in Scotland. These are essentially the remains of folded sedimentary rocks that were deposited between 1,000 Ma and 670 Ma over the gneiss on what was then the floor of the Iapetus Ocean. Palaeozoic At 520 Ma, what is now Great Britain was split between two continents; the north of Scotland was located on the continent of Laurentia at about 20° south of the equator, while the rest of the country was on the continent of Gondwana near the Antarctic Circle. In Gondwana, England and Wales were largely submerged under a shallow sea studded with volcanic islands. The remains of these islands underlie much of central England with small outcrops visible in many places. About 500 Ma southern Britain, the east coast of North America and south-east Newfoundland broke away from Gondwana to form the continent of Avalonia, which by 440 Ma had drifted to about 30° south. During this period north Wales was subject to volcanic activity. The remains of these volcanoes are still visible, one example of which is Rhobell Fawr dating from 510 Ma. Large quantities of volcanic lava and ash known as the Borrowdale Volcanics covered the Lake District and this can still be seen in the form of mountains such as Helvellyn and Scafell Pike. Between 425 and 400 Ma Avalonia had joined with the continent of Baltica, and the combined landmass collided with Laurentia at about 20° south, joining the southern and northern halves of Great Britain together. The resulting Caledonian Orogeny produced an Alpine-style mountain range in much of north and west Britain. The collision between continents continued during the Devonian period, producing uplift and subsequent erosion, resulting in the deposition of numerous sedimentary rock layers in lowlands and seas. The Old Red Sandstone and the contemporary volcanics and marine sediments found in Devon originated from these processes. Around 360 Ma Great Britain was lying at the equator, covered by the warm shallow waters of the Rheic Ocean, during which time the Carboniferous Limestone was deposited, as found in the Mendip Hills and the Peak District of Derbyshire. Later, river deltas formed and the sediments deposited were colonised by swamps and rain forest. It was in this environment that the Coal Measures were formed, the source of the majority of Britain's extensive coal reserves. Around 280 Ma the Variscan orogeny mountain-building period occurred, again due to collision of continental plates, causing major deformation