British peninsula from the European mainland by around 6500 BC. Geology 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. 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 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 My and 670 My ago over the gneiss on what was then the floor of the Iapetus Ocean. In the current era the north of the island is rising as a result of the weight of Devensian ice being lifted. Counterbalanced, the south and east is sinking, generally estimated at 1 mm ( inch) per year, with the London area sinking at double this partly due to the continuing compaction of the recent clay deposits. Fauna Animal diversity is modest, as a result of factors including the island's small land area, the relatively recent age of the habitats developed since the last glacial period and the island's physical separation from continental Europe, and the effects of seasonal variability. Great Britain also experienced early industrialisation and is subject to continuing urbanisation, which have contributed towards the overall loss of species. A DEFRA (Department for Environment, Food and Rural Affairs) study from 2006 suggested that 100 species have become extinct in the UK during the 20th century, about 100 times the background extinction rate. However, some species, such as the brown rat, red fox, and introduced grey squirrel, are well adapted to urban areas. Rodents make up 40% of the mammal species. These include squirrels, mice, voles, rats and the recently reintroduced European beaver. There is also an abundance of European rabbit, European hare, shrews, European mole and several species of bat. Carnivorous mammals include