lake in the Permian landscape shaped by volcanoes and located close to the equator at the time approx. 290 million years ago.

The abundant fossils allowed conclusions to be made about the former plant and animal communities in the lake and their food chain as well as about the ecological changes caused by volcanic influences. This can best be shown by the amphibian and fish fossils that were found. The previously examined prospect profile reveals ten lake phases with each one being interrupted by volcanogenic events. Ideally the lake developed from an amphibian lake, to an amphibian and fish lake, and then to a fish only lake. The time frame of these lake phases can be given with 2,000 to no less than 4,000 years. This time is “stored” in the examined layer of rocks with a thickness of just five metres.

The fossils of the Börtewitz “fossilised lake” are also of great significance internationally since they provide valuable comparison material on the global development of Permian fauna.

Portrait of the “fossilised” lake, Tschernay

Fossil conifer Culmitzschia parvifolia, Photo: H. Walter (LfULG)

Tooth of a sweet water shark, Photo: H. Walter (LfULG)

Exposure site during the 2010 excavation campaign, Photo: LfULG

Börtewitz rock succession with the discovered fossils, Walter + Tschernay (LfULG)