Orsa, Sweden Orsa, Sweden Orsa is a locality and the seat of Orsa Municipality, Dalarna County, Sweden, with 5,308 inhabitants (2010). It is located in the northern part of lake Siljan, about north of the town Mora. Inlandsbanan and European route E45 run through the town and the railroad Bollnäs-Orsa connects. The southern parts of Orsa are characterized by the Siljan Ring, the ring of lakes and rivers in northern Dalarna formed by the big meteorite strike 365 million years ago. In central Orsa, at the ouflow of river Oreälven a sand bank has been built up, in these days enjoyed by
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