Physical geography of the UK
The overall geomorphology of the UK was shaped by the combined forces of tectonics and climate change, in particular glaciation.
The exact centre of the island of Great Britain is disputed. Depending upon how it is calculated it can be either Haltwhistle in Northumberland, or Dunsop Bridge in Lancashire.
The geology of the United Kingdom is varied and complex. This gives rise to the wide variety of landscapes found across the UK. This variety, coupled with the early efforts of UK based scientists and geologists to understand it, has influenced the naming of many geological concepts, including many of the geological periods (for example, the Ordovician period is named after the Ordovices, a people of early Britain; the Devonian period is named after the county of Devon in south-west England).
The oldest rocks in the UK are gneisses which date from at least 2,700 Ma ("Ma" means "millions of years ago") in the Archaean Period, which are found in the far north west of Scotland and in the Hebrides, with a few small outcrops elsewhere. South of the gneisses are a complex mixture of rocks forming the North West Highlands and Grampian Highlands in Scotland, as well as the Connemara, Donegal and Mayo mountains of north Ireland. These are essentially the remains of folded sedimentary rock, deposited over the gneiss, from 1,000 Ma, with a notable 7 km thick layer of Torridon Sandstone being deposited about 800 Ma, as well as the debris deposited by an ice sheet 670 Ma.
The remains of ancient volcanic islands underlie much of central England with small outcrops visible in many places. Around 600 Ma, the Cadomian Orogeny (mountain building period) caused the English and Welsh landscape to be transformed into a mountainous region, along with much of north west Europe.
The Welsh Skiddaw slate deposits formed at around 500 Ma, during the Ordovician Period. At about this time, around 425 Ma, north Wales (and south Mayo in Ireland) experienced volcanic activity. The remains of these volcanoes are still visible, for example Rhobell Fwar, dating from 510 Ma. Large quantities of volcanic lava and ash known as the Borrowdale Volcanics covered both Wales and the Lake District, still seen in the form of mountains such as Helvellyn and Scafell Pike.
In the Silurian Period, between 425 and 400 Ma, the Caledonian fold mountains formed (the Caledonian Orogeny), covering much of what is now the UK to perhaps 8,000 feet (2,500 m) thick. Volcanic ashes and lavas deposited during this period are still found in the Mendip Hills and in Pembrokeshire.
Volcanic deposits formed Ben Nevis in the Devonian Period. Sea levels varied considerably, with the coastline advancing and retreating from north to south across England, and with the deposition of numerous sedimentary rock layers. The Old Red Sandstone of Devon gave the period its name, though deposits are found in many other places.
During the Carboniferous Period, around 360 Ma, the UK was lying at the equator, covered by the warm shallow waters of the Rheic Ocean, during which time the Carboniferous limestone was deposited, still found in areas such as the Mendip Hills and the Pennines. The coal measures were formed at this time, in river deltas, swamps and rain forests. Coal can be found in many areas of the UK, as far North as Sutherland and as far south as Kent, though it has largely been mined in the Midlands, northern England and Wales. Also formed were the Millstone Grits.
During the Permian and Triassic Periods, much of the UK was beneath shallow seas, leading to the deposition of sedimentary rocks such as shale, limestone, gravel, and marl. The seas finally receded to leave a flat desert with salt pans.
At the beginning of the Jurassic Period, the UK was under-water again, leading to the deposition of sedimentary rocks which now underlie much of England from the Cleveland Hills of Yorkshire to the Jurassic Coast in Dorset, including clays, sandstones, and the oolitic limestone of the Cotswold Hills. The burial of algae and bacteria below the mud of the sea floor during this time resulted in the formation of North Sea oil and natural gas.
In the Cretaceous Period, much of the UK was again below the sea and chalk and flints were deposited over much of Great Britain. These are now notably exposed at the White Cliffs of Dover, and form Salisbury Plain, the Chiltern Hills, the South Downs and other similar features.
The last volcanic rocks in the UK were formed in the early Tertiary Period, between 63 and 52 Ma, with the major eruptions that formed the Antrim Plateau and the basaltic columns of the Giant's Causeway. Further sediments were deposited over southern England, including the London clay, while the English Channel consisted of mud flats and river deposited sands.
The major changes during the last few million years, during the Quaternary Period, have been brought about by several recent ice ages, leaving a legacy of U-shaped valleys in highland areas, and fertile (if often stoney) soil in southern England.