Siljan (lake) Siljan (lake) Siljan, in Dalarna in central Sweden, is Sweden's sixth largest lake. The cumulative area of Siljan and the adjacent, smaller lakes Orsasjön and Insjön is . Siljan reaches a maximum depth of , and its surface is situated above sea level. The largest town on its shore is Mora. The lake is located around the southwestern perimeter of the Siljan Ring (Swedish: "Siljansringen"), a circular geological formation which was formed 377 million years ago in the Devonian by a major meteorite impact. The original crater, now mostly eroded, is estimated to have been about 52 km (32 mi)
Siljan Ring an annular outcrop of Lower Paleozoic sedimentary rocks within Proterozoic granites of the Dala series. In Dalarna, the basement rocks consist of granites dated as 1.6 billion years old, putting them at the boundary between the Paleoproterozoic and the Mesoproterozoic. They were emplaced shortly after the Svecokarelian orogeny. The oldest sedimentary rocks that outcrop in the Siljan area are of Ordovician age. The sequence is dominated by limestone formations with one prominent black shale, the Fjäcka Shale, which is bituminous and has generated petroleum, sourcing the oil found in limestone cavities in the same area. The Ordovician sequence is overlain