England and Scotland each remained legally separate countries, each with its own parliament, until 1707, when each parliament passed an Act of Union to ratify the Treaty of Union that had been agreed the previous year. This created a single kingdom with one parliament with effect from 1 May 1707. The Treaty of Union specified the name of the new all-island state as "Great Britain", while describing it as "One Kingdom" and "the United Kingdom". To most historians, therefore, the all-island state that existed between 1707 and 1800 is either "Great Britain" or the "Kingdom of Great Britain". Geography Great Britain lies on the European continental shelf, part of the Eurasian Plate and off the north-west coast of continental Europe, separated from this European mainland by the North Sea and by the English Channel, which narrows to at the Straits of Dover. It stretches over about ten degrees of latitude on its longer, north–south axis and covers , excluding the much 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 since 1993 joined, via one structure, with continental Europe: the Channel Tunnel, the longest undersea rail tunnel in the world. 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 (between Land's End, Cornwall and John o' Groats, Caithness), by road. 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 north-western Europe, lying partially underneath the Eurasian ice sheet. The sea level was about 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 reflooded cutting off what was the