into 'highland' to the north and west, and 'lowland' to the south and east. The Highlands of Scotland are largely mountainous, and form the highest ground in the UK: they are bisected by the Great Glen into the Grampian Mountains to the southeast and the Northwest Highlands. The Scottish Lowlands can be further subdivided into the Southern Uplands, an area of rolling farmland and high moorland, and the lowland farmland of the Central Belt and eastern Scotland. Scotland has an incomparable variety of geology for an area of its size. The oldest rocks of Scotland are the Lewisian gneisses, which were formed in the Precambrian period, up to 3 billion years ago. They are among the oldest rocks in the world. During the Precambrian, the Torridonian sandstones and the Moine were also laid down. Further sedimentary deposits were formed through the Cambrian period, some of which metamorphosed into the Dalradian series. The area which would become Scotland was at this time close to the south pole. During the Silurian period (439–409 Ma), the area which became Scotland was part of the continent of Laurentia. Across the Iapetus Ocean to the south, was the continent of Baltica. The two continents gradually collided, joining Scotland to the area which would become England and Europe. This event is known as the Caledonian Orogeny, and the Highland Boundary Fault marks this stitching together of continents. Silurian rocks form the Southern Uplands of Scotland, which was pushed up from the seabed during the collision. The highlands were also pushed up as a result of this collision, and may have been as high as the modern-day Alps at this time. The Old Red Sandstones were laid down in low-lying areas during this period. 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. During the Carboniferous period (363–290 Ma), Scotland lay close to the equator. Several changes in sea level occurred during this time. The coal deposits of Lanarkshire, and further sedimentary deposits, date from this time. More volcanic activity formed Arthur's Seat in Edinburgh, among other hills. By the Triassic, Scotland was a desert, the origin of large sandstone outcrops of the southwest. Although large deposits of Cretaceous rocks would have been laid down over Scotland, these have not