The Southern Alps divide the South Island into east and west. The rock which forms them was laid down on the sea floor off the coast of Gondwanaland between two and four hundred million years ago, then baked under immense pressure deep in the crust, folded, shuffled and finally thrust high by the collision between the Pacific and Indo-Australian tectonic plates. They are still being pushed upwards, but they have been ground and carved by glaciers and earthquakes, and erosion is always wearing them down. Vast quantities of ground-up rock and shingle have been carried out in rivers, creating the coastal plains of the South Island.

Arthur's Pass National Park is in the heart of the mountains, about 150 kilometres from the city of Christchurch, which sits on deposits of mountain shingle over 100 metres deep. Ground-up mountain rock is the key to the fertility of the Canterbury Plains, which stretch from the foothills to the sea for about 300 kilometres of the east coast.

Europeans arrived in the 1830s and 40s to find the plains largely depopulated by Maori inter-tribal warfare and cleared of forest by the fires of early Maori. Land was purchased and planned importation of a slice of British society and culture ensued. 'Gentleman' colonists practised large-scale pastoralism in the foothills and valleys of the Alps, producing large quantities of wool and high profits, while the lower social classes farmed what they could of the fertile flat lands close to the coast. By the late 1850s it was a reasonably prosperous piece of Britain in the South Pacific.

In 1863 gold was discovered on the west coast of the South Island and by 1865 it had become the commercial focus of the colony. Gold diggers rushed to get there and businesses scrambled for a piece of the action. The businesses of Christchurch and the Canterbury Plains, however, were on the wrong side of the mountains and needed a route to get a road through to the goldfields of the West Coast.

Maori had long crossed the Alps to find or trade for West Coast greenstone, which is an extremely hard and beautiful form of nephrite jade. One Maori name for the South Island is Te Wai Pounamou or greenstone water. They knew about the route of Arthur's Pass, but preferred the easier route up the Hurunui Valley with its abundant food sources to the swampy saddle of what we now call Harper Pass, about 30 kilometres north-east of Arthur's Pass.

In the summer of 1865, with goldrush fever at its peak, George Dobson was selecting the best road route through the mountains for the Canterbury authorities, when thousands of men and their supplies poured through the Harper Pass route, nearly destroying the path and rendering it almost impassable. George decided that the only option was a difficult and barely roadworthy route that his younger brother Arthur had explored the year before. He declared that "Arthur's pass is the best route", and so we get the name - Arthur's Pass.

Within a year a thousand men had cut a roadway out of the side of the mountains. It cost one hundred and fourty-five thousand pounds to build, but even more to maintain due to the unstable nature of the rock and the high rainfall. Actually, the government has just recently paid for a great big viaduct to be built to reduce the long-term cost of maintaining the highway so it's still an expensive place to have a road. Anyway, specially toughened coaches were making the 37 hour trip between the West Coast and Christchurch by 1866, but in the end the gold that the road had been built to transport was sent off on ships rather than carried through Arthur's Pass.

In 1923 the Otira Tunnel was opened and train services started. Because of the spectacular scenery, tourism boomed. Conservationists and botanists who had studied the area campaigned for it to become a park and in 1929 Arthur's Pass National Park was created. Now it covers over 100,000 hectares of mountains, valleys and great braided rivers with shingle beds. Dozens of tracks and routes link valleys through alpine passes and there is a good network of DOC-maintained huts and bivouacs throughout the park. It's extremely popular in summer, though heavy rain can make rivers unsafe to cross at any time of the year and the changeable nature of the weather is its most constant feature. In winter it is a challenging area for climbing and skiing enthusiasts, with 16 mountains over 2,000 metres.

The rain shadow effect of the Alps is so strong that while western slopes receive 6,000 to 9,000 mm of rain a year, eastern slopes only twenty kilometres away receive 1,500 to 2,000 mm. This leads to a great range of ecosystems and vegetation in the park. The west side has wet, dense rainforest with rimu, matai, miro, kamahi and rata, while the drier, colder east side has predominantly mountain beech forest and grasslands of tussock. East-side rivers are wide and braided with shingle beds, while those on the west are deeper, narrower and prone to flooding. Spring is generally the wettest season, with February and March the best months.