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Architecture of the orogen
In the most general of terms, the Scandinavian Caledonides can be divided in autochthonous basement of the Baltican cratonic margin and a nappe stack that was thrust over the autochthon.

Caledonized autochthonous basement
See e.g. Western Gneiss Region

Caledonian Allochthons
The Caledonian allochthons were put in place during the Scandian orogeny and make up most of the bedrock now seen in the Scandinavian Mountains. The allochthons overly the much older Svecokarelien and Sveconorwegian provinces of the Baltic shield. Much of the Caledonian rocks have been eroded since they were put in place meaning that they were once thicker and more contiguous. It is also implied from the erosion that the nappes of Caledonian rock originally reached further east than they do today. The erosion has left remaining massifs of Caledonian rocks and windows of Precambrian-aged rock.

While there are some disagreements, geologists generally recognize four units among the nappes: an Uppermost, an Upper, a Middle and a Lower one. The last one is made up Ediacaran (Vendian), Cambrian, Ordovician and Silurian-aged sedimentary rocks. Pieces of Precambrian shield rocks are in some places also incorporated into the Lower nappes.