<P> Great Britain lies on the European continental shelf, part of the Eurasian Plate . Situated off the north - west coast of continental Europe, it is separated from the mainland by the North Sea and by the English Channel, which narrows to 34 km (18 nmi; 21 mi) at the Straits of Dover . It stretches over about ten degrees of latitude on its longer, north - south axis and occupies an area of 209,331 km (80,823 sq mi), excluding the smaller surrounding islands . The North Channel, Irish Sea, St George's Channel and Celtic Sea separate the island from the island of Ireland to its west . The island is physically connected with continental Europe via the Channel Tunnel, the longest undersea rail tunnel in the world, completed in 1993 . The island is marked by low, rolling countryside in the east and south, while hills and mountains predominate in the western and northern regions . It is surrounded by over 1,000 smaller islands and islets . The greatest distance between two points is 968.0 km (601 ⁄ mi) (between Land's End, Cornwall and John o' Groats, Caithness), 838 miles (1,349 km) by road . </P> <P> The English Channel is thought to have been created between 450,000 and 180,000 years ago by two catastrophic glacial lake outburst floods caused by the breaching of the Weald - Artois Anticline, a ridge that held back a large proglacial lake, now submerged under the North Sea . Around 10,000 years ago, during the Devensian glaciation with its lower sea level, Great Britain was not an island, but an upland region of continental northwestern Europe, lying partially underneath the Eurasian ice sheet . The sea level was about 120 metres (390 ft) lower than today, and the bed of the North Sea was dry and acted as a land bridge, now known as Doggerland, to the Continent . It is generally thought that as sea levels gradually rose after the end of the last glacial period of the current ice age, Doggerland became submerged beneath the North Sea, cutting off what was previously the British peninsula from the European mainland by around 6500 BC . </P> <P> Great Britain has been subject to a variety of plate tectonic processes over a very extended period of time . Changing latitude and sea levels have been important factors in the nature of sedimentary sequences, whilst successive continental collisions have affected its geological structure with major faulting and folding being a legacy of each orogeny (mountain - building period), often associated with volcanic activity and the metamorphism of existing rock sequences . As a result of this eventful geological history, the island shows a rich variety of landscapes . </P> <P> The oldest rocks in Great Britain are the Lewisian gneisses, metamorphic rocks found in the far north west of the island and in the Hebrides (with a few small outcrops elsewhere), which date from at least 2,700 Ma (Ma = million years ago). South of the gneisses are a complex mixture of rocks forming the North West Highlands and Grampian Highlands 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 . </P>

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