Silurian rocks form the Southern Uplands of Scotland, which were pushed up from the sea bed during the collision with Baltica/Avalonia. The majority of the rocks are weakly metamorphosed coarse greywacke. The Highlands were also affected by these collisions, creating a series of thrust faults in the northwest Highlands including the Moine Thrust, the understanding of which played an important role in 19th century geological thinking. Volcanic activity occurred across Scotland as a result of the collision of the tectonic plates, with volcanoes in southern Scotland, and magma chambers in the north, which today form the granite mountains such as the Cairngorms.